# QString 类

QString 类提供 Unicode 字符串。 更多...

 Header: #include CMake: find_package(Qt6 COMPONENTS Core REQUIRED) target_link_libraries(mytarget PRIVATE Qt6::Core) qmake: QT += core

## 公共类型

 ConstIterator Iterator enum NormalizationForm { NormalizationForm_D, NormalizationForm_C, NormalizationForm_KD, NormalizationForm_KC } enum SectionFlag { SectionDefault, SectionSkipEmpty, SectionIncludeLeadingSep, SectionIncludeTrailingSep, SectionCaseInsensitiveSeps } flags SectionFlags const_iterator const_pointer const_reference const_reverse_iterator difference_type iterator pointer reference reverse_iterator size_type value_type

## 公共函数

 QString (const QByteArray & ba ) QString (const char * str ) QString (QString && other ) QString (const QString & other ) QString (const char8_t * str ) QString (QLatin1String str ) QString (qsizetype size , QChar ch ) QString (QChar ch ) QString (const QChar * unicode , qsizetype size = -1) QString () QString & operator= (const QByteArray & ba ) QString & operator= (QString && other ) QString & operator= (const QString & other ) ~QString () QString & append (const QString & str ) QString & append (QChar ch ) QString & append (const QChar * str , qsizetype len ) QString & append (QLatin1String str ) QString & append (const char * str ) QString & append (const QByteArray & ba ) QString arg (const QString & a , int fieldWidth = 0, QChar fillChar = QLatin1Char(' ')) const QString arg (qlonglong a , int fieldWidth = 0, int base = 10, QChar fillChar = QLatin1Char(' ')) const QString arg (qulonglong a , int fieldWidth = 0, int base = 10, QChar fillChar = QLatin1Char(' ')) const QString arg (long a , int fieldWidth = 0, int base = 10, QChar fillChar = QLatin1Char(' ')) const QString arg (ulong a , int fieldWidth = 0, int base = 10, QChar fillChar = QLatin1Char(' ')) const QString arg (int a , int fieldWidth = 0, int base = 10, QChar fillChar = QLatin1Char(' ')) const QString arg (uint a , int fieldWidth = 0, int base = 10, QChar fillChar = QLatin1Char(' ')) const QString arg (short a , int fieldWidth = 0, int base = 10, QChar fillChar = QLatin1Char(' ')) const QString arg (ushort a , int fieldWidth = 0, int base = 10, QChar fillChar = QLatin1Char(' ')) const QString arg (double a , int fieldWidth = 0, char format = 'g', int precision = -1, QChar fillChar = QLatin1Char(' ')) const QString arg (char a , int fieldWidth = 0, QChar fillChar = QLatin1Char(' ')) const QString arg (QChar a , int fieldWidth = 0, QChar fillChar = QLatin1Char(' ')) const QString arg (QStringView a , int fieldWidth = 0, QChar fillChar = QLatin1Char(' ')) const QString arg (QLatin1String a , int fieldWidth = 0, QChar fillChar = QLatin1Char(' ')) const QString arg (Args &&... args ) const const QChar at (qsizetype position ) const QChar back () const QChar & back () QString::iterator begin () QString::const_iterator begin () const qsizetype capacity () const QString::const_iterator cbegin () const QString::const_iterator cend () const void chop (qsizetype n ) QString chopped (qsizetype len ) const void clear () int compare (const QString & other , Qt::CaseSensitivity cs = Qt::CaseSensitive) const int compare (QLatin1String other , Qt::CaseSensitivity cs = Qt::CaseSensitive) const int compare (QStringView s , Qt::CaseSensitivity cs = Qt::CaseSensitive) const int compare (QChar ch , Qt::CaseSensitivity cs = Qt::CaseSensitive) const QString::const_iterator constBegin () const const QChar * constData () const QString::const_iterator constEnd () const bool contains (const QString & str , Qt::CaseSensitivity cs = Qt::CaseSensitive) const bool contains (QChar ch , Qt::CaseSensitivity cs = Qt::CaseSensitive) const bool contains (QLatin1String str , Qt::CaseSensitivity cs = Qt::CaseSensitive) const bool contains (QStringView str , Qt::CaseSensitivity cs = Qt::CaseSensitive) const bool contains (const QRegularExpression & re , QRegularExpressionMatch * rmatch = nullptr) const qsizetype count (const QString & str , Qt::CaseSensitivity cs = Qt::CaseSensitive) const qsizetype count () const qsizetype count (QChar ch , Qt::CaseSensitivity cs = Qt::CaseSensitive) const qsizetype count (QStringView str , Qt::CaseSensitivity cs = Qt::CaseSensitive) const qsizetype count (const QRegularExpression & re ) const QString::const_reverse_iterator crbegin () const QString::const_reverse_iterator crend () const QChar * data () const QChar * data () const QString::iterator end () QString::const_iterator end () const bool endsWith (const QString & s , Qt::CaseSensitivity cs = Qt::CaseSensitive) const bool endsWith (QStringView str , Qt::CaseSensitivity cs = Qt::CaseSensitive) const bool endsWith (QLatin1String s , Qt::CaseSensitivity cs = Qt::CaseSensitive) const bool endsWith (QChar c , Qt::CaseSensitivity cs = Qt::CaseSensitive) const QString::iterator erase (QString::const_iterator first , QString::const_iterator last ) QString & fill (QChar ch , qsizetype size = -1) QString first (qsizetype n ) const QChar front () const QChar & front () qsizetype indexOf (QLatin1String str , qsizetype from = 0, Qt::CaseSensitivity cs = Qt::CaseSensitive) const qsizetype indexOf (QChar ch , qsizetype from = 0, Qt::CaseSensitivity cs = Qt::CaseSensitive) const qsizetype indexOf (const QString & str , qsizetype from = 0, Qt::CaseSensitivity cs = Qt::CaseSensitive) const qsizetype indexOf (QStringView str , qsizetype from = 0, Qt::CaseSensitivity cs = Qt::CaseSensitive) const qsizetype indexOf (const QRegularExpression & re , qsizetype from = 0, QRegularExpressionMatch * rmatch = nullptr) const QString & insert (qsizetype position , const QString & str ) QString & insert (qsizetype position , QChar ch ) QString & insert (qsizetype position , const QChar * unicode , qsizetype size ) QString & insert (qsizetype position , QStringView str ) QString & insert (qsizetype position , QLatin1String str ) QString & insert (qsizetype position , const char * str ) QString & insert (qsizetype position , const QByteArray & str ) bool isEmpty () const bool isLower () const bool isNull () const bool isRightToLeft () const bool isUpper () const bool isValidUtf16 () const QString last (qsizetype n ) const qsizetype lastIndexOf (const QString & str , qsizetype from = -1, Qt::CaseSensitivity cs = Qt::CaseSensitive) const qsizetype lastIndexOf (QChar ch , qsizetype from = -1, Qt::CaseSensitivity cs = Qt::CaseSensitive) const qsizetype lastIndexOf (QLatin1String str , qsizetype from = -1, Qt::CaseSensitivity cs = Qt::CaseSensitive) const qsizetype lastIndexOf (QStringView str , qsizetype from = -1, Qt::CaseSensitivity cs = Qt::CaseSensitive) const qsizetype lastIndexOf (const QRegularExpression & re , qsizetype from = -1, QRegularExpressionMatch * rmatch = nullptr) const QString left (qsizetype n ) const QString leftJustified (qsizetype width , QChar fill = QLatin1Char(' '), bool truncate = false) const qsizetype length () const int localeAwareCompare (const QString & other ) const int localeAwareCompare (QStringView other ) const QString mid (qsizetype position , qsizetype n = -1) const QString normalized (QString::NormalizationForm mode , QChar::UnicodeVersion version = QChar::Unicode_Unassigned) const QString & prepend (const QString & str ) QString & prepend (QChar ch ) QString & prepend (const QChar * str , qsizetype len ) QString & prepend (QStringView str ) QString & prepend (QLatin1String str ) QString & prepend (const char * str ) QString & prepend (const QByteArray & ba ) void push_back (const QString & other ) void push_back (QChar ch ) void push_front (const QString & other ) void push_front (QChar ch ) QString::reverse_iterator rbegin () QString::const_reverse_iterator rbegin () const QString & remove (qsizetype position , qsizetype n ) QString & remove (QChar ch , Qt::CaseSensitivity cs = Qt::CaseSensitive) QString & remove (QLatin1String str , Qt::CaseSensitivity cs = Qt::CaseSensitive) QString & remove (const QString & str , Qt::CaseSensitivity cs = Qt::CaseSensitive) QString & remove (const QRegularExpression & re ) QString & removeIf (Predicate pred ) QString::reverse_iterator rend () QString::const_reverse_iterator rend () const QString repeated (qsizetype times ) const QString & replace (qsizetype position , qsizetype n , const QString & after ) QString & replace (qsizetype position , qsizetype n , QChar after ) QString & replace (qsizetype position , qsizetype n , const QChar * unicode , qsizetype size ) QString & replace (QChar before , QChar after , Qt::CaseSensitivity cs = Qt::CaseSensitive) QString & replace (const QChar * before , qsizetype blen , const QChar * after , qsizetype alen , Qt::CaseSensitivity cs = Qt::CaseSensitive) QString & replace (QLatin1String before , QLatin1String after , Qt::CaseSensitivity cs = Qt::CaseSensitive) QString & replace (QLatin1String before , const QString & after , Qt::CaseSensitivity cs = Qt::CaseSensitive) QString & replace (const QString & before , QLatin1String after , Qt::CaseSensitivity cs = Qt::CaseSensitive) QString & replace (const QString & before , const QString & after , Qt::CaseSensitivity cs = Qt::CaseSensitive) QString & replace (QChar ch , const QString & after , Qt::CaseSensitivity cs = Qt::CaseSensitive) QString & replace (QChar c , QLatin1String after , Qt::CaseSensitivity cs = Qt::CaseSensitive) QString & replace (const QRegularExpression & re , const QString & after ) void reserve (qsizetype size ) void resize (qsizetype size ) void resize (qsizetype size , QChar fillChar ) QString right (qsizetype n ) const QString rightJustified (qsizetype width , QChar fill = QLatin1Char(' '), bool truncate = false) const QString section (QChar sep , qsizetype start , qsizetype end = -1, QString::SectionFlags flags = SectionDefault) const QString section (const QString & sep , qsizetype start , qsizetype end = -1, QString::SectionFlags flags = SectionDefault) const QString section (const QRegularExpression & re , qsizetype start , qsizetype end = -1, QString::SectionFlags flags = SectionDefault) const QString & setNum (int n , int base = 10) QString & setNum (short n , int base = 10) QString & setNum (ushort n , int base = 10) QString & setNum (uint n , int base = 10) QString & setNum (long n , int base = 10) QString & setNum (ulong n , int base = 10) QString & setNum (qlonglong n , int base = 10) QString & setNum (qulonglong n , int base = 10) QString & setNum (float n , char format = 'g', int precision = 6) QString & setNum (double n , char format = 'g', int precision = 6) QString & setRawData (const QChar * unicode , qsizetype size ) QString & setUnicode (const QChar * unicode , qsizetype size ) QString & setUtf16 (const ushort * unicode , qsizetype size ) void shrink_to_fit () QString simplified () const qsizetype size () const QString sliced (qsizetype pos , qsizetype n ) const QString sliced (qsizetype pos ) const QStringList split (const QString & sep , Qt::SplitBehavior behavior = Qt::KeepEmptyParts, Qt::CaseSensitivity cs = Qt::CaseSensitive) const QStringList split (QChar sep , Qt::SplitBehavior behavior = Qt::KeepEmptyParts, Qt::CaseSensitivity cs = Qt::CaseSensitive) const QStringList split (const QRegularExpression & re , Qt::SplitBehavior behavior = Qt::KeepEmptyParts) const void squeeze () bool startsWith (const QString & s , Qt::CaseSensitivity cs = Qt::CaseSensitive) const bool startsWith (QStringView str , Qt::CaseSensitivity cs = Qt::CaseSensitive) const bool startsWith (QLatin1String s , Qt::CaseSensitivity cs = Qt::CaseSensitive) const bool startsWith (QChar c , Qt::CaseSensitivity cs = Qt::CaseSensitive) const void swap (QString & other ) CFStringRef toCFString () const QString toCaseFolded () const double toDouble (bool * ok = nullptr) const float toFloat (bool * ok = nullptr) const QString toHtmlEscaped () const int toInt (bool * ok = nullptr, int base = 10) const QByteArray toLatin1 () const QByteArray toLocal8Bit () const long toLong (bool * ok = nullptr, int base = 10) const qlonglong toLongLong (bool * ok = nullptr, int base = 10) const QString toLower () const NSString * toNSString () const short toShort (bool * ok = nullptr, int base = 10) const std::string toStdString () const std::u16string toStdU16String () const std::u32string toStdU32String () const std::wstring toStdWString () const uint toUInt (bool * ok = nullptr, int base = 10) const ulong toULong (bool * ok = nullptr, int base = 10) const qulonglong toULongLong (bool * ok = nullptr, int base = 10) const ushort toUShort (bool * ok = nullptr, int base = 10) const QList toUcs4 () const QString toUpper () const QByteArray toUtf8 () const qsizetype toWCharArray (wchar_t * array ) const decltype(qTokenize(*this, std::forward(needle), flags...)) tokenize (Needle && sep , Flags... flags ) const & decltype(qTokenize(std::move(*this), std::forward(needle), flags...)) tokenize (Needle && sep , Flags... flags ) && QString trimmed () const void truncate (qsizetype position ) const QChar * unicode () const const ushort * utf16 () const bool operator!= (const char * other ) const bool operator!= (const QByteArray & other ) const QString & operator+= (const QString & other ) QString & operator+= (QChar ch ) QString & operator+= (QStringView str ) QString & operator+= (QLatin1String str ) QString & operator+= (const char * str ) QString & operator+= (const QByteArray & ba ) bool operator< (const char * other ) const bool operator< (const QByteArray & other ) const bool operator<= (const char * other ) const bool operator<= (const QByteArray & other ) const QString & operator= (QChar ch ) QString & operator= (QLatin1String str ) QString & operator= (const char * str ) bool operator== (const char * other ) const bool operator== (const QByteArray & other ) const bool operator> (const char * other ) const bool operator> (const QByteArray & other ) const bool operator>= (const char * other ) const bool operator>= (const QByteArray & other ) const QChar & operator[] (qsizetype position ) const QChar operator[] (qsizetype position ) const

