QAnyStringView Class | Qt Core | Qt 6.10.1
Description: The QAnyStringView class provides a unified view on Latin-1, UTF-8, or UTF-16 strings with a read-only subset of the QString API.

- Qt 6.10
- Qt Core
- C++ Classes
- QAnyStringView
- Qt 6.10.1 Reference Documentation

# QAnyStringView Class

The QAnyStringView class provides a unified view on Latin-1, UTF-8, or UTF-16 strings with a read-only subset of the QString API. More...

Header: | `#include <QAnyStringView>`
CMake: | `find_package(Qt6 REQUIRED COMPONENTS Core)` `target_link_libraries(mytarget PRIVATE Qt6::Core)`
qmake: | `QT += core`
Since: | Qt 6.0

- List of all members, including inherited members
- Deprecated members
- QAnyStringView is part of Classes for string data.

This class is strongly comparable.

This class is strongly comparable with char16_t, QChar, const char16_t *, const char *, QByteArray, QByteArrayView, QString, QStringView, QUtf8StringView, and QLatin1StringView.

Note: All functions in this class are reentrant.

## Public Types

| difference_type
| size_type

## Public Functions

| QAnyStringView()
| QAnyStringView(const Char &ch)
| QAnyStringView(const Char (&)[N] string)
| QAnyStringView(const Char *str)
| QAnyStringView(const Container &str)
| QAnyStringView(const QByteArray &str)
| QAnyStringView(const QString &str)
| QAnyStringView(std::nullptr_t)
| QAnyStringView(const Char *first, const Char *last)
| QAnyStringView(const Char *str, qsizetype len)
`(since 6.9)` QString | arg(Args &&... args) const
QChar | back() const
`(since 6.5)` void | chop(qsizetype n)
`(since 6.5)` QAnyStringView | chopped(qsizetype n) const
const void * | data() const
bool | empty() const
`(since 6.5)` QAnyStringView | first(qsizetype n) const
QChar | front() const
bool | isEmpty() const
bool | isNull() const
`(since 6.5)` QAnyStringView | last(qsizetype n) const
qsizetype | length() const
`(since 6.8)` qsizetype | max_size() const
qsizetype | size() const
qsizetype | size_bytes() const
`(since 6.8)` QAnyStringView & | slice(qsizetype pos, qsizetype n)
`(since 6.8)` QAnyStringView & | slice(qsizetype pos)
`(since 6.5)` QAnyStringView | sliced(qsizetype pos) const
`(since 6.5)` QAnyStringView | sliced(qsizetype pos, qsizetype n) const
QString | toString() const
`(since 6.5)` void | truncate(qsizetype n)
decltype(auto) | visit(Visitor &&v) const

## Static Public Members

int | compare(QAnyStringView lhs, QAnyStringView rhs, Qt::CaseSensitivity cs = Qt::CaseSensitive)
QAnyStringView | fromArray(const Char (&)[Size] string)

## Related Non-Members

bool | operator!=(const QAnyStringView &lhs, const QAnyStringView &rhs)
bool | operator<(const QAnyStringView &lhs, const QAnyStringView &rhs)
`(since 6.7)` QDebug | operator<<(QDebug d, QAnyStringView s)
bool | operator<=(const QAnyStringView &lhs, const QAnyStringView &rhs)
bool | operator==(const QAnyStringView &lhs, const QAnyStringView &rhs)
bool | operator>(const QAnyStringView &lhs, const QAnyStringView &rhs)
bool | operator>=(const QAnyStringView &lhs, const QAnyStringView &rhs)

## Detailed Description

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

Unlike QStringView and QUtf8StringView, QAnyStringView can hold strings of any of the following encodings: UTF-8, UTF-16, and Latin-1. The latter is supported because Latin-1, unlike UTF-8, can be efficiently compared to UTF-16 data: a length mismatch already means the strings cannot be equal. This is not true for UTF-8/UTF-16 comparisons, because UTF-8 is a variable-length encoding.

