Qt 5.13 | Qt Core | C++ Classes | QStringView

Qt 5.13.2 Reference Documentation

# QStringView Class

The QStringView class provides a unified view on UTF-16 strings with a read-only subset of the QString API. More...

Header: | #include <QStringView>
qmake: | QT += core
Since: | Qt 5.10

This class was introduced in Qt 5.10.

- List of all members, including inherited members

Note: All functions in this class are reentrant.

## Public Types

typedef | const_iterator
typedef | const_pointer
typedef | const_reference
typedef | difference_type
typedef | iterator
typedef | pointer
typedef | reference
typedef | size_type
typedef | storage_type
typedef | value_type

## Public Functions

| QStringView(const StdBasicString &str)
| QStringView(const QStringRef &str)
| QStringView(const QString &str)
| QStringView(const Char *str)
| QStringView()
QChar | at(int n) const
QChar | back() const
QStringView::const_iterator | begin() const
QStringView::const_iterator | cbegin() const
QStringView::const_iterator | cend() const
void | chop(int length)
QStringView | chopped(int length) const
int | compare(QStringView other, Qt::CaseSensitivity cs = Qt::CaseSensitive) const
int | crbegin() const
int | crend() const
QStringView::const_pointer | data() const
bool | empty() const
QStringView::const_iterator | end() const
bool | endsWith(QStringView str, Qt::CaseSensitivity cs = Qt::CaseSensitive) const
bool | endsWith(QLatin1String l1, Qt::CaseSensitivity cs = Qt::CaseSensitive) const
bool | endsWith(QChar ch) const
bool | endsWith(QChar ch, Qt::CaseSensitivity cs) const
QChar | first() const
QChar | front() const
bool | isEmpty() const
bool | isNull() const
bool | isRightToLeft() const
QChar | last() const
QStringView | left(int length) const
int | length() const
QStringView | mid(int start) const
QStringView | mid(int start, int length) const
int | rbegin() const
int | rend() const
QStringView | right(int length) const
int | size() const
bool | startsWith(QStringView str, Qt::CaseSensitivity cs = Qt::CaseSensitive) const
bool | startsWith(QLatin1String l1, Qt::CaseSensitivity cs = Qt::CaseSensitive) const
bool | startsWith(QChar ch) const
bool | startsWith(QChar ch, Qt::CaseSensitivity cs) const
QByteArray | toLatin1() const
QByteArray | toLocal8Bit() const
QString | toString() const
QVector<uint> | toUcs4() const
QByteArray | toUtf8() const
QStringView | trimmed() const
void | truncate(int length)
const QStringView::storage_type * | utf16() const
QChar | operator[](int n) const

## Related Non-Members

uint | qHash(QStringView key, uint seed = ...)

## Detailed Description

A QStringView references a contiguous portion of a UTF-16 string it does not own. It acts as an interface type to all kinds of UTF-16 string, without the need to construct a QString first.

The UTF-16 string may be represented as an array (or an array-compatible data-structure such as QString, std::basic_string, etc.) of QChar, `ushort`, `char16_t` (on compilers that support C++11 Unicode strings) or (on platforms, such as Windows, where it is a 16-bit type) `wchar_t`.

QStringView is designed as an interface type; its main use-case is as a function parameter type. When QStringViews are used as automatic variables or data members, care must be taken to ensure that the referenced string data (for example, owned by a QString) outlives the QStringView on all code paths, lest the string view ends up referencing deleted data.

When used as an interface type, QStringView allows a single function to accept a wide variety of UTF-16 string data sources. One function accepting QStringView thus replaces three function overloads (taking QString, QStringRef, and `(const QChar*, int)`), while at the same time enabling even more string data sources to be passed to the function, such as `u"Hello World"`, a `char16_t` string literal.

QStringViews should be passed by value, not by reference-to-const:

          void myfun1(QStringView sv);        // preferred
          void myfun2(const QStringView &sv); // compiles and works, but slower

If you want to give your users maximum freedom in what strings they can pass to your function, accompany the QStringView overload with overloads for

- QChar: this overload can delegate to the QStringView version:

            void fun(QChar ch) { fun(QStringView(&ch, 1)); }

  even though, for technical reasons, QStringView cannot provide a QChar constructor by itself.