## 静态公共成员

 QString asprintf (const char * cformat , ...) int compare (const QString & s1 , const QString & s2 , Qt::CaseSensitivity cs = Qt::CaseSensitive) int compare (const QString & s1 , QLatin1String s2 , Qt::CaseSensitivity cs = Qt::CaseSensitive) int compare (QLatin1String s1 , const QString & s2 , Qt::CaseSensitivity cs = Qt::CaseSensitive) int compare (const QString & s1 , QStringView s2 , Qt::CaseSensitivity cs = Qt::CaseSensitive) int compare (QStringView s1 , const QString & s2 , Qt::CaseSensitivity cs = Qt::CaseSensitive) QString fromCFString (CFStringRef string ) QString fromLatin1 (const char * str , qsizetype size ) QString fromLatin1 (QByteArrayView str ) QString fromLatin1 (const QByteArray & str ) QString fromLocal8Bit (const char * str , qsizetype size ) QString fromLocal8Bit (QByteArrayView str ) QString fromLocal8Bit (const QByteArray & str ) QString fromNSString (const NSString * string ) QString fromRawData (const QChar * unicode , qsizetype size ) QString fromStdString (const std::string & str ) QString fromStdU16String (const std::u16string & str ) QString fromStdU32String (const std::u32string & str ) QString fromStdWString (const std::wstring & str ) QString fromUcs4 (const char32_t * unicode , qsizetype size = -1) QString fromUtf8 (const char * str , qsizetype size ) QString fromUtf8 (QByteArrayView str ) QString fromUtf8 (const QByteArray & str ) QString fromUtf8 (const char8_t * str ) QString fromUtf8 (const char8_t * str , qsizetype size ) QString fromUtf16 (const char16_t * unicode , qsizetype size = -1) QString fromWCharArray (const wchar_t * string , qsizetype size = -1) int localeAwareCompare (const QString & s1 , const QString & s2 ) int localeAwareCompare (QStringView s1 , QStringView s2 ) QString number (long n , int base = 10) QString number (int n , int base = 10) QString number (uint n , int base = 10) QString number (ulong n , int base = 10) QString number (qlonglong n , int base = 10) QString number (qulonglong n , int base = 10) QString number (double n , char format = 'g', int precision = 6) QString vasprintf (const char * cformat , va_list ap )
 qsizetype erase (QString & s , const T & t ) qsizetype erase_if (QString & s , Predicate pred ) bool operator!= (const QString & s1 , const QString & s2 ) bool operator!= (const QString & s1 , QLatin1String s2 ) bool operator!= (const char * s1 , const QString & s2 ) const QString operator+ (const QString & s1 , const QString & s2 ) const QString operator+ (const QString & s1 , const char * s2 ) const QString operator+ (const char * s1 , const QString & s2 ) bool operator< (const QString & s1 , const QString & s2 ) bool operator< (const QString & s1 , QLatin1String s2 ) bool operator< (QLatin1String s1 , const QString & s2 ) bool operator< (const char * s1 , const QString & s2 ) QDataStream & operator<< (QDataStream & stream , const QString & string ) bool operator<= (const QString & s1 , const QString & s2 ) bool operator<= (const QString & s1 , QLatin1String s2 ) bool operator<= (QLatin1String s1 , const QString & s2 ) bool operator<= (const char * s1 , const QString & s2 ) bool operator== (const QString & s1 , const QString & s2 ) bool operator== (const QString & s1 , QLatin1String s2 ) bool operator== (QLatin1String s1 , const QString & s2 ) bool operator== (const char * s1 , const QString & s2 ) bool operator> (const QString & s1 , const QString & s2 ) bool operator> (const QString & s1 , QLatin1String s2 ) bool operator> (QLatin1String s1 , const QString & s2 ) bool operator> (const char * s1 , const QString & s2 ) bool operator>= (const QString & s1 , const QString & s2 ) bool operator>= (const QString & s1 , QLatin1String s2 ) bool operator>= (QLatin1String s1 , const QString & s2 ) bool operator>= (const char * s1 , const QString & s2 ) QDataStream & operator>> (QDataStream & stream , QString & string )

## 宏

 QStringLiteral ( str ) QT_NO_CAST_FROM_ASCII QT_NO_CAST_TO_ASCII QT_RESTRICTED_CAST_FROM_ASCII

## 详细描述

QString 存储字符串的 16 位 QChar ，其中各 QChar corresponds to one UTF-16 code unit. (Unicode characters with code values above 65535 are stored using surrogate pairs, i.e., two consecutive QChar s.)

Unicode is an international standard that supports most of the writing systems in use today. It is a superset of US-ASCII (ANSI X3.4-1986) and Latin-1 (ISO 8859-1), and all the US-ASCII/Latin-1 characters are available at the same code positions.

Behind the scenes, QString uses 隐式共享 (copy-on-write) to reduce memory usage and to avoid the needless copying of data. This also helps reduce the inherent overhead of storing 16-bit characters instead of 8-bit characters.

In addition to QString, Qt also provides the QByteArray class to store raw bytes and traditional 8-bit '\0'-terminated strings. For most purposes, QString is the class you want to use. It is used throughout the Qt API, and the Unicode support ensures that your applications will be easy to translate if you want to expand your application's market at some point. The two main cases where QByteArray is appropriate are when you need to store raw binary data, and when memory conservation is critical (like in embedded systems).

### 初始化字符串

One way to initialize a QString is simply to pass a  const char *  to its constructor. For example, the following code creates a QString of size 5 containing the data "Hello":

QString str = "Hello";


QString 转换  const char *  数据成 Unicode 使用 fromUtf8 () 函数。

In all of the QString functions that take  const char *  parameters, the  const char *  is interpreted as a classic C-style '\0'-terminated string encoded in UTF-8. It is legal for the  const char *  parameter to be  nullptr  .

You can also provide string data as an array of QChar s:

static const QChar data[4] = { 0x0055, 0x006e, 0x10e3, 0x03a3 };
QString str(data, 4);


QString makes a deep copy of the QChar data, so you can modify it later without experiencing side effects. (If for performance reasons you don't want to take a deep copy of the character data, use QString::fromRawData () instead.)

Another approach is to set the size of the string using resize () and to initialize the data character per character. QString uses 0-based indexes, just like C++ arrays. To access the character at a particular index position, you can use operator[] (). On non-  const  strings, operator[] () returns a reference to a character that can be used on the left side of an assignment. For example:

QString str;
str.resize(4);
str[0] = QChar('U');
str[1] = QChar('n');
str[2] = QChar(0x10e3);
str[3] = QChar(0x03a3);


For read-only access, an alternative syntax is to use the at () 函数：

QString str;
for (qsizetype i = 0; i < str.size(); ++i) {
if (str.at(i) >= QChar('a') && str.at(i) <= QChar('f'))
qDebug() << "Found character in range [a-f]";
}


at () function can be faster than operator[] (), because it never causes a deep copy to occur. Alternatively, use the first (), last ()，或 sliced () functions to extract several characters at a time.

A QString can embed '\0' characters ( QChar::Null ). The size () function always returns the size of the whole string, including embedded '\0' characters.

After a call to the resize () function, newly allocated characters have undefined values. To set all the characters in the string to a particular value, use the fill () 函数。

QString provides dozens of overloads designed to simplify string usage. For example, if you want to compare a QString with a string literal, you can write code like this and it will work as expected:

QString str;
if (str == "auto" || str == "extern"
|| str == "static" || str == "register") {
// ...
}


You can also pass string literals to functions that take QStrings as arguments, invoking the QString(const char *) constructor. Similarly, you can pass a QString to a function that takes a  const char *  argument using the qPrintable () macro which returns the given QString as a  const char *  . This is equivalent to calling <QString>. toLocal8Bit (). constData ().

### 操纵字符串数据

QString provides the following basic functions for modifying the character data: append (), prepend (), insert (), replace ()，和 remove ()。例如：

QString str = "and";
str.prepend("rock ");     // str == "rock and"
str.append(" roll");        // str == "rock and roll"
str.replace(5, 3, "&");   // str == "rock & roll"


In the above example the replace () function's first two arguments are the position from which to start replacing and the number of characters that should be replaced.

When data-modifying functions increase the size of the string, they may lead to reallocation of memory for the QString object. When this happens, QString expands by more than it immediately needs so as to have space for further expansion without reallocation until the size of the string has greatly increased.

insert (), remove () and, when replacing a sub-string with one of different size, replace () functions can be slow ( linear time ) for large strings, because they require moving many characters in the string by at least one position in memory.

If you are building a QString gradually and know in advance approximately how many characters the QString will contain, you can call reserve (), asking QString to preallocate a certain amount of memory. You can also call capacity () to find out how much memory the QString actually has allocated.

QString provides STL 样式迭代器 ( QString::const_iterator and QString::iterator ). In practice, iterators are handy when working with generic algorithms provided by the C++ standard library.

A frequent requirement is to remove whitespace characters from a string ('\n', '\t', ' ', etc.). If you want to remove whitespace from both ends of a QString, use the trimmed () function. If you want to remove whitespace from both ends and replace multiple consecutive whitespaces with a single space character within the string, use simplified ().

If you want to find all occurrences of a particular character or substring in a QString, use the indexOf () 或 lastIndexOf () functions. The former searches forward starting from a given index position, the latter searches backward. Both return the index position of the character or substring if they find it; otherwise, they return -1. For example, here is a typical loop that finds all occurrences of a particular substring:

QString str = "We must be <b>bold</b>, very <b>bold</b>";
qsizetype j = 0;
while ((j = str.indexOf("<b>", j)) != -1) {
qDebug() << "Found <b> tag at index position" << j;
++j;
}


QString provides many functions for converting numbers into strings and strings into numbers. See the arg () functions, the setNum () functions, the number () static functions, and the toInt (), toDouble (), and similar functions.

To get an upper- or lowercase version of a string use toUpper () 或 toLower ().

Lists of strings are handled by the QStringList class. You can split a string into a list of strings using the split () function, and join a list of strings into a single string with an optional separator using QStringList::join (). You can obtain a list of strings from a string list that contain a particular substring or that match a particular QRegularExpression 使用 QStringList::filter () 函数。

### 查询字符串数据

If you want to see if a QString starts or ends with a particular substring use startsWith () 或 endsWith (). If you simply want to check whether a QString contains a particular character or substring, use the contains () function. If you want to find out how many times a particular character or substring occurs in the string, use count ().

To obtain a pointer to the actual character data, call data () 或 constData (). These functions return a pointer to the beginning of the QChar data. The pointer is guaranteed to remain valid until a non-  const  function is called on the QString.

#### Comparing Strings

QStrings can be compared using overloaded operators such as operator< (), operator<= (), operator== (), operator>= (), and so on. Note that the comparison is based exclusively on the numeric Unicode values of the characters. It is very fast, but is not what a human would expect; the QString::localeAwareCompare () function is usually a better choice for sorting user-interface strings, when such a comparison is available.

On Unix-like platforms (including Linux, macOS and iOS), when Qt is linked with the ICU library (which it usually is), its locale-aware sorting is used. Otherwise, on macOS and iOS, localeAwareCompare () compares according the "Order for sorted lists" setting in the International preferences panel. On other Unix-like systems without ICU, the comparison falls back to the system library's  strcoll()  ,

### 在编码字符串数据和 QString 之间转换

QString provides the following three functions that return a  const char *  version of the string as QByteArray : toUtf8 (), toLatin1 ()，和 toLocal8Bit ().

• toLatin1 () returns a Latin-1 (ISO 8859-1) encoded 8-bit string.
• toUtf8 () returns a UTF-8 encoded 8-bit string. UTF-8 is a superset of US-ASCII (ANSI X3.4-1986) that supports the entire Unicode character set through multibyte sequences.
• toLocal8Bit () returns an 8-bit string using the system's local encoding. This is the same as toUtf8 () on Unix systems.

To convert from one of these encodings, QString provides fromLatin1 (), fromUtf8 ()，和 fromLocal8Bit (). Other encodings are supported through the QStringEncoder and QStringDecoder 类。

As mentioned above, QString provides a lot of functions and operators that make it easy to interoperate with  const char *  strings. But this functionality is a double-edged sword: It makes QString more convenient to use if all strings are US-ASCII or Latin-1, but there is always the risk that an implicit conversion from or to  const char *  is done using the wrong 8-bit encoding. To minimize these risks, you can turn off these implicit conversions by defining some of the following preprocessor symbols:

You then need to explicitly call fromUtf8 (), fromLatin1 ()，或 fromLocal8Bit () to construct a QString from an 8-bit string, or use the lightweight QLatin1String class, for example:

QString url = QLatin1String("http://www.unicode.org/");


Similarly, you must call toLatin1 (), toUtf8 ()，或 toLocal8Bit () explicitly to convert the QString to an 8-bit string.

Note for C Programmers
Due to C++'s type system and the fact that QString is 隐式共享 , QStrings may be treated like  int  s or other basic types. For example:
QString Widget::boolToString(bool b)
{

QString

result;

if

(b)
result

=

"True"

;

else

result

=

"False"

;

return

result;
}


 result  variable, is a normal variable allocated on the stack. When  return  is called, and because we're returning by value, the copy constructor is called and a copy of the string is returned. No actual copying takes place thanks to the implicit sharing.

### Null 和空字符串之间的区别

For historical reasons, QString distinguishes between a null string and an empty string. A null string is a string that is initialized using QString's default constructor or by passing (  const char *  )0 to the constructor. An empty string is any string with size 0. A null string is always empty, but an empty string isn't necessarily null:

QString().isNull();               // returns true
QString().isEmpty();              // returns true
QString("").isNull();             // returns false
QString("").isEmpty();            // returns true
QString("abc").isNull();          // returns false
QString("abc").isEmpty();         // returns false


All functions except isNull () treat null strings the same as empty strings. For example, toUtf8 (). constData () returns a valid pointer ( not nullptr) to a '\0' character for a null string. We recommend that you always use the isEmpty () function and avoid isNull ().

### 自变量格式

In member functions where an argument format can be specified (e.g., arg (), number ()), the argument format can be one of the following:

 e  format as [-]9.9e[+|-]999
 E  format as [-]9.9E[+|-]999
 f  format as [-]9.9
 g  use  e  or  f  format, whichever is the most concise
 G  use  E  or  f  format, whichever is the most concise

A precision is also specified with the argument format . For the 'e', 'E', and 'f' formats, the precision represents the number of digits after the decimal point. For the 'g' and 'G' formats, the precision represents the maximum number of significant digits (trailing zeroes are omitted).

### 更高效的字符串构造

Many strings are known at compile time. But the trivial constructor QString("Hello"), will copy the contents of the string, treating the contents as Latin-1. To avoid this one can use the QStringLiteral macro to directly create the required data at compile time. Constructing a QString out of the literal does then not cause any overhead at runtime.

A slightly less efficient way is to use QLatin1String . This class wraps a C string literal, precalculates it length at compile time and can then be used for faster comparison with QStrings and conversion to QStrings than a regular C string literal.

Using the QString  '+'  operator, it is easy to construct a complex string from multiple substrings. You will often write code like this:

    QString foo;
QString type = "long";
foo = QLatin1String("vector<") + type + QLatin1String(">::iterator");
if (foo.startsWith("(" + type + ") 0x"))
...


There is nothing wrong with either of these string constructions, but there are a few hidden inefficiencies. Beginning with Qt 4.6, you can eliminate them.

First, multiple uses of the  '+'  operator usually means multiple memory allocations. When concatenating n substrings, where n > 2 , there can be as many as n - 1 calls to the memory allocator.

In 4.6, an internal template class  QStringBuilder  has been added along with a few helper functions. This class is marked internal and does not appear in the documentation, because you aren't meant to instantiate it in your code. Its use will be automatic, as described below. The class is found in  src/corelib/tools/qstringbuilder.cpp  if you want to have a look at it.

 QStringBuilder  uses expression templates and reimplements the  '%'  operator so that when you use  '%'  for string concatenation instead of  '+'  , multiple substring concatenations will be postponed until the final result is about to be assigned to a QString. At this point, the amount of memory required for the final result is known. The memory allocator is then called once to get the required space, and the substrings are copied into it one by one.

Additional efficiency is gained by inlining and reduced reference counting (the QString created from a  QStringBuilder  typically has a ref count of 1, whereas QString::append () needs an extra test).

There are two ways you can access this improved method of string construction. The straightforward way is to include  QStringBuilder  wherever you want to use it, and use the  '%'  operator instead of  '+'  when concatenating strings:

    #include <QStringBuilder>
QString hello("hello");
QStringView el = QStringView{ hello }.mid(2, 3);
QLatin1String world("world");
QString message =  hello % el % world % QChar('!');


A more global approach which is the most convenient but not entirely source compatible, is to this define in your .pro file:

    DEFINES *= QT_USE_QSTRINGBUILDER


 '+'  will automatically be performed as the  QStringBuilder   '%'  everywhere.

### 最大尺寸和内存不足情况

The maximum size of QString depends on the architecture. Most 64-bit systems can allocate more than 2 GB of memory, with a typical limit of 2^63 bytes. The actual value also depends on the overhead required for managing the data block. As a result, you can expect the maximum size of 2 GB minus overhead on 32-bit platforms, and 2^63 bytes minus overhead on 64-bit platforms. The number of elements that can be stored in a QString is this maximum size divided by the size of QChar .

When memory allocation fails, QString throws a  std::bad_alloc  exception if the application was compiled with exception support. Out of memory conditions in Qt containers are the only case where Qt will throw exceptions. If exceptions are disabled, then running out of memory is undefined behavior.

Note that the operating system may impose further limits on applications holding a lot of allocated memory, especially large, contiguous blocks. Such considerations, the configuration of such behavior or any mitigation are outside the scope of the Qt API.

## 成员类型文档编制

### QString:: ConstIterator

Qt 样式同义词 QString::const_iterator .

### QString:: Iterator

Qt 样式同义词 QString::iterator .

### enum QString:: NormalizationForm

 QString::NormalizationForm_D   0  典型分解
 QString::NormalizationForm_C   1  典型分解，紧接着是典型合成
 QString::NormalizationForm_KD   2  兼容性分解
 QString::NormalizationForm_KC   3  兼容性分解，紧接着是典型合成

### enum QString:: SectionFlag flags QString:: SectionFlags

This enum specifies flags that can be used to affect various aspects of the section () function's behavior with respect to separators and empty fields.

 QString::SectionDefault   0x00  Empty fields are counted, leading and trailing separators are not included, and the separator is compared case sensitively.
 QString::SectionSkipEmpty   0x01  Treat empty fields as if they don't exist, i.e. they are not considered as far as start and end are concerned.
 QString::SectionIncludeLeadingSep   0x02  Include the leading separator (if any) in the result string.
 QString::SectionIncludeTrailingSep   0x04  Include the trailing separator (if any) in the result string.
 QString::SectionCaseInsensitiveSeps   0x08  Compare the separator case-insensitively.

The SectionFlags type is a typedef for QFlags <SectionFlag>. It stores an OR combination of SectionFlag values.

### QString:: const_pointer

The QString::const_pointer typedef provides an STL-style const pointer to a QString 元素 ( QChar ).

### QString:: pointer

The QString::pointer typedef provides an STL-style pointer to a QString 元素 ( QChar ).

## 成员函数文档编制

### template <typename Needle, typename Flags> decltype ( qTokenize (* this , std::forward < Needle >( needle ), flags ...)) QString:: tokenize ( Needle && sep , Flags ... flags ) const &

Splits the string into substring views wherever sep occurs, and returns a lazy sequence of those strings.

return QStringTokenizer{std::forward<Needle>(sep), flags...};


except it works without C++17 Class Template Argument Deduction (CTAD) enabled in the compiler.

QStringTokenizer for how sep and flags interact to form the result.

QStringTokenizer result = sv.tokenize(sep);


(without template arguments). If you can't use C++17 CTAD, you must store the return value only in  auto  variables:

auto result = sv.tokenize(sep);


This is because the template arguments of QStringTokenizer have a very subtle dependency on the specific tokenize () overload from which they are returned, and they don't usually correspond to the type used for the separator.

### template <typename Args> QString QString:: arg ( Args &&... args ) const

Replaces occurrences of  %N  in this string with the corresponding argument from args . The arguments are not positional: the first of the args replaces the  %N  with the lowest  N  (all of them), the second of the args the  %N  with the next-lowest  N  etc.

 Args  can consist of anything that implicitly converts to QString , QStringView or QLatin1String .

In addition, the following types are also supported: QChar , QLatin1Char .

### QString:: QString (const QByteArray & ba )

Constructs a string initialized with the byte array ba . The given byte array is converted to Unicode using fromUtf8 (). Stops copying at the first 0 character, otherwise copies the entire byte array.

You can disable this constructor by defining QT_NO_CAST_FROM_ASCII when you compile your applications. This can be useful if you want to ensure that all user-visible strings go through QObject::tr (), for example.

### QString:: QString (const char * str )

Constructs a string initialized with the 8-bit string str . The given const char pointer is converted to Unicode using the fromUtf8 () 函数。

You can disable this constructor by defining QT_NO_CAST_FROM_ASCII when you compile your applications. This can be useful if you want to ensure that all user-visible strings go through QObject::tr (), for example.

###  [since 5.2]  QString:: QString ( QString && other )

Move-constructs a QString instance, making it point at the same object that other 所指向的。

### QString:: QString (const QString & other )

This operation takes constant time , because QString is 隐式共享 . This makes returning a QString from a function very fast. If a shared instance is modified, it will be copied (copy-on-write), and that takes linear time .

###  [since 6.1]  template <typename> QString:: QString (const char8_t * str )

Constructs a string initialized with the UTF-8 string str . The given const char8_t pointer is converted to Unicode using the fromUtf8 () 函数。

### QString:: QString ( QLatin1String str )

Constructs a copy of the Latin-1 string str .

### QString:: QString ( qsizetype size , QChar ch )

Constructs a string of the given size with every character set to ch .

### QString:: QString ( QChar ch )

Constructs a string of size 1 containing the character ch .

### QString:: QString (const QChar * unicode , qsizetype size = -1)

Constructs a string initialized with the first size characters of the QChar array unicode .

unicode is 0, a null string is constructed.

size is negative, unicode is assumed to point to a \0'-terminated array and its length is determined dynamically. The terminating null character is not considered part of the string.

QString makes a deep copy of the string data. The unicode data is copied as is and the Byte Order Mark is preserved if present.

### QString:: QString ()

Constructs a null string. Null strings are also empty.

### QString &QString:: operator= (const QByteArray & ba )

You can disable this operator by defining QT_NO_CAST_FROM_ASCII when you compile your applications. This can be useful if you want to ensure that all user-visible strings go through QObject::tr (), for example.

### QString &QString:: append (const QString & str )

QString x = "free";
QString y = "dom";
x.append(y);
// x == "freedom"


x.insert(x.size(), y);


The append() function is typically very fast ( constant time ), because QString preallocates extra space at the end of the string data so it can grow without reallocating the entire string each time.

### QString &QString:: append ( QLatin1String str )

Appends the Latin-1 string str 到此字符串。

### QString &QString:: append (const char * str )

You can disable this function by defining QT_NO_CAST_FROM_ASCII when you compile your applications. This can be useful if you want to ensure that all user-visible strings go through QObject::tr (), for example.

### QString &QString:: append (const QByteArray & ba )

You can disable this function by defining QT_NO_CAST_FROM_ASCII when you compile your applications. This can be useful if you want to ensure that all user-visible strings go through QObject::tr (), for example.

### QString QString:: arg (const QString & a , int fieldWidth = 0, QChar fillChar = QLatin1Char(' ')) const

Returns a copy of this string with the lowest numbered place marker replaced by string a , i.e.,  %1  ,  %2  , ...,  %99  .

fieldWidth specifies the minimum amount of space that argument a shall occupy. If a requires less space than fieldWidth , it is padded to fieldWidth with character fillChar . A positive fieldWidth produces right-aligned text. A negative fieldWidth produces left-aligned text.

This example shows how we might create a  status  string for reporting progress while processing a list of files:

QString i;           // current file's number
QString total;       // number of files to process
QString fileName;    // current file's name
QString status = QString("Processing file %1 of %2: %3")
.arg(i).arg(total).arg(fileName);


First,  arg(i)  replaces  %1  . Then  arg(total)  replaces  %2  . Finally,  arg(fileName)  replaces  %3  .

One advantage of using arg() over asprintf () is that the order of the numbered place markers can change, if the application's strings are translated into other languages, but each arg() will still replace the lowest numbered unreplaced place marker, no matter where it appears. Also, if place marker  %i  appears more than once in the string, the arg() replaces all of them.

If there is no unreplaced place marker remaining, a warning message is output and the result is undefined. Place marker numbers must be in the range 1 to 99.

### QString QString:: arg ( qlonglong a , int fieldWidth = 0, int base = 10, QChar fillChar = QLatin1Char(' ')) const

This function overloads arg().

fieldWidth specifies the minimum amount of space that a is padded to and filled with the character fillChar . A positive value produces right-aligned text; a negative value produces left-aligned text.

base argument specifies the base to use when converting the integer a into a string. The base must be between 2 and 36, with 8 giving octal, 10 decimal, and 16 hexadecimal numbers.

fillChar is '0' (the number 0, ASCII 48), the locale's zero is used. For negative numbers, zero padding might appear before the minus sign.

### QString QString:: arg ( qulonglong a , int fieldWidth = 0, int base = 10, QChar fillChar = QLatin1Char(' ')) const

This function overloads arg().

fieldWidth specifies the minimum amount of space that a is padded to and filled with the character fillChar . A positive value produces right-aligned text; a negative value produces left-aligned text.

base argument specifies the base to use when converting the integer a into a string. base must be between 2 and 36, with 8 giving octal, 10 decimal, and 16 hexadecimal numbers.

fillChar is '0' (the number 0, ASCII 48), the locale's zero is used. For negative numbers, zero padding might appear before the minus sign.

### QString QString:: arg ( long a , int fieldWidth = 0, int base = 10, QChar fillChar = QLatin1Char(' ')) const

This function overloads arg().

fieldWidth specifies the minimum amount of space that a is padded to and filled with the character fillChar . A positive value produces right-aligned text; a negative value produces left-aligned text.

a argument is expressed in the given base , which is 10 by default and must be between 2 and 36.

The '%' can be followed by an 'L', in which case the sequence is replaced with a localized representation of a . The conversion uses the default locale. The default locale is determined from the system's locale settings at application startup. It can be changed using QLocale::setDefault (). The 'L' flag is ignored if base is not 10.

QString str;
str = QString("Decimal 63 is %1 in hexadecimal")
.arg(63, 0, 16);
// str == "Decimal 63 is 3f in hexadecimal"
QLocale::setDefault(QLocale(QLocale::English, QLocale::UnitedStates));
str = QString("%1 %L2 %L3")
.arg(12345)
.arg(12345)
.arg(12345, 0, 16);
// str == "12345 12,345 3039"


fillChar is '0' (the number 0, ASCII 48), the locale's zero is used. For negative numbers, zero padding might appear before the minus sign.

### QString QString:: arg ( ulong a , int fieldWidth = 0, int base = 10, QChar fillChar = QLatin1Char(' ')) const

This function overloads arg().

fieldWidth specifies the minimum amount of space that a is padded to and filled with the character fillChar . A positive value produces right-aligned text; a negative value produces left-aligned text.

base argument specifies the base to use when converting the integer a to a string. The base must be between 2 and 36, with 8 giving octal, 10 decimal, and 16 hexadecimal numbers.

fillChar is '0' (the number 0, ASCII 48), the locale's zero is used. For negative numbers, zero padding might appear before the minus sign.

### QString QString:: arg ( int a , int fieldWidth = 0, int base = 10, QChar fillChar = QLatin1Char(' ')) const

This function overloads arg().

a argument is expressed in base base , which is 10 by default and must be between 2 and 36. For bases other than 10, a is treated as an unsigned integer.

fieldWidth specifies the minimum amount of space that a is padded to and filled with the character fillChar . A positive value produces right-aligned text; a negative value produces left-aligned text.

The '%' can be followed by an 'L', in which case the sequence is replaced with a localized representation of a . The conversion uses the default locale, set by QLocale::setDefault (). If no default locale was specified, the "C" locale is used. The 'L' flag is ignored if base is not 10.