- QString: if you store an unmodified copy of the string and thus would like to take advantage of QString's implicit sharing.
- QLatin1String: if you can implement the function without converting the QLatin1String to UTF-16 first; users expect a function overloaded on QLatin1String to perform strictly less memory allocations than the semantically equivalent call of the QStringView version, involving construction of a QString from the QLatin1String.

QStringView can also be used as the return value of a function. If you call a function returning QStringView, take extra care to not keep the QStringView around longer than the function promises to keep the referenced string data alive. If in doubt, obtain a strong reference to the data by calling toString() to convert the QStringView into a QString.

QStringView is a Literal Type, but since it stores data as `char16_t`, iteration is not `constexpr` (casts from `const char16_t*` to `const QChar*`, which is not allowed in `constexpr` functions). You can use an indexed loop and/or utf16() in `constexpr` contexts instead.

Note: We strongly discourage the use of QList<QStringView>, because QList is a very inefficient container for QStringViews (it would heap-allocate every element). Use QVector (or std::vector) to hold QStringViews instead.

See also QString and QStringRef.

## Member Type Documentation

### typedef QStringView::const_iterator

This typedef provides an STL-style const iterator for QStringView.

See also iterator and const_reverse_iterator.

### typedef QStringView::const_pointer

Alias for `value_type *`. Provided for compatibility with the STL.

### typedef QStringView::const_reference

Alias for `value_type &`. Provided for compatibility with the STL.

### typedef QStringView::difference_type

Alias for `std::ptrdiff_t`. Provided for compatibility with the STL.

### typedef QStringView::iterator

This typedef provides an STL-style const iterator for QStringView.

QStringView does not support mutable iterators, so this is the same as const_iterator.

See also const_iterator and reverse_iterator.

### typedef QStringView::pointer

Alias for `value_type *`. Provided for compatibility with the STL.

QStringView does not support mutable pointers, so this is the same as const_pointer.

### typedef QStringView::reference

Alias for `value_type &`. Provided for compatibility with the STL.

QStringView does not support mutable references, so this is the same as const_reference.

### typedef QStringView::size_type

Alias for qsizetype. Provided for compatibility with the STL.

Unlike other Qt classes, QStringView uses qsizetype as its `size_type`, to allow accepting data from `std::basic_string` without truncation. The Qt API functions, for example length(), return `int`, while the STL-compatible functions, for example size(), return `size_type`.

### typedef QStringView::storage_type

Alias for `char16_t` for non-Windows or if Q_COMPILER_UNICODE_STRINGS is defined. Otherwise, alias for `wchar_t`.

### typedef QStringView::value_type

Alias for `const QChar`. Provided for compatibility with the STL.

## Member Function Documentation

### bool QStringView::endsWith(QStringView str, Qt::CaseSensitivity cs = Qt::CaseSensitive) const

### bool QStringView::endsWith(QLatin1String l1, Qt::CaseSensitivity cs = Qt::CaseSensitive) const

### bool QStringView::endsWith(QChar ch) const

### bool QStringView::endsWith(QChar ch, Qt::CaseSensitivity cs) const

Returns `true` if this string-view ends with string-view str, Latin-1 string l1, or character ch, respectively; otherwise returns `false`.

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

See also startsWith().

### bool QStringView::startsWith(QStringView str, Qt::CaseSensitivity cs = Qt::CaseSensitive) const

### bool QStringView::startsWith(QLatin1String l1, Qt::CaseSensitivity cs = Qt::CaseSensitive) const

### bool QStringView::startsWith(QChar ch) const

### bool QStringView::startsWith(QChar ch, Qt::CaseSensitivity cs) const

Returns `true` if this string-view starts with string-view str, Latin-1 string l1, or character ch, respectively; otherwise returns `false`.

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

See also endsWith().

### QStringView::QStringView(const StdBasicString &str)

Constructs a string view on str. The length is taken from `str.size()`.

`str.data()` must remain valid for the lifetime of this string view object.

This constructor only participates in overload resolution if `StdBasicString` is an instantiation of `std::basic_string` with a compatible character type. The compatible character types are: `QChar`, `ushort`, `char16_t` and (on platforms, such as Windows, where it is a 16-bit type) `wchar_t`.

The string view will be empty if and only if `str.empty()`. It is unspecified whether this constructor can result in a null string view (`str.data()` would have to return `nullptr` for this).

See also isNull() and isEmpty().

### QStringView::QStringView(const QStringRef &str)

Constructs a string view on str.

`str.data()` must remain valid for the lifetime of this string view object.

The string view will be null if and only if `str.isNull()`.

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

Constructs a string view on str.

`str.data()` must remain valid for the lifetime of this string view object.

The string view will be null if and only if `str.isNull()`.

### QStringView::QStringView(const Char *str)

Constructs a string view on str. The length is determined by scanning for the first `Char(0)`.

str must remain valid for the lifetime of this string view object.

Passing `nullptr` as str is safe and results in a null string view.

This constructor only participates in overload resolution if str is not an array and if `Char` is a compatible character type. The compatible character types are: `QChar`, `ushort`, `char16_t` and (on platforms, such as Windows, where it is a 16-bit type) `wchar_t`.

### QStringView::QStringView()

Constructs a null string view.

See also isNull().

### QChar QStringView::at(int n) const

Returns the character at position n in this string view.

The behavior is undefined if n is negative or not less than size().

See also operator[](), front(), and back().

### QChar QStringView::back() const

Returns the last character in the string. Same as last().

This function is provided for STL compatibility.

Warning: Calling this function on an empty string view constitutes undefined behavior.

See also front(), first(), and last().

### QStringView::const_iterator QStringView::begin() const

Returns a const STL-style iterator pointing to the first character in the string.

This function is provided for STL compatibility.

See also end(), cbegin(), rbegin(), and data().

### QStringView::const_iterator QStringView::cbegin() const

Same as begin().

This function is provided for STL compatibility.

See also cend(), begin(), crbegin(), and data().

### QStringView::const_iterator QStringView::cend() const

Same as end().

This function is provided for STL compatibility.

See also cbegin(), end(), and crend().

### void QStringView::chop(int length)

Truncates this string view by length characters.

Same as `*this = left(size() - length)`.

Note: The behavior is undefined when length < 0 or length > size().

See also mid(), left(), right(), chopped(), and truncate().

### QStringView QStringView::chopped(int length) const

Returns the substring of length size() - length starting at the beginning of this object.

Same as `left(size() - length)`.

Note: The behavior is undefined when length < 0 or length > size().

See also mid(), left(), right(), chop(), and truncate().

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

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

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

This function was introduced in Qt 5.12.

See also operator==(), operator<(), and operator>().

### int QStringView::crbegin() const

Same as rbegin().

This function is provided for STL compatibility.

See also crend(), rbegin(), and cbegin().

### int QStringView::crend() const

Same as rend().

This function is provided for STL compatibility.

See also crbegin(), rend(), and cend().

### QStringView::const_pointer QStringView::data() const

Returns a const pointer to the first character in the string.

Note: The character array represented by the return value is not null-terminated.

See also begin(), end(), and utf16().

### bool QStringView::empty() const

Returns whether this string view is empty - that is, whether `size() == 0`.

This function is provided for STL compatibility.

See also isEmpty(), isNull(), size(), and length().

### QStringView::const_iterator QStringView::end() const

Returns a const STL-style iterator pointing to the imaginary character after the last character in the list.

This function is provided for STL compatibility.

See also begin(), cend(), and rend().

### QChar QStringView::first() const

Returns the first character in the string. Same as front().

This function is provided for compatibility with other Qt containers.

Warning: Calling this function on an empty string view constitutes undefined behavior.

See also front(), back(), and last().

### QChar QStringView::front() const

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

This function is provided for STL compatibility.

Warning: Calling this function on an empty string view constitutes undefined behavior.

See also back(), first(), and last().

### bool QStringView::isEmpty() const

Returns whether this string view is empty - that is, whether `size() == 0`.

This function is provided for compatibility with other Qt containers.

See also empty(), isNull(), size(), and length().

### bool QStringView::isNull() const

Returns whether this string view is null - that is, whether `data() == nullptr`.

This functions is provided for compatibility with other Qt containers.

See also empty(), isEmpty(), size(), and length().

### bool QStringView::isRightToLeft() const

Returns `true` if the string is read right to left.

This function was introduced in Qt 5.11.

See also QString::isRightToLeft().