QString str;
str = QString("Decimal 63 is %1 in hexadecimal")
.arg(63, 0, 16);
// str == "Decimal 63 is 3f in hexadecimal"
QLocale::setDefault(QLocale(QLocale::English, QLocale::UnitedStates));
str = QString("%1 %L2 %L3")
.arg(12345)
.arg(12345)
.arg(12345, 0, 16);
// str == "12345 12,345 3039"


fillChar is '0' (the number 0, ASCII 48), the locale's zero is used. For negative numbers, zero padding might appear before the minus sign.

### QString QString:: arg ( uint a , int fieldWidth = 0, int base = 10, QChar fillChar = QLatin1Char(' ')) const

This function overloads arg().

base argument specifies the base to use when converting the integer a into a string. The base must be between 2 and 36.

fillChar is '0' (the number 0, ASCII 48), the locale's zero is used. For negative numbers, zero padding might appear before the minus sign.

### QString QString:: arg ( short a , int fieldWidth = 0, int base = 10, QChar fillChar = QLatin1Char(' ')) const

This function overloads arg().

fieldWidth specifies the minimum amount of space that a is padded to and filled with the character fillChar . A positive value produces right-aligned text; a negative value produces left-aligned text.

base argument specifies the base to use when converting the integer a into a string. The base must be between 2 and 36, with 8 giving octal, 10 decimal, and 16 hexadecimal numbers.

fillChar is '0' (the number 0, ASCII 48), the locale's zero is used. For negative numbers, zero padding might appear before the minus sign.

### QString QString:: arg ( ushort a , int fieldWidth = 0, int base = 10, QChar fillChar = QLatin1Char(' ')) const

This function overloads arg().

fieldWidth specifies the minimum amount of space that a is padded to and filled with the character fillChar . A positive value produces right-aligned text; a negative value produces left-aligned text.

base argument specifies the base to use when converting the integer a into a string. The base must be between 2 and 36, with 8 giving octal, 10 decimal, and 16 hexadecimal numbers.

fillChar is '0' (the number 0, ASCII 48), the locale's zero is used. For negative numbers, zero padding might appear before the minus sign.

### QString QString:: arg ( double a , int fieldWidth = 0, char format = 'g', int precision = -1, QChar fillChar = QLatin1Char(' ')) const

This function overloads arg().

Argument a is formatted according to the specified format and precision 。见 自变量格式 了解细节。

fieldWidth specifies the minimum amount of space that a is padded to and filled with the character fillChar . A positive value produces right-aligned text; a negative value produces left-aligned text.

double d = 12.34;
QString str = QString("delta: %1").arg(d, 0, 'E', 3);
// str == "delta: 1.234E+01"


The '%' can be followed by an 'L', in which case the sequence is replaced with a localized representation of a . The conversion uses the default locale, set by QLocale::setDefault (). If no default locale was specified, the "C" locale is used.

fillChar is '0' (the number 0, ASCII 48), this function will use the locale's zero to pad. For negative numbers, the zero padding will probably appear before the minus sign.

### QString QString:: arg ( char a , int fieldWidth = 0, QChar fillChar = QLatin1Char(' ')) const

This function overloads arg().

a argument is interpreted as a Latin-1 character.

### QString QString:: arg ( QChar a , int fieldWidth = 0, QChar fillChar = QLatin1Char(' ')) const

This function overloads arg().

###  [since 5.10]  QString QString:: arg ( QStringView a , int fieldWidth = 0, QChar fillChar = QLatin1Char(' ')) const

Returns a copy of this string with the lowest-numbered place-marker replaced by string a , i.e.,  %1  ,  %2  , ...,  %99  .

fieldWidth specifies the minimum amount of space that a shall occupy. If a requires less space than fieldWidth , it is padded to fieldWidth with character fillChar . A positive fieldWidth produces right-aligned text. A negative fieldWidth produces left-aligned text.

This example shows how we might create a  status  string for reporting progress while processing a list of files:

int i;                // current file's number
int total;            // number of files to process
QStringView fileName; // current file's name
QString status = QString("Processing file %1 of %2: %3")
.arg(i).arg(total).arg(fileName);


First,  arg(i)  replaces  %1  . Then  arg(total)  replaces  %2  . Finally,  arg(fileName)  replaces  %3  .

One advantage of using arg() over asprintf () is that the order of the numbered place markers can change, if the application's strings are translated into other languages, but each arg() will still replace the lowest-numbered unreplaced place-marker, no matter where it appears. Also, if place-marker  %i  appears more than once in the string, arg() replaces all of them.

If there is no unreplaced place-marker remaining, a warning message is printed and the result is undefined. Place-marker numbers must be in the range 1 to 99.

###  [since 5.10]  QString QString:: arg ( QLatin1String a , int fieldWidth = 0, QChar fillChar = QLatin1Char(' ')) const

Returns a copy of this string with the lowest-numbered place-marker replaced by string a , i.e.,  %1  ,  %2  , ...,  %99  .

fieldWidth specifies the minimum amount of space that a shall occupy. If a requires less space than fieldWidth , it is padded to fieldWidth with character fillChar . A positive fieldWidth produces right-aligned text. A negative fieldWidth produces left-aligned text.

One advantage of using arg() over asprintf () is that the order of the numbered place markers can change, if the application's strings are translated into other languages, but each arg() will still replace the lowest-numbered unreplaced place-marker, no matter where it appears. Also, if place-marker  %i  appears more than once in the string, arg() replaces all of them.

If there is no unreplaced place-marker remaining, a warning message is printed and the result is undefined. Place-marker numbers must be in the range 1 to 99.

###  [static, since 5.5]  QString QString:: asprintf (const char * cformat , ...)

Safely builds a formatted string from the format string cformat and an arbitrary list of arguments.

The format string supports the conversion specifiers, length modifiers, and flags provided by printf() in the standard C++ library. The cformat string and  %s  arguments must be UTF-8 encoded.

QString result;
QTextStream(&result) << "pi = " << 3.14;
// result == "pi = 3.14"


For translations , especially if the strings contains more than one escape sequence, you should consider using the arg () function instead. This allows the order of the replacements to be controlled by the translator.

### const QChar QString:: at ( qsizetype position ) const

Returns the character at the given index position in the string.

position must be a valid index position in the string (i.e., 0 <= position < size ()).

###  [since 5.10]  QChar QString:: back () const

Returns the last character in the string. Same as  at(size() - 1)  .

This function is provided for STL compatibility.

###  [since 5.10]  QChar &QString:: back ()

Returns a reference to the last character in the string. Same as  operator[](size() - 1)  .

This function is provided for STL compatibility.

### qsizetype QString:: capacity () const

Returns the maximum number of characters that can be stored in the string without forcing a reallocation.

The sole purpose of this function is to provide a means of fine tuning QString 's memory usage. In general, you will rarely ever need to call this function. If you want to know how many characters are in the string, call size ().

### void QString:: chop ( qsizetype n )

n is greater than or equal to size (), the result is an empty string; if n is negative, it is equivalent to passing zero.

QString str("LOGOUT\r\n");
str.chop(2);
// str == "LOGOUT"


If you want to remove characters from the beginning of the string, use remove () 代替。

###  [since 5.10]  QString QString:: chopped ( qsizetype len ) const

Returns a string that contains the size () - len leftmost characters of this string.

Clears the contents of the string and makes it null.

###  [static]  int QString:: compare (const QString & s1 , const QString & s2 , Qt::CaseSensitivity cs = Qt::CaseSensitive)

cs is Qt::CaseSensitive , the comparison is case sensitive; otherwise the comparison is case insensitive.

Case sensitive comparison is based exclusively on the numeric Unicode values of the characters and is very fast, but is not what a human would expect. Consider sorting user-visible strings with localeAwareCompare ().

int x = QString::compare("aUtO", "AuTo", Qt::CaseInsensitive);  // x == 0
int y = QString::compare("auto", "Car", Qt::CaseSensitive);     // y > 0
int z = QString::compare("auto", "Car", Qt::CaseInsensitive);   // z < 0


### int QString:: compare (const QString & other , Qt::CaseSensitivity cs = Qt::CaseSensitive) const

This function overloads compare().

Lexically compares this string with the other string and returns an integer less than, equal to, or greater than zero if this string is less than, equal to, or greater than the other string.

Same as compare(*this, other , cs ).

### int QString:: compare ( QLatin1String other , Qt::CaseSensitivity cs = Qt::CaseSensitive) const

This function overloads compare().

Same as compare(*this, other , cs ).

###  [since 5.12]  int QString:: compare ( QStringView s , Qt::CaseSensitivity cs = Qt::CaseSensitive) const

This function overloads compare().

Performs a comparison of this with s , using the case sensitivity setting cs .

###  [since 5.14]  int QString:: compare ( QChar ch , Qt::CaseSensitivity cs = Qt::CaseSensitive) const

This function overloads compare().

Performs a comparison of this with ch , using the case sensitivity setting cs .

###  [static]  int QString:: compare (const QString & s1 , QLatin1String s2 , Qt::CaseSensitivity cs = Qt::CaseSensitive)

This function overloads compare().

Performs a comparison of s1 and s2 , using the case sensitivity setting cs .

###  [static]  int QString:: compare ( QLatin1String s1 , const QString & s2 , Qt::CaseSensitivity cs = Qt::CaseSensitive)

This function overloads compare().

Performs a comparison of s1 and s2 , using the case sensitivity setting cs .

###  [static]  int QString:: compare (const QString & s1 , QStringView s2 , Qt::CaseSensitivity cs = Qt::CaseSensitive)

This function overloads compare().

###  [static]  int QString:: compare ( QStringView s1 , const QString & s2 , Qt::CaseSensitivity cs = Qt::CaseSensitive)

This function overloads compare().

### const QChar *QString:: constData () const

Returns a pointer to the data stored in the QString . The pointer can be used to access the characters that compose the string.

Note that the pointer remains valid only as long as the string is not modified.

### bool QString:: contains (const QString & str , Qt::CaseSensitivity cs = Qt::CaseSensitive) const

cs is Qt::CaseSensitive (default), the search is case sensitive; otherwise the search is case insensitive.

QString str = "Peter Pan";
str.contains("peter", Qt::CaseInsensitive);    // returns true


### bool QString:: contains ( QChar ch , Qt::CaseSensitivity cs = Qt::CaseSensitive) const

This function overloads contains().

###  [since 5.3]  bool QString:: contains ( QLatin1String str , Qt::CaseSensitivity cs = Qt::CaseSensitive) const

This function overloads contains().

###  [since 5.14]  bool QString:: contains ( QStringView str , Qt::CaseSensitivity cs = Qt::CaseSensitive) const

This function overloads contains().

cs is Qt::CaseSensitive (default), the search is case sensitive; otherwise the search is case insensitive.

###  [since 5.1]  bool QString:: contains (const QRegularExpression & re , QRegularExpressionMatch * rmatch = nullptr) const

If the match is successful and rmatch 不是  nullptr  , it also writes the results of the match into the QRegularExpressionMatch object pointed to by rmatch .

### qsizetype QString:: count (const QString & str , Qt::CaseSensitivity cs = Qt::CaseSensitive) const

Returns the number of (potentially overlapping) occurrences of the string str in this string.

cs is Qt::CaseSensitive (default), the search is case sensitive; otherwise the search is case insensitive.

### qsizetype QString:: count () const

This function overloads count().

### qsizetype QString:: count ( QChar ch , Qt::CaseSensitivity cs = Qt::CaseSensitive) const

This function overloads count().

Returns the number of occurrences of character ch in the string.

cs is Qt::CaseSensitive (default), the search is case sensitive; otherwise the search is case insensitive.

###  [since 6.0]  qsizetype QString:: count ( QStringView str , Qt::CaseSensitivity cs = Qt::CaseSensitive) const

This function overloads count().

Returns the number of (potentially overlapping) occurrences of the string view str in this string.

cs is Qt::CaseSensitive (default), the search is case sensitive; otherwise the search is case insensitive.

###  [since 5.0]  qsizetype QString:: count (const QRegularExpression & re ) const

This function overloads count().

Returns the number of times the regular expression re matches in the string.

For historical reasons, this function counts overlapping matches, so in the example below, there are four instances of "ana" or "ama":

QString str = "banana and panama";
str.count(QRegularExpression("a[nm]a"));    // returns 4


This behavior is different from simply iterating over the matches in the string using QRegularExpressionMatchIterator .

### QChar *QString:: data ()

Returns a pointer to the data stored in the QString . The pointer can be used to access and modify the characters that compose the string.

QString str = "Hello world";
QChar *data = str.data();
while (!data->isNull()) {
qDebug() << data->unicode();
++data;
}


Note that the pointer remains valid only as long as the string is not modified by other means. For read-only access, constData () is faster because it never causes a deep copy to occur.

### bool QString:: endsWith (const QString & s , Qt::CaseSensitivity cs = Qt::CaseSensitive) const

cs is Qt::CaseSensitive (default), the search is case sensitive; otherwise the search is case insensitive.

QString str = "Bananas";
str.endsWith("anas");         // returns true
str.endsWith("pple");         // returns false


###  [since 5.10]  bool QString:: endsWith ( QStringView str , Qt::CaseSensitivity cs = Qt::CaseSensitive) const

This function overloads endsWith().

cs is Qt::CaseSensitive (default), the search is case sensitive; otherwise the search is case insensitive.

### bool QString:: endsWith ( QLatin1String s , Qt::CaseSensitivity cs = Qt::CaseSensitive) const

This function overloads endsWith().

### bool QString:: endsWith ( QChar c , Qt::CaseSensitivity cs = Qt::CaseSensitive) const

This function overloads endsWith().

###  [since 6.1]  QString::iterator QString:: erase ( QString::const_iterator first , QString::const_iterator last )

Removes from the string the characters in the half-open range [ first , last ). Returns an iterator to the character referred to by last before the erase.

### QString &QString:: fill ( QChar ch , qsizetype size = -1)

Sets every character in the string to character ch 。若 size is different from -1 (default), the string is resized to size beforehand.

QString str = "Berlin";
str.fill('z');
// str == "zzzzzz"
str.fill('A', 2);
// str == "AA"


###  [since 6.0]  QString QString:: first ( qsizetype n ) const

Returns a string that contains the first n characters of this string.

QString x = "Pineapple";
QString y = x.first(4);      // y == "Pine"


###  [static]  QString QString:: fromLatin1 (const char * str , qsizetype size )

size is  -1  ,  strlen(str)  is used instead.

###  [static]  QString QString:: fromLocal8Bit (const char * str , qsizetype size )

size is  -1  ,  strlen(str)  is used instead.