### QChar QStringView::last() const

Returns the last character in the string. Same as back().

This function is provided for compatibility with other Qt containers.

Warning: Calling this function on an empty string view constitutes undefined behavior.

See also back(), front(), and first().

### QStringView QStringView::left(int length) const

Returns the substring of length length starting at position 0 in this object.

Note: The behavior is undefined when length < 0 or length > size().

See also mid(), right(), chopped(), chop(), and truncate().

### int QStringView::length() const

Same as size(), except returns the result as an `int`.

This function is provided for compatibility with other Qt containers.

Warning: QStringView can represent strings with more than 231 characters. Calling this function on a string view for which size() returns a value greater than `INT_MAX` constitutes undefined behavior.

See also empty(), isEmpty(), isNull(), and size().

### QStringView QStringView::mid(int start) const

Returns the substring starting at position start in this object, and extending to the end of the string.

Note: The behavior is undefined when start < 0 or start > size().

See also left(), right(), chopped(), chop(), and truncate().

### QStringView QStringView::mid(int start, int length) const

This is an overloaded function.

Returns the substring of length length starting at position start in this object.

Note: The behavior is undefined when start < 0, length < 0, or start + length > size().

See also left(), right(), chopped(), chop(), and truncate().

### int QStringView::rbegin() const

Returns a const STL-style reverse iterator pointing to the first character in the string, in reverse order.

This function is provided for STL compatibility.

See also rend(), crbegin(), and begin().

### int QStringView::rend() const

Returns a STL-style reverse iterator pointing to one past the last character in the string, in reverse order.

This function is provided for STL compatibility.

See also rbegin(), crend(), and end().

### QStringView QStringView::right(int length) const

Returns the substring of length length starting at position size() - length in this object.

Note: The behavior is undefined when length < 0 or length > size().

See also mid(), left(), chopped(), chop(), and truncate().

### int QStringView::size() const

Returns the size of this string view, in UTF-16 code points (that is, surrogate pairs count as two for the purposes of this function, the same as in QString and QStringRef).

See also empty(), isEmpty(), isNull(), and length().

### QByteArray QStringView::toLatin1() const

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

The behavior is undefined if the string contains non-Latin1 characters.

See also toUtf8(), toLocal8Bit(), and QTextCodec.

### QByteArray QStringView::toLocal8Bit() const

Returns a local 8-bit representation of the string as a QByteArray.

QTextCodec::codecForLocale() is used to perform the conversion from Unicode. If the locale's encoding could not be determined, this function does the same as toLatin1().

The behavior is undefined if the string contains characters not supported by the locale's 8-bit encoding.

See also toLatin1(), toUtf8(), and QTextCodec.

### QString QStringView::toString() const

Returns a deep copy of this string view's data as a QString.

The return value will be the null QString if and only if this string view is null.

Warning: QStringView can store strings with more than 230 characters while QString cannot. Calling this function on a string view for which size() returns a value greater than `INT_MAX / 2` constitutes undefined behavior.

### QVector<uint> QStringView::toUcs4() const

Returns a UCS-4/UTF-32 representation of the string as a QVector<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 replacement character (QChar::ReplacementCharacter, which corresponds to `U+FFFD`).

The returned vector is not 0-terminated.

See also toUtf8(), toLatin1(), toLocal8Bit(), and QTextCodec.

### QByteArray QStringView::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.

See also toLatin1(), toLocal8Bit(), and QTextCodec.

### QStringView QStringView::trimmed() const

Strips leading and trailing whitespace and returns the result.

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

### void QStringView::truncate(int length)

Truncates this string view to length length.

Same as `*this = left(length)`.

Note: The behavior is undefined when length < 0 or length > size().

See also mid(), left(), right(), chopped(), and chop().

### const QStringView::storage_type *QStringView::utf16() const

Returns a const pointer to the first character in the string.

`storage_type` is `char16_t`.

Note: The character array represented by the return value is not null-terminated.

See also begin(), end(), and data().

### QChar QStringView::operator[](int n) const

Returns the character at position n in this string view.

The behavior is undefined if n is negative or not less than size().

See also at(), front(), and back().

## Related Non-Members

### uint qHash(QStringView key, uint seed = ...)

Returns the hash value for the key, using seed to seed the calculation.

This function was introduced in Qt 5.10.