On Unix systems this is equivalen to fromUtf8 (), on Windows the systems current code page is being used.

###  [static]  QString QString:: fromRawData (const QChar * unicode , qsizetype size )

Any attempts to modify the QString or copies of it will cause it to create a deep copy of the data, ensuring that the raw data isn't modified.

Here is an example of how we can use a QRegularExpression on raw data in memory without requiring to copy the data into a QString :

QRegularExpression pattern("\u00A4");
static const QChar unicode[] = {
0x005A, 0x007F, 0x00A4, 0x0060,
0x1009, 0x0020, 0x0020};
qsizetype size = sizeof(unicode) / sizeof(QChar);
QString str = QString::fromRawData(unicode, size);
if (str.contains(pattern) {
// ...
}


###  [static, since 5.3]  QString QString:: fromUcs4 (const char32_t * unicode , qsizetype size = -1)

size is -1 (default), unicode must be \0'-terminated.

###  [static]  QString QString:: fromUtf8 (const char * str , qsizetype size )

size is  -1  ,  strlen(str)  is used instead.

UTF-8 is a Unicode codec and can represent all characters in a Unicode string like QString . However, invalid sequences are possible with UTF-8 and, if any such are found, they will be replaced with one or more "replacement characters", or suppressed. These include non-Unicode sequences, non-characters, overlong sequences or surrogate codepoints encoded into UTF-8.

This function can be used to process incoming data incrementally as long as all UTF-8 characters are terminated within the incoming data. Any unterminated characters at the end of the string will be replaced or suppressed. In order to do stateful decoding, please use QStringDecoder .

###  [static, since 6.1]  template <typename> QString QString:: fromUtf8 (const char8_t * str )

This overload is only available when compiling in C++20 mode.

###  [static, since 6.0]  template <typename> QString QString:: fromUtf8 (const char8_t * str , qsizetype size )

This overload is only available when compiling in C++20 mode.

###  [static, since 5.3]  QString QString:: fromUtf16 (const char16_t * unicode , qsizetype size = -1)

size is -1 (default), unicode must be \0'-terminated.

This function checks for a Byte Order Mark (BOM). If it is missing, host byte order is assumed.

This function is slow compared to the other Unicode conversions. Use QString (const QChar *, int) or QString (const QChar *) if possible.

QString makes a deep copy of the Unicode data.

###  [static]  QString QString:: fromWCharArray (const wchar_t * string , qsizetype size = -1)

size is -1 (default), the string must be '\0'-terminated.

###  [since 5.10]  QChar QString:: front () const

Returns the first character in the string. Same as  at(0)  .

This function is provided for STL compatibility.

###  [since 5.10]  QChar &QString:: front ()

Returns a reference to the first character in the string. Same as  operator[](0)  .

This function is provided for STL compatibility.

### qsizetype QString:: indexOf ( QLatin1String str , qsizetype from = 0, Qt::CaseSensitivity cs = Qt::CaseSensitive) const

Returns the index position of the first occurrence of the string str in this string, searching forward from index position from . Returns -1 if str 找不到。

cs is Qt::CaseSensitive (default), the search is case sensitive; otherwise the search is case insensitive.

QString x = "sticky question";
QString y = "sti";
x.indexOf(y);               // returns 0
x.indexOf(y, 1);            // returns 10
x.indexOf(y, 10);           // returns 10
x.indexOf(y, 11);           // returns -1


from is -1, the search starts at the last character; if it is -2, at the next to last character and so on.

### qsizetype QString:: indexOf ( QChar ch , qsizetype from = 0, Qt::CaseSensitivity cs = Qt::CaseSensitive) const

Returns the index position of the first occurrence of the character ch in the string, searching forward from index position from . Returns -1 if ch could not be found.

### qsizetype QString:: indexOf (const QString & str , qsizetype from = 0, Qt::CaseSensitivity cs = Qt::CaseSensitive) const

Returns the index position of the first occurrence of the string str in this string, searching forward from index position from . Returns -1 if str 找不到。

cs is Qt::CaseSensitive (default), the search is case sensitive; otherwise the search is case insensitive.

QString x = "sticky question";
QString y = "sti";
x.indexOf(y);               // returns 0
x.indexOf(y, 1);            // returns 10
x.indexOf(y, 10);           // returns 10
x.indexOf(y, 11);           // returns -1


from is -1, the search starts at the last character; if it is -2, at the next to last character and so on.

###  [since 5.14]  qsizetype QString:: indexOf ( QStringView str , qsizetype from = 0, Qt::CaseSensitivity cs = Qt::CaseSensitive) const

Returns the index position of the first occurrence of the string view str in this string, searching forward from index position from . Returns -1 if str 找不到。

cs is Qt::CaseSensitive (default), the search is case sensitive; otherwise the search is case insensitive.

from is -1, the search starts at the last character; if it is -2, at the next to last character and so on.

###  [since 5.5]  qsizetype QString:: indexOf (const QRegularExpression & re , qsizetype from = 0, QRegularExpressionMatch * rmatch = nullptr) const

Returns the index position of the first match of the regular expression re in the string, searching forward from index position from . Returns -1 if re didn't match anywhere.

If the match is successful and rmatch 不是  nullptr  , it also writes the results of the match into the QRegularExpressionMatch object pointed to by rmatch .

QString str = "the minimum";
str.indexOf(QRegularExpression("m[aeiou]"), 0);       // returns 4
QString str = "the minimum";
QRegularExpressionMatch match;
str.indexOf(QRegularExpression("m[aeiou]"), 0, &match);       // returns 4
// match.captured() == mi


### QString &QString:: insert ( qsizetype position , const QString & str )

Inserts the string str at the given index position and returns a reference to this string.

QString str = "Meal";
str.insert(1, QString("ontr"));
// str == "Montreal"


This string grows to accommodate the insertion. If position is beyond the end of the string, space characters are appended to the string to reach this position , followed by str .

### QString &QString:: insert ( qsizetype position , QChar ch )

This function overloads insert().

This string grows to accommodate the insertion. If position is beyond the end of the string, space characters are appended to the string to reach this position , followed by ch .

### QString &QString:: insert ( qsizetype position , const QChar * unicode , qsizetype size )

This function overloads insert().

Inserts the first size characters of the QChar array unicode at the given index position in the string.

This string grows to accommodate the insertion. If position is beyond the end of the string, space characters are appended to the string to reach this position , followed by size characters of the QChar array unicode .

###  [since 6.0]  QString &QString:: insert ( qsizetype position , QStringView str )

This function overloads insert().

Inserts the string view str at the given index position and returns a reference to this string.

This string grows to accommodate the insertion. If position is beyond the end of the string, space characters are appended to the string to reach this position , followed by str .

### QString &QString:: insert ( qsizetype position , QLatin1String str )

This function overloads insert().

Inserts the Latin-1 string str at the given index position .

This string grows to accommodate the insertion. If position is beyond the end of the string, space characters are appended to the string to reach this position , followed by str .

###  [since 5.5]  QString &QString:: insert ( qsizetype position , const char * str )

This function overloads insert().

Inserts the C string str at the given index position and returns a reference to this string.

This string grows to accommodate the insertion. If position is beyond the end of the string, space characters are appended to the string to reach this position , followed by str .

This function is not available when QT_NO_CAST_FROM_ASCII is defined.

###  [since 5.5]  QString &QString:: insert ( qsizetype position , const QByteArray & str )

This function overloads insert().

Interprets the contents of str as UTF-8, inserts the Unicode string it encodes at the given index position and returns a reference to this string.

This string grows to accommodate the insertion. If position is beyond the end of the string, space characters are appended to the string to reach this position , followed by str .

This function is not available when QT_NO_CAST_FROM_ASCII is defined.

### bool QString:: isEmpty () const

QString().isEmpty();            // returns true
QString("").isEmpty();          // returns true
QString("x").isEmpty();         // returns false
QString("abc").isEmpty();       // returns false


###  [since 5.12]  bool QString:: isLower () const

Note that this does not mean that the string does not contain uppercase letters (some uppercase letters do not have a lowercase folding; they are left unchanged by toLower ()). For more information, refer to the Unicode standard, section 3.13.

### bool QString:: isNull () const

QString().isNull();             // returns true
QString("").isNull();           // returns false
QString("abc").isNull();        // returns false


Qt makes a distinction between null strings and empty strings for historical reasons. For most applications, what matters is whether or not a string contains any data, and this can be determined using the isEmpty () 函数。

###  [since 5.12]  bool QString:: isUpper () const

Note that this does not mean that the string does not contain lowercase letters (some lowercase letters do not have a uppercase folding; they are left unchanged by toUpper ()). For more information, refer to the Unicode standard, section 3.13.

###  [since 5.15]  bool QString:: isValidUtf16 () const

Note that this function does not perform any special validation of the data; it merely checks if it can be successfully decoded from UTF-16. The data is assumed to be in host byte order; the presence of a BOM is meaningless.

###  [since 6.0]  QString QString:: last ( qsizetype n ) const

Returns the string that contains the last n characters of this string.

QString x = "Pineapple";
QString y = x.last(5);      // y == "apple"


### qsizetype QString:: lastIndexOf (const QString & str , qsizetype from = -1, Qt::CaseSensitivity cs = Qt::CaseSensitive) const

Returns the index position of the last occurrence of the string str in this string, searching backward from index position from 。若 from is -1 (default), the search starts at the last character; if from is -2, at the next to last character and so on. Returns -1 if str 找不到。

cs is Qt::CaseSensitive (default), the search is case sensitive; otherwise the search is case insensitive.

QString x = "crazy azimuths";
QString y = "az";
x.lastIndexOf(y);           // returns 6
x.lastIndexOf(y, 6);        // returns 6
x.lastIndexOf(y, 5);        // returns 2
x.lastIndexOf(y, 1);        // returns -1


### qsizetype QString:: lastIndexOf ( QChar ch , qsizetype from = -1, Qt::CaseSensitivity cs = Qt::CaseSensitive) const

This function overloads lastIndexOf().

Returns the index position of the last occurrence of the character ch , searching backward from position from .

### qsizetype QString:: lastIndexOf ( QLatin1String str , qsizetype from = -1, Qt::CaseSensitivity cs = Qt::CaseSensitive) const

This function overloads lastIndexOf().

Returns the index position of the last occurrence of the string str in this string, searching backward from index position from 。若 from is -1 (default), the search starts at the last character; if from is -2, at the next to last character and so on. Returns -1 if str 找不到。

cs is Qt::CaseSensitive (default), the search is case sensitive; otherwise the search is case insensitive.

QString x = "crazy azimuths";
QString y = "az";
x.lastIndexOf(y);           // returns 6
x.lastIndexOf(y, 6);        // returns 6
x.lastIndexOf(y, 5);        // returns 2
x.lastIndexOf(y, 1);        // returns -1


###  [since 5.14]  qsizetype QString:: lastIndexOf ( QStringView str , qsizetype from = -1, Qt::CaseSensitivity cs = Qt::CaseSensitive) const

This function overloads lastIndexOf().

Returns the index position of the last occurrence of the string view str in this string, searching backward from index position from 。若 from is -1 (default), the search starts at the last character; if from is -2, at the next to last character and so on. Returns -1 if str 找不到。

cs is Qt::CaseSensitive (default), the search is case sensitive; otherwise the search is case insensitive.

###  [since 5.5]  qsizetype QString:: lastIndexOf (const QRegularExpression & re , qsizetype from = -1, QRegularExpressionMatch * rmatch = nullptr) const

Returns the index position of the last match of the regular expression re in the string, which starts before the index position from . Returns -1 if re didn't match anywhere.

If the match is successful and rmatch 不是  nullptr  , it also writes the results of the match into the QRegularExpressionMatch object pointed to by rmatch .

QString str = "the minimum";
str.lastIndexOf(QRegularExpression("m[aeiou]"));      // returns 8
QString str = "the minimum";
QRegularExpressionMatch match;
str.lastIndexOf(QRegularExpression("m[aeiou]"), -1, &match);      // returns 8
// match.captured() == mu


### QString QString:: left ( qsizetype n ) const

Returns a substring that contains the n leftmost characters of the string.

If you know that n cannot be out of bounds, use first () instead in new code, because it is faster.

The entire string is returned if n is greater than or equal to size (), or less than zero.

### QString QString:: leftJustified ( qsizetype width , QChar fill = QLatin1Char(' '), bool truncate = false) const

Returns a string of size width that contains this string padded by the fill character.

truncate is  false  size () of the string is more than width , then the returned string is a copy of the string.

QString s = "apple";
QString t = s.leftJustified(8, '.');    // t == "apple..."


truncate is  true  size () of the string is more than width , then any characters in a copy of the string after position width are removed, and the copy is returned.

QString str = "Pineapple";
str = str.leftJustified(5, '.', true);    // str == "Pinea"


### qsizetype QString:: length () const

Returns the number of characters in this string. Equivalent to size ().

###  [static]  int QString:: localeAwareCompare (const QString & s1 , const QString & s2 )

The comparison is performed in a locale- and also platform-dependent manner. Use this function to present sorted lists of strings to the user.

### int QString:: localeAwareCompare (const QString & other ) const

This function overloads localeAwareCompare().

Compares this string with the other string and returns an integer less than, equal to, or greater than zero if this string is less than, equal to, or greater than the other string.

The comparison is performed in a locale- and also platform-dependent manner. Use this function to present sorted lists of strings to the user.

###  [since 6.0]  int QString:: localeAwareCompare ( QStringView other ) const

This function overloads localeAwareCompare().

Compares this string with the other string and returns an integer less than, equal to, or greater than zero if this string is less than, equal to, or greater than the other string.

The comparison is performed in a locale- and also platform-dependent manner. Use this function to present sorted lists of strings to the user.

###  [static, since 6.0]  int QString:: localeAwareCompare ( QStringView s1 , QStringView s2 )

This function overloads localeAwareCompare().

The comparison is performed in a locale- and also platform-dependent manner. Use this function to present sorted lists of strings to the user.

### QString QString:: mid ( qsizetype position , qsizetype n = -1) const

Returns a string that contains n characters of this string, starting at the specified position index.

If you know that position and n cannot be out of bounds, use sliced () instead in new code, because it is faster.

Returns a null string if the position index exceeds the length of the string. If there are less than n characters available in the string starting at the given position ，或者若 n is -1 (default), the function returns all characters that are available from the specified position .

### QString QString:: normalized ( QString::NormalizationForm mode , QChar::UnicodeVersion version = QChar::Unicode_Unassigned) const

Returns the string in the given Unicode normalization mode , according to the given version of the Unicode standard.

###  [static]  QString QString:: number ( long n , int base = 10)

Returns a string equivalent of the number n 根据指定 base .

The base is 10 by default and must be between 2 and 36. For bases other than 10, n is treated as an unsigned integer.

long a = 63;
QString s = QString::number(a, 16);             // s == "3f"
QString t = QString::number(a, 16).toUpper();     // t == "3F"


###  [static]  QString QString:: number ( double n , char format = 'g', int precision = 6)

Returns a string equivalent of the number n , formatted according to the specified format and precision 。见 自变量格式 了解细节。

### QString &QString:: prepend (const QString & str )

This operation is typically very fast ( constant time ), because QString preallocates extra space at the beginning of the string data, so it can grow without reallocating the entire string each time.

QString x = "ship";
QString y = "air";
x.prepend(y);
// x == "airship"


### QString &QString:: prepend ( QChar ch )

This function overloads prepend().

###  [since 5.5]  QString &QString:: prepend (const QChar * str , qsizetype len )

This function overloads prepend().

###  [since 6.0]  QString &QString:: prepend ( QStringView str )

This function overloads prepend().

Prepends the string view str to the beginning of this string and returns a reference to this string.

### QString &QString:: prepend ( QLatin1String str )

This function overloads prepend().

Prepends the Latin-1 string str 到此字符串。

### QString &QString:: prepend (const char * str )

This function overloads prepend().

You can disable this function by defining QT_NO_CAST_FROM_ASCII when you compile your applications. This can be useful if you want to ensure that all user-visible strings go through QObject::tr (), for example.

### QString &QString:: prepend (const QByteArray & ba )

This function overloads prepend().

You can disable this function by defining QT_NO_CAST_FROM_ASCII when you compile your applications. This can be useful if you want to ensure that all user-visible strings go through QObject::tr (), for example.

### void QString:: push_back (const QString & other )

This function is provided for STL compatibility, appending the given other string onto the end of this string. It is equivalent to  append(other)  .

### void QString:: push_back ( QChar ch )

Appends the given ch character onto the end of this string.

### void QString:: push_front (const QString & other )

This function is provided for STL compatibility, prepending the given other string to the beginning of this string. It is equivalent to  prepend(other)  .

### void QString:: push_front ( QChar ch )

Prepends the given ch character to the beginning of this string.

### QString &QString:: remove ( qsizetype position , qsizetype n )

If the specified position index is within the string, but position + n is beyond the end of the string, the string is truncated at the specified position .

QString s = "Montreal";
s.remove(1, 4);
// s == "Meal"


Element removal will preserve the string's capacity and not reduce the amount of allocated memory. To shed extra capacity and free as much memory as possible, call squeeze () after the last change to the string's size.

### QString &QString:: remove ( QChar ch , Qt::CaseSensitivity cs = Qt::CaseSensitive)

Removes every occurrence of the character ch in this string, and returns a reference to this string.

cs is Qt::CaseSensitive (default), the search is case sensitive; otherwise the search is case insensitive.

QString t = "Ali Baba";
t.remove(QChar('a'), Qt::CaseInsensitive);
// t == "li Bb"


Element removal will preserve the string's capacity and not reduce the amount of allocated memory. To shed extra capacity and free as much memory as possible, call squeeze () after the last change to the string's size.

###  [since 5.11]  QString &QString:: remove ( QLatin1String str , Qt::CaseSensitivity cs = Qt::CaseSensitive)

Removes every occurrence of the given str string in this string, and returns a reference to this string.

cs is Qt::CaseSensitive (default), the search is case sensitive; otherwise the search is case insensitive.

Element removal will preserve the string's capacity and not reduce the amount of allocated memory. To shed extra capacity and free as much memory as possible, call squeeze () after the last change to the string's size.

### QString &QString:: remove (const QString & str , Qt::CaseSensitivity cs = Qt::CaseSensitive)

Removes every occurrence of the given str string in this string, and returns a reference to this string.

cs is Qt::CaseSensitive (default), the search is case sensitive; otherwise the search is case insensitive.

Element removal will preserve the string's capacity and not reduce the amount of allocated memory. To shed extra capacity and free as much memory as possible, call squeeze () after the last change to the string's size.

###  [since 5.0]  QString &QString:: remove (const QRegularExpression & re )

Removes every occurrence of the regular expression re in the string, and returns a reference to the string. For example:

QString r = "Telephone";
r.remove(QRegularExpression("[aeiou]."));
// r == "The"


Element removal will preserve the string's capacity and not reduce the amount of allocated memory. To shed extra capacity and free as much memory as possible, call squeeze () after the last change to the string's size.

###  [since 6.1]  template <typename Predicate> QString &QString:: removeIf ( Predicate pred )

Removes all elements for which the predicate pred returns true from the string. Returns a reference to the string.

### QString QString:: repeated ( qsizetype times ) const

Returns a copy of this string repeated the specified number of times .

times is less than 1, an empty string is returned.

QString str("ab");
str.repeated(4);            // returns "abababab"


### QString &QString:: replace ( qsizetype position , qsizetype n , const QString & after )

QString x = "Say yes!";
QString y = "no";
x.replace(4, 3, y);
// x == "Say no!"


### QString &QString:: replace ( qsizetype position , qsizetype n , QChar after )

This function overloads replace().

### QString &QString:: replace ( qsizetype position , qsizetype n , const QChar * unicode , qsizetype size )

This function overloads replace().

### QString &QString:: replace ( QChar before , QChar after , Qt::CaseSensitivity cs = Qt::CaseSensitive)

This function overloads replace().

Replaces every occurrence of the character before with the character after and returns a reference to this string.

cs is Qt::CaseSensitive (default), the search is case sensitive; otherwise the search is case insensitive.

### QString &QString:: replace (const QChar * before , qsizetype blen , const QChar * after , qsizetype alen , Qt::CaseSensitivity cs = Qt::CaseSensitive)

This function overloads replace().

Replaces each occurrence in this string of the first blen characters of before with the first alen characters of after and returns a reference to this string.

cs is Qt::CaseSensitive (default), the search is case sensitive; otherwise the search is case insensitive.

### QString &QString:: replace ( QLatin1String before , QLatin1String after , Qt::CaseSensitivity cs = Qt::CaseSensitive)

This function overloads replace().

Replaces every occurrence of the string before with the string after and returns a reference to this string.

cs is Qt::CaseSensitive (default), the search is case sensitive; otherwise the search is case insensitive.

### QString &QString:: replace ( QLatin1String before , const QString & after , Qt::CaseSensitivity cs = Qt::CaseSensitive)

This function overloads replace().

Replaces every occurrence of the string before with the string after and returns a reference to this string.

cs is Qt::CaseSensitive (default), the search is case sensitive; otherwise the search is case insensitive.

### QString &QString:: replace (const QString & before , QLatin1String after , Qt::CaseSensitivity cs = Qt::CaseSensitive)

This function overloads replace().

Replaces every occurrence of the string before with the string after and returns a reference to this string.

cs is Qt::CaseSensitive (default), the search is case sensitive; otherwise the search is case insensitive.

### QString &QString:: replace (const QString & before , const QString & after , Qt::CaseSensitivity cs = Qt::CaseSensitive)

This function overloads replace().

Replaces every occurrence of the string before with the string after and returns a reference to this string.

cs is Qt::CaseSensitive (default), the search is case sensitive; otherwise the search is case insensitive.

QString str = "colour behaviour flavour neighbour";
str.replace(QString("ou"), QString("o"));
// str == "color behavior flavor neighbor"


QString equis = "xxxxxx";
equis.replace("xx", "x");
// equis == "xxx"


### QString &QString:: replace ( QChar ch , const QString & after , Qt::CaseSensitivity cs = Qt::CaseSensitive)

This function overloads replace().

Replaces every occurrence of the character ch in the string with after and returns a reference to this string.

cs is Qt::CaseSensitive (default), the search is case sensitive; otherwise the search is case insensitive.

### QString &QString:: replace ( QChar c , QLatin1String after , Qt::CaseSensitivity cs = Qt::CaseSensitive)

This function overloads replace().

Replaces every occurrence of the character c with the string after and returns a reference to this string.

cs is Qt::CaseSensitive (default), the search is case sensitive; otherwise the search is case insensitive.

###  [since 5.0]  QString &QString:: replace (const QRegularExpression & re , const QString & after )

This function overloads replace().

Replaces every occurrence of the regular expression re in the string with after . Returns a reference to the string. For example:

QString s = "Banana";
s.replace(QRegularExpression("a[mn]"), "ox");
// s == "Boxoxa"


For regular expressions containing capturing groups, occurrences of \1 , \2 , ..., in after are replaced with the string captured by the corresponding capturing group.

QString t = "A <i>bon mot</i>.";
t.replace(QRegularExpression("<i>([^<]*)</i>"), "\\emph{\\1}");
// t == "A \\emph{bon mot}."


### void QString:: reserve ( qsizetype size )

Ensures the string has space for at least size 字符。

If you know in advance how large the string will be, you can call this function to save repeated reallocation in the course of building it. This can improve performance when building a string incrementally. A long sequence of operations that add to a string may trigger several reallocations, the last of which may leave you with significantly more space than you really need, which is less efficient than doing a single allocation of the right size at the start.

If in doubt about how much space shall be needed, it is usually better to use an upper bound as size , or a high estimate of the most likely size, if a strict upper bound would be much bigger than this. If size is an underestimate, the string will grow as needed once the reserved size is exceeded, which may lead to a larger allocation than your best overestimate would have and will slow the operation that triggers it.

This function is useful for code that needs to build up a long string and wants to avoid repeated reallocation. In this example, we want to add to the string until some condition is  true  , and we're fairly sure that size is large enough to make a call to reserve() worthwhile:

QString result;
qsizetype maxSize;
bool condition;
QChar nextChar;
result.reserve(maxSize);
while (condition)
result.append(nextChar);
result.squeeze();


### void QString:: resize ( qsizetype size )

Sets the size of the string to size 字符。

size is greater than the current size, the string is extended to make it size characters long with the extra characters added to the end. The new characters are uninitialized.

size is less than the current size, characters beyond position size are excluded from the string.

QString s = "Hello world";
s.resize(5);
// s == "Hello"
s.resize(8);
// s == "Hello???" (where ? stands for any character)


If you want to append a certain number of identical characters to the string, use the resize (qsizetype, QChar ) overload.

If you want to expand the string so that it reaches a certain width and fill the new positions with a particular character, use the leftJustified () 函数：

size is negative, it is equivalent to passing zero.

QString r = "Hello";
r = r.leftJustified(10, ' ');
// r == "Hello     "


###  [since 5.7]  void QString:: resize ( qsizetype size , QChar fillChar )

QString t = "Hello";
r.resize(t.size() + 10, 'X');
// t == "HelloXXXXXXXXXX"


Returns a substring that contains the n rightmost characters of the string.

If you know that n cannot be out of bounds, use last () instead in new code, because it is faster.

The entire string is returned if n is greater than or equal to size (), or less than zero.

### QString QString:: rightJustified ( qsizetype width , QChar fill = QLatin1Char(' '), bool truncate = false) const

Returns a string of size () width that contains the fill character followed by the string. For example:

QString s = "apple";
QString t = s.rightJustified(8, '.');    // t == "...apple"


truncate is  false  size () of the string is more than width , then the returned string is a copy of the string.

truncate is true and the size () of the string is more than width , then the resulting string is truncated at position width .

QString str = "Pineapple";
str = str.rightJustified(5, '.', true);    // str == "Pinea"


### QString QString:: section ( QChar sep , qsizetype start , qsizetype end = -1, QString::SectionFlags flags = SectionDefault) const

This function returns a section of the string.

This string is treated as a sequence of fields separated by the character, sep . The returned string consists of the fields from position start to position end inclusive. If end is not specified, all fields from position start to the end of the string are included. Fields are numbered 0, 1, 2, etc., counting from the left, and -1, -2, etc., counting from right to left.

flags argument can be used to affect some aspects of the function's behavior, e.g. whether to be case sensitive, whether to skip empty fields and how to deal with leading and trailing separators; see SectionFlags .

QString str;
QString csv = "forename,middlename,surname,phone";
QString path = "/usr/local/bin/myapp"; // First field is empty
QString::SectionFlag flag = QString::SectionSkipEmpty;
str = csv.section(',', 2, 2);   // str == "surname"
str = path.section('/', 3, 4);  // str == "bin/myapp"
str = path.section('/', 3, 3, flag); // str == "myapp"


start or end is negative, we count fields from the right of the string, the right-most field being -1, the one from right-most field being -2, and so on.

str = csv.section(',', -3, -2);  // str == "middlename,surname"
str = path.section('/', -1); // str == "myapp"


### QString QString:: section (const QString & sep , qsizetype start , qsizetype end = -1, QString::SectionFlags flags = SectionDefault) const

This function overloads section().

QString str;
QString data = "forename**middlename**surname**phone";
str = data.section("**", 2, 2); // str == "surname"
str = data.section("**", -3, -2); // str == "middlename**surname"


###  [since 5.0]  QString QString:: section (const QRegularExpression & re , qsizetype start , qsizetype end = -1, QString::SectionFlags flags = SectionDefault) const

This function overloads section().

This string is treated as a sequence of fields separated by the regular expression, re .

QString line = "forename\tmiddlename  surname \t \t phone";
QRegularExpression sep("\\s+");
str = line.section(sep, 2, 2); // str == "surname"
str = line.section(sep, -3, -2); // str == "middlename  surname"


### QString &QString:: setNum ( int n , int base = 10)

Sets the string to the printed value of n 以指定 base , and returns a reference to the string.

The base is 10 by default and must be between 2 and 36. For bases other than 10, n is treated as an unsigned integer.

QString str;
str.setNum(1234);       // str == "1234"


### QString &QString:: setNum ( float n , char format = 'g', int precision = 6)

Sets the string to the printed value of n , formatted according to the given format and precision , and returns a reference to the string.

### QString &QString:: setNum ( double n , char format = 'g', int precision = 6)

Sets the string to the printed value of n , formatted according to the given format and precision , and returns a reference to the string.

format can be 'e', 'E', 'f', 'g' or 'G' (see 自变量格式 for an explanation of the formats).

### QString &QString:: setRawData (const QChar * unicode , qsizetype size )

Resets the QString to use the first size Unicode characters in the array unicode . The data in unicode is not copied. The caller must be able to guarantee that unicode will not be deleted or modified as long as the QString (or an unmodified copy of it) exists.

This function can be used instead of fromRawData () to re-use existings QString objects to save memory re-allocations.

### QString &QString:: setUnicode (const QChar * unicode , qsizetype size )

Resizes the string to size characters and copies unicode into the string.

unicode is  nullptr  , nothing is copied, but the string is still resized to size .

### QString &QString:: setUtf16 (const ushort * unicode , qsizetype size )

Resizes the string to size characters and copies unicode into the string.

unicode is  nullptr  , nothing is copied, but the string is still resized to size .

Note that unlike fromUtf16 (), this function does not consider BOMs and possibly differing byte ordering.

### QString QString:: simplified () const

Returns a string that has whitespace removed from the start and the end, and that has each sequence of internal whitespace replaced with a single space.

Whitespace means any character for which QChar::isSpace () 返回  true  . This includes the ASCII characters '\t', '\n', '\v', '\f', '\r', and ' '.

QString str = "  lots\t of\nwhitespace\r\n ";
str = str.simplified();
// str == "lots of whitespace";


### qsizetype QString:: size () const

Returns the number of characters in this string.

The last character in the string is at position size() - 1.

QString str = "World";
qsizetype n = str.size();   // n == 5
str.data()[0];              // returns 'W'
str.data()[4];              // returns 'd'


###  [since 6.0]  QString QString:: sliced ( qsizetype pos , qsizetype n ) const

Returns a string that contains n characters of this string, starting at position pos .

QString x = "Nine pineapples";
QString y = x.sliced(5, 4);            // y == "pine"
QString z = x.sliced(5);               // z == "pineapples"


###  [since 6.0]  QString QString:: sliced ( qsizetype pos ) const

Returns a string that contains the portion of this string starting at position pos and extending to its end.

###  [since 5.14]  QStringList QString:: split (const QString & sep , Qt::SplitBehavior behavior = Qt::KeepEmptyParts, Qt::CaseSensitivity cs = Qt::CaseSensitive) const

Splits the string into substrings wherever sep occurs, and returns the list of those strings. If sep does not match anywhere in the string, split() returns a single-element list containing this string.

cs specifies whether sep should be matched case sensitively or case insensitively.

behavior is Qt::SkipEmptyParts , empty entries don't appear in the result. By default, empty entries are kept.

QString str = QStringLiteral("a,,b,c");
QStringList list1 = str.split(QLatin1Char(','));
// list1: [ "a", "", "b", "c" ]
QStringList list2 = str.split(QLatin1Char(','), Qt::SkipEmptyParts);
// list2: [ "a", "b", "c" ]


sep is empty, split() returns an empty string, followed by each of the string's characters, followed by another empty string:

QString str = "abc";
auto parts = str.split(QString());
// parts: {"", "a", "b", "c", ""}


To understand this behavior, recall that the empty string matches everywhere, so the above is qualitatively the same as:

QString str = "/a/b/c/";
auto parts = str.split(QLatin1Char('/'));
// parts: {"", "a", "b", "c", ""}


###  [since 5.14]  QStringList QString:: split (const QRegularExpression & re , Qt::SplitBehavior behavior = Qt::KeepEmptyParts) const

Splits the string into substrings wherever the regular expression re matches, and returns the list of those strings. If re does not match anywhere in the string, split() returns a single-element list containing this string.

Here is an example where we extract the words in a sentence using one or more whitespace characters as the separator:

QString str;
QStringList list;
str = "Some  text\n\twith  strange whitespace.";
list = str.split(QRegularExpression("\\s+"));
// list: [ "Some", "text", "with", "strange", "whitespace." ]


Here is a similar example, but this time we use any sequence of non-word characters as the separator:

str = "This time, a normal English sentence.";
list = str.split(QRegularExpression("\\W+"), Qt::SkipEmptyParts);
// list: [ "This", "time", "a", "normal", "English", "sentence" ]


Here is a third example where we use a zero-length assertion, \b (word boundary), to split the string into an alternating sequence of non-word and word tokens:

str = "Now: this sentence fragment.";
list = str.split(QRegularExpression("\\b"));
// list: [ "", "Now", ": ", "this", " ", "sentence", " ", "fragment", "." ]


### void QString:: squeeze ()

Releases any memory not required to store the character data.

The sole purpose of this function is to provide a means of fine tuning QString 's memory usage. In general, you will rarely ever need to call this function.

### bool QString:: startsWith (const QString & s , Qt::CaseSensitivity cs = Qt::CaseSensitive) const

cs is Qt::CaseSensitive (default), the search is case sensitive; otherwise the search is case insensitive.

QString str = "Bananas";
str.startsWith("Ban");     // returns true
str.startsWith("Car");     // returns false


###  [since 5.10]  bool QString:: startsWith ( QStringView str , Qt::CaseSensitivity cs = Qt::CaseSensitive) const

cs is Qt::CaseSensitive (default), the search is case-sensitive; otherwise the search is case insensitive.

### bool QString:: startsWith ( QLatin1String s , Qt::CaseSensitivity cs = Qt::CaseSensitive) const

This function overloads startsWith().

### bool QString:: startsWith ( QChar c , Qt::CaseSensitivity cs = Qt::CaseSensitive) const

This function overloads startsWith().

### void QString:: swap ( QString & other )

Swaps string other with this string. This operation is very fast and never fails.

###  [since 5.2]  CFStringRef QString:: toCFString () const

Creates a CFString from a QString .

The caller owns the CFString and is responsible for releasing it.

### QString QString:: toCaseFolded () const

Returns the case folded equivalent of the string. For most Unicode characters this is the same as toLower ().

### double QString:: toDouble ( bool * ok = nullptr) const

Returns the string converted to a  double  值。

Returns an infinity if the conversion overflows or 0.0 if the conversion fails for other reasons (e.g. underflow).

ok 不是  nullptr  , failure is reported by setting * ok to  false  , and success by setting * ok to  true  .

QString str = "1234.56";
double val = str.toDouble();   // val == 1234.56


bool ok;
double d;
d = QString( "1234.56e-02" ).toDouble(&ok); // ok == true, d == 12.3456
d = QString( "1234.56e-02 Volt" ).toDouble(&ok); // ok == false, d == 0


The string conversion will always happen in the 'C' locale. For locale-dependent conversion use QLocale::toDouble ()

d = QString( "1234,56" ).toDouble(&ok); // ok == false
d = QString( "1234.56" ).toDouble(&ok); // ok == true, d == 1234.56


For historical reasons, this function does not handle thousands group separators. If you need to convert such numbers, use QLocale::toDouble ().

d = QString( "1,234,567.89" ).toDouble(&ok); // ok == false
d = QString( "1234567.89" ).toDouble(&ok); // ok == true


### float QString:: toFloat ( bool * ok = nullptr) const

Returns the string converted to a  float  值。

Returns an infinity if the conversion overflows or 0.0 if the conversion fails for other reasons (e.g. underflow).

ok 不是  nullptr  , failure is reported by setting * ok to  false  , and success by setting * ok to  true  .

The string conversion will always happen in the 'C' locale. For locale-dependent conversion use QLocale::toFloat ()

For historical reasons, this function does not handle thousands group separators. If you need to convert such numbers, use QLocale::toFloat ().

QString str1 = "1234.56";
str1.toFloat();             // returns 1234.56
bool ok;
QString str2 = "R2D2";
str2.toFloat(&ok);          // returns 0.0, sets ok to false
QString str3 = "1234.56 Volt";
str3.toFloat(&ok);          // returns 0.0, sets ok to false


###  [since 5.0]  QString QString:: toHtmlEscaped () const

Converts a plain text string to an HTML string with HTML metacharacters  <  ,  >  ,  &  ，和  "  replaced by HTML entities.

QString plain = "#include <QtCore>"
QString html = plain.toHtmlEscaped();
// html == "#include <QtCore>"


### int QString:: toInt ( bool * ok = nullptr, int base = 10) const

Returns the string converted to an  int  using base base , which is 10 by default and must be between 2 and 36, or 0. Returns 0 if the conversion fails.

ok 不是  nullptr  , failure is reported by setting * ok to  false  , and success by setting * ok to  true  .

base is 0, the C language convention is used: If the string begins with "0x", base 16 is used; if the string begins with "0", base 8 is used; otherwise, base 10 is used.

The string conversion will always happen in the 'C' locale. For locale-dependent conversion use QLocale::toInt ()

QString str = "FF";
bool ok;
int hex = str.toInt(&ok, 16);       // hex == 255, ok == true
int dec = str.toInt(&ok, 10);       // dec == 0, ok == false


### QByteArray QString:: toLatin1 () const

Returns a Latin-1 representation of the string as a QByteArray .

The returned byte array is undefined if the string contains non-Latin1 characters. Those characters may be suppressed or replaced with a question mark.

### QByteArray QString:: toLocal8Bit () const

Returns the local 8-bit representation of the string as a QByteArray . The returned byte array is undefined if the string contains characters not supported by the local 8-bit encoding.

On Unix systems this is equivalen to toUtf8 (), on Windows the systems current code page is being used.

If this string contains any characters that cannot be encoded in the locale, the returned byte array is undefined. Those characters may be suppressed or replaced by another.

### long QString:: toLong ( bool * ok = nullptr, int base = 10) const

Returns the string converted to a  long  using base base , which is 10 by default and must be between 2 and 36, or 0. Returns 0 if the conversion fails.

ok 不是  nullptr  , failure is reported by setting * ok to  false  , and success by setting * ok to  true  .

base is 0, the C language convention is used: If the string begins with "0x", base 16 is used; if the string begins with "0", base 8 is used; otherwise, base 10 is used.

The string conversion will always happen in the 'C' locale. For locale-dependent conversion use QLocale::toLongLong ()

QString str = "FF";
bool ok;
long hex = str.toLong(&ok, 16);     // hex == 255, ok == true
long dec = str.toLong(&ok, 10);     // dec == 0, ok == false


### qlonglong QString:: toLongLong ( bool * ok = nullptr, int base = 10) const

Returns the string converted to a  long long  using base base , which is 10 by default and must be between 2 and 36, or 0. Returns 0 if the conversion fails.

ok 不是  nullptr  , failure is reported by setting * ok to  false  , and success by setting * ok to  true  .

base is 0, the C language convention is used: If the string begins with "0x", base 16 is used; if the string begins with "0", base 8 is used; otherwise, base 10 is used.

The string conversion will always happen in the 'C' locale. For locale-dependent conversion use QLocale::toLongLong ()

QString str = "FF";
bool ok;
qint64 hex = str.toLongLong(&ok, 16);      // hex == 255, ok == true
qint64 dec = str.toLongLong(&ok, 10);      // dec == 0, ok == false


### QString QString:: toLower () const

Returns a lowercase copy of the string.

QString str = "The Qt PROJECT";
str = str.toLower();        // str == "the qt project"


The case conversion will always happen in the 'C' locale. For locale-dependent case folding use QLocale::toLower ()

###  [since 5.2]  NSString *QString:: toNSString () const

Creates a NSString from a QString .

The NSString is autoreleased.

### short QString:: toShort ( bool * ok = nullptr, int base = 10) const

Returns the string converted to a  short  using base base , which is 10 by default and must be between 2 and 36, or 0. Returns 0 if the conversion fails.

ok 不是  nullptr  , failure is reported by setting * ok to  false  , and success by setting * ok to  true  .

base is 0, the C language convention is used: If the string begins with "0x", base 16 is used; if the string begins with "0", base 8 is used; otherwise, base 10 is used.

The string conversion will always happen in the 'C' locale. For locale-dependent conversion use QLocale::toShort ()

QString str = "FF";
bool ok;
short hex = str.toShort(&ok, 16);   // hex == 255, ok == true
short dec = str.toShort(&ok, 10);   // dec == 0, ok == false


### std::string QString:: toStdString () const

Returns a std::string object with the data contained in this QString . The Unicode data is converted into 8-bit characters using the toUtf8 () 函数。

This method is mostly useful to pass a QString to a function that accepts a std::string object.

###  [since 5.5]  std::u16string QString:: toStdU16String () const

Returns a std::u16string object with the data contained in this QString . The Unicode data is the same as returned by the utf16 () 方法。

###  [since 5.5]  std::u32string QString:: toStdU32String () const

Returns a std::u32string object with the data contained in this QString . The Unicode data is the same as returned by the toUcs4 () 方法。

### std::wstring QString:: toStdWString () const

Returns a std::wstring object with the data contained in this QString . The std::wstring is encoded in utf16 on platforms where wchar_t is 2 bytes wide (e.g. windows) and in ucs4 on platforms where wchar_t is 4 bytes wide (most Unix systems).

This method is mostly useful to pass a QString to a function that accepts a std::wstring object.

### uint QString:: toUInt ( bool * ok = nullptr, int base = 10) const

Returns the string converted to an  无符号 int  using base base , which is 10 by default and must be between 2 and 36, or 0. Returns 0 if the conversion fails.

ok 不是  nullptr  , failure is reported by setting * ok to  false  , and success by setting * ok to  true  .

base is 0, the C language convention is used: If the string begins with "0x", base 16 is used; if the string begins with "0", base 8 is used; otherwise, base 10 is used.

The string conversion will always happen in the 'C' locale. For locale-dependent conversion use QLocale::toUInt ()

QString str = "FF";
bool ok;
uint hex = str.toUInt(&ok, 16);     // hex == 255, ok == true
uint dec = str.toUInt(&ok, 10);     // dec == 0, ok == false


### ulong QString:: toULong ( bool * ok = nullptr, int base = 10) const

Returns the string converted to an  unsigned long  using base base , which is 10 by default and must be between 2 and 36, or 0. Returns 0 if the conversion fails.

ok 不是  nullptr  , failure is reported by setting * ok to  false  , and success by setting * ok to  true  .

base is 0, the C language convention is used: If the string begins with "0x", base 16 is used; if the string begins with "0", base 8 is used; otherwise, base 10 is used.

The string conversion will always happen in the 'C' locale. For locale-dependent conversion use QLocale::toULongLong ()

QString str = "FF";
bool ok;
ulong hex = str.toULong(&ok, 16);   // hex == 255, ok == true
ulong dec = str.toULong(&ok, 10);   // dec == 0, ok == false


### qulonglong QString:: toULongLong ( bool * ok = nullptr, int base = 10) const

Returns the string converted to an  unsigned long long  using base base , which is 10 by default and must be between 2 and 36, or 0. Returns 0 if the conversion fails.

ok 不是  nullptr  , failure is reported by setting * ok to  false  , and success by setting * ok to  true  .

base is 0, the C language convention is used: If the string begins with "0x", base 16 is used; if the string begins with "0", base 8 is used; otherwise, base 10 is used.

The string conversion will always happen in the 'C' locale. For locale-dependent conversion use QLocale::toULongLong ()

QString str = "FF";
bool ok;
quint64 hex = str.toULongLong(&ok, 16);    // hex == 255, ok == true
quint64 dec = str.toULongLong(&ok, 10);    // dec == 0, ok == false


### ushort QString:: toUShort ( bool * ok = nullptr, int base = 10) const

Returns the string converted to an  unsigned short  using base base , which is 10 by default and must be between 2 and 36, or 0. Returns 0 if the conversion fails.

ok 不是  nullptr  , failure is reported by setting * ok to  false  , and success by setting * ok to  true  .

base is 0, the C language convention is used: If the string begins with "0x", base 16 is used; if the string begins with "0", base 8 is used; otherwise, base 10 is used.

The string conversion will always happen in the 'C' locale. For locale-dependent conversion use QLocale::toUShort ()

QString str = "FF";
bool ok;
ushort hex = str.toUShort(&ok, 16);     // hex == 255, ok == true
ushort dec = str.toUShort(&ok, 10);     // dec == 0, ok == false


### QList < uint > QString:: toUcs4 () const

Returns a UCS-4/UTF-32 representation of the string as a QList <uint>.

UCS-4 is a Unicode codec and therefore it is lossless. All characters from this string will be encoded in UCS-4. Any invalid sequence of code units in this string is replaced by the Unicode's replacement character ( QChar::ReplacementCharacter , which corresponds to  U+FFFD  ).

The returned list is not \0'-terminated.

### QString QString:: toUpper () const

Returns an uppercase copy of the string.

QString str = "TeXt";
str = str.toUpper();        // str == "TEXT"


The case conversion will always happen in the 'C' locale. For locale-dependent case folding use QLocale::toUpper ()

### QByteArray QString:: toUtf8 () const

Returns a UTF-8 representation of the string as a QByteArray .

UTF-8 is a Unicode codec and can represent all characters in a Unicode string like QString .

### qsizetype QString:: toWCharArray ( wchar_t * array ) const

Fills the array with the data contained in this QString object. The array is encoded in UTF-16 on platforms where wchar_t is 2 bytes wide (e.g. windows) and in UCS-4 on platforms where wchar_t is 4 bytes wide (most Unix systems).

array has to be allocated by the caller and contain enough space to hold the complete string (allocating the array with the same length as the string is always sufficient).

This function returns the actual length of the string in array .

### QString QString:: trimmed () const

Returns a string that has whitespace removed from the start and the end.

Whitespace means any character for which QChar::isSpace () 返回  true  . This includes the ASCII characters '\t', '\n', '\v', '\f', '\r', and ' '.

QString str = "  lots\t of\nwhitespace\r\n ";
str = str.trimmed();
// str == "lots\t of\nwhitespace"


### void QString:: truncate ( qsizetype position )

Truncates the string at the given position index.

If the specified position index is beyond the end of the string, nothing happens.

QString str = "Vladivostok";
str.truncate(4);
// str == "Vlad"


position is negative, it is equivalent to passing zero.

### const QChar *QString:: unicode () const

Returns a Unicode representation of the string. The result remains valid until the string is modified.

### const ushort *QString:: utf16 () const

The returned string is in host byte order.

###  [static, since 5.5]  QString QString:: vasprintf (const char * cformat , va_list ap )

Equivalent method to asprintf (), but takes a va_list ap instead a list of variable arguments. See the asprintf () documentation for an explanation of cformat .

This method does not call the va_end macro, the caller is responsible to call va_end on ap .

### bool QString:: operator!= (const char * other ) const

This function overloads operator!=().

other const char pointer is converted to a QString 使用 fromUtf8 () 函数。

You can disable this operator by defining QT_NO_CAST_FROM_ASCII when you compile your applications. This can be useful if you want to ensure that all user-visible strings go through QObject::tr (), for example.

### bool QString:: operator!= (const QByteArray & other ) const

This function overloads operator!=().

other byte array is converted to a QString 使用 fromUtf8 () function. If any NUL characters ('\0') are embedded in the byte array, they will be included in the transformation.

You can disable this operator by defining QT_NO_CAST_FROM_ASCII when you compile your applications. This can be useful if you want to ensure that all user-visible strings go through QObject::tr (), for example.

### QString &QString:: operator+= (const QString & other )

QString x = "free";
QString y = "dom";
x += y;
// x == "freedom"


This operation is typically very fast ( constant time ), because QString preallocates extra space at the end of the string data so it can grow without reallocating the entire string each time.

### QString &QString:: operator+= ( QChar ch )

This function overloads operator+=().

###  [since 6.0]  QString &QString:: operator+= ( QStringView str )

This function overloads operator+=().

Appends the string view str 到此字符串。

### QString &QString:: operator+= ( QLatin1String str )

This function overloads operator+=().

Appends the Latin-1 string str 到此字符串。

### QString &QString:: operator+= (const char * str )

This function overloads operator+=().

You can disable this function by defining QT_NO_CAST_FROM_ASCII when you compile your applications. This can be useful if you want to ensure that all user-visible strings go through QObject::tr (), for example.

### QString &QString:: operator+= (const QByteArray & ba )

This function overloads operator+=().

You can disable this function by defining QT_NO_CAST_FROM_ASCII when you compile your applications. This can be useful if you want to ensure that all user-visible strings go through QObject::tr (), for example.

### bool QString:: operator< (const char * other ) const

This function overloads operator<().

other const char pointer is converted to a QString 使用 fromUtf8 () 函数。

You can disable this operator by defining QT_NO_CAST_FROM_ASCII when you compile your applications. This can be useful if you want to ensure that all user-visible strings go through QObject::tr (), for example.

### bool QString:: operator< (const QByteArray & other ) const

This function overloads operator<().

other byte array is converted to a QString 使用 fromUtf8 () function. If any NUL characters ('\0') are embedded in the byte array, they will be included in the transformation.

You can disable this operator QT_NO_CAST_FROM_ASCII when you compile your applications. This can be useful if you want to ensure that all user-visible strings go through QObject::tr (), for example.

### bool QString:: operator<= (const char * other ) const

This function overloads operator<=().

other const char pointer is converted to a QString 使用 fromUtf8 () 函数。

You can disable this operator by defining QT_NO_CAST_FROM_ASCII when you compile your applications. This can be useful if you want to ensure that all user-visible strings go through QObject::tr (), for example.

### bool QString:: operator<= (const QByteArray & other ) const

This function overloads operator<=().

other byte array is converted to a QString 使用 fromUtf8 () function. If any NUL characters ('\0') are embedded in the byte array, they will be included in the transformation.

You can disable this operator by defining QT_NO_CAST_FROM_ASCII when you compile your applications. This can be useful if you want to ensure that all user-visible strings go through QObject::tr (), for example.

### QString &QString:: operator= ( QChar ch )

Sets the string to contain the single character ch .

### QString &QString:: operator= ( QLatin1String str )

Assigns the Latin-1 string str 到此字符串。

### QString &QString:: operator= (const char * str )

You can disable this operator by defining QT_NO_CAST_FROM_ASCII or QT_RESTRICTED_CAST_FROM_ASCII when you compile your applications. This can be useful if you want to ensure that all user-visible strings go through QObject::tr (), for example.

### bool QString:: operator== (const char * other ) const

This function overloads operator==().

other const char pointer is converted to a QString 使用 fromUtf8 () 函数。

You can disable this operator by defining QT_NO_CAST_FROM_ASCII when you compile your applications. This can be useful if you want to ensure that all user-visible strings go through QObject::tr (), for example.

### bool QString:: operator== (const QByteArray & other ) const

This function overloads operator==().

other byte array is converted to a QString 使用 fromUtf8 () function. This function stops conversion at the first NUL character found, or the end of the byte array.

You can disable this operator by defining QT_NO_CAST_FROM_ASCII when you compile your applications. This can be useful if you want to ensure that all user-visible strings go through QObject::tr (), for example.

### bool QString:: operator> (const char * other ) const

This function overloads operator>().

other const char pointer is converted to a QString 使用 fromUtf8 () 函数。

You can disable this operator by defining QT_NO_CAST_FROM_ASCII when you compile your applications. This can be useful if you want to ensure that all user-visible strings go through QObject::tr (), for example.

### bool QString:: operator> (const QByteArray & other ) const

This function overloads operator>().

other byte array is converted to a QString 使用 fromUtf8 () function. If any NUL characters ('\0') are embedded in the byte array, they will be included in the transformation.

You can disable this operator by defining QT_NO_CAST_FROM_ASCII when you compile your applications. This can be useful if you want to ensure that all user-visible strings go through QObject::tr (), for example.

### bool QString:: operator>= (const char * other ) const

This function overloads operator>=().

other const char pointer is converted to a QString 使用 fromUtf8 () 函数。

You can disable this operator by defining QT_NO_CAST_FROM_ASCII when you compile your applications. This can be useful if you want to ensure that all user-visible strings go through QObject::tr (), for example.

### bool QString:: operator>= (const QByteArray & other ) const

This function overloads operator>=().

other byte array is converted to a QString 使用 fromUtf8 () function. If any NUL characters ('\0') are embedded in the byte array, they will be included in the transformation.

You can disable this operator by defining QT_NO_CAST_FROM_ASCII when you compile your applications. This can be useful if you want to ensure that all user-visible strings go through QObject::tr (), for example.

### QChar &QString:: operator[] ( qsizetype position )

Returns the character at the specified position in the string as a modifiable reference.

QString str;
if (str[0] == QChar('?'))
str[0] = QChar('_');


### const QChar QString:: operator[] ( qsizetype position ) const

This function overloads operator[]().

## 相关非成员

###  [since 6.1]  template <typename T> qsizetype erase ( QString & s , const T & t )

Removes all elements that compare equal to t 从字符串 s . Returns the number of elements removed, if any.

###  [since 6.1]  template <typename Predicate> qsizetype erase_if ( QString & s , Predicate pred )

Removes all elements for which the predicate pred returns true from the string s . Returns the number of elements removed, if any.

### bool operator!= (const QString & s1 , QLatin1String s2 )

This function overloads operator!=().

### bool operator!= (const char * s1 , const QString & s2 )

For s1 != 0, this is equivalent to  compare(  s1 , s2  ) != 0  . Note that no string is equal to s1 being 0.

### const QString operator+ (const QString & s1 , const QString & s2 )

Returns a string which is the result of concatenating s1 and s2 .

### const QString operator+ (const QString & s1 , const char * s2 )

Returns a string which is the result of concatenating s1 and s2 ( s2 is converted to Unicode using the QString::fromUtf8 () function).

### const QString operator+ (const char * s1 , const QString & s2 )

Returns a string which is the result of concatenating s1 and s2 ( s1 is converted to Unicode using the QString::fromUtf8 () function).

### bool operator< (const QString & s1 , const QString & s2 )

This function overloads operator<().

### bool operator< (const QString & s1 , QLatin1String s2 )

This function overloads operator<().

### bool operator< ( QLatin1String s1 , const QString & s2 )

This function overloads operator<().

### bool operator<= (const QString & s1 , QLatin1String s2 )

This function overloads operator<=().

### bool operator<= ( QLatin1String s1 , const QString & s2 )

This function overloads operator<=().

### bool operator== (const QString & s1 , const QString & s2 )

This function overloads operator==().

### bool operator== (const QString & s1 , QLatin1String s2 )

This function overloads operator==().

### bool operator== ( QLatin1String s1 , const QString & s2 )

This function overloads operator==().

### bool operator== (const char * s1 , const QString & s2 )

This function overloads operator==().

### bool operator> (const QString & s1 , QLatin1String s2 )

This function overloads operator>().

### bool operator> ( QLatin1String s1 , const QString & s2 )

This function overloads operator>().

### bool operator>= (const QString & s1 , QLatin1String s2 )

This function overloads operator>=().

### bool operator>= ( QLatin1String s1 , const QString & s2 )

This function overloads operator>=().

### QDataStream & operator>> ( QDataStream & stream , QString & string )

Reads a string from the specified stream 进给定 string .

## 宏文档编制

### QStringLiteral ( str )

The macro generates the data for a QString out of the string literal str at compile time. Creating a QString from it is free in this case, and the generated string data is stored in the read-only segment of the compiled object file.

If you have code that looks like this:

// hasAttribute takes a QString argument
if (node.hasAttribute("http-contents-length")) //...


then a temporary QString will be created to be passed as the  hasAttribute  function parameter. This can be quite expensive, as it involves a memory allocation and the copy/conversion of the data into QString 's internal encoding.

This cost can be avoided by using QStringLiteral instead:

if (node.hasAttribute(QStringLiteral(u"http-contents-length"))) //...


In this case, QString 's internal data will be generated at compile time; no conversion or allocation will occur at runtime.

Using QStringLiteral instead of a double quoted plain C++ string literal can significantly speed up creation of QString instances from data known at compile time.

if (attribute.name() == QLatin1String("http-contents-length")) //...


### QT_NO_CAST_FROM_ASCII

Disables automatic conversions from 8-bit strings (  char *  ) to Unicode QStrings, as well as from 8-bit  char  types (  char  and  unsigned char  ) 到 QChar .

### QT_NO_CAST_TO_ASCII

Disables automatic conversion from QString to 8-bit strings (  char *  ).

### QT_RESTRICTED_CAST_FROM_ASCII

Disables most automatic conversions from source literals and 8-bit data to unicode QStrings, but allows the use of the  QChar(char)  and  QString(const char (&ch)[N]  constructors, and the  QString::operator=(const char (&ch)[N])  assignment operator. This gives most of the type-safety benefits of QT_NO_CAST_FROM_ASCII but does not require user code to wrap character and string literals with QLatin1Char , QLatin1String or similar.

Using this macro together with source strings outside the 7-bit range, non-literals, or literals with embedded NUL characters is undefined.