The string may be represented as an array (or an array-compatible data-structure such as QString, std::basic_string, etc.) of `char`, `char8_t`, QChar, `ushort`, `char16_t` or (on platforms, such as Windows, where it is a 16-bit type) `wchar_t`.

QAnyStringView is designed as an interface type; its main use-case is as a function parameter type. When QAnyStringViews 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 QAnyStringView on all code paths, lest the string view ends up referencing deleted data.

For example,

     QAnyStringView str = funcReturningQString(); // return value is a temp

would leave `str` referencing the deleted temporary (which constitutes undefined behavior). This is particularly true for the single-character constructors:

     QAnyStringView ch = u' '; // u' ' is a temporary
     // oops, ch references deleted temporary

In both cases, the solution is to "pin" the temporary to an lvalue and only then create a QAnyStringView from it:

     const auto r = funcReturningQString();
     QAnyStringView str = r; // ok, `r` outlives `str`
     const auto sp = u' ';
     QAnyStringView ch = sp; // ok, `sp` outlives `ch`

However, using QAnyStringView as the interface type that it is intended to be is always safe, provided the called function's documentation is not asking for a longer lifetime:

     void func(QAnyStringView s);
     func(u' ');
     func(functionReturningQString());

This is why QAnyStringView supports these conversions in the first place.

When used as an interface type, QAnyStringView allows a single function to accept a wide variety of string data sources. One function accepting QAnyStringView thus replaces five function overloads (taking QString, `(const QChar*, qsizetype)`, QUtf8StringView, QLatin1StringView (but see above), and QChar), while at the same time enabling even more string data sources to be passed to the function, such as `u8"Hello World"`, a `char8_t` string literal.

Like elsewhere in Qt, QAnyStringView assumes `char` data is encoded in UTF-8, unless it is presented as a QLatin1StringView.

Since Qt 6.4, however, UTF-8 string literals that are pure US-ASCII are automatically stored as Latin-1. This is a compile-time check with no runtime overhead. The feature requires compiling in C++20, or with a recent GCC.

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

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

QAnyStringView can also be used as the return value of a function, but this is not recommended. QUtf8StringView or QStringView are better suited as function return values. If you call a function returning QAnyStringView, take extra care to not keep the QAnyStringView 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 QAnyStringView into a QString.

QAnyStringView is a Literal Type.

#### Compatible Character Types

QAnyStringView accepts strings over a variety of character types:

- `char` (both signed and unsigned)
- `char8_t` (C++20 only)
- `char16_t`
- `wchar_t` (where it's a 16-bit type, e.g. Windows)
- `ushort`
- `QChar`

The 8-bit character types are interpreted as UTF-8 data (except when presented as a QLatin1StringView) while the 16-bit character types are interpreted as UTF-16 data in host byte order (the same as QString).

The following character types are only supported by the single-character constructor:

- `QLatin1Char`
- `QChar::SpecialCharacter`
- `wchar_t` (where it's a 32-bit type, i.e. Unix) (since 6.10)
- `char32_t`

These character types are internally decomposed into a UTF-16 sequence (using QChar::fromUcs4() for the last).

#### Sizes and Sub-Strings

All sizes and positions in QAnyStringView functions are in the encoding's code units (that is, UTF-16 surrogate pairs count as two for the purposes of these functions, the same as in QString, and UTF-8 multibyte sequences count as two, three or four, depending on their length).

See also Which string class to use?, QUtf8StringView, and QStringView.

## Member Type Documentation

### QAnyStringView::difference_type

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

### QAnyStringView::size_type

Alias for qsizetype. Provided for compatibility with the STL.

## Member Function Documentation

### `[constexpr noexcept]` QAnyStringView::QAnyStringView()

Constructs a null string view.

See also isNull().

### `[constexpr noexcept]` template <typename Char, QAnyStringView::if_compatible_char<Char> = true> QAnyStringView::QAnyStringView(const Char &ch)

Constructs a string view on the single character ch. The length is usually `1` (but see below).

In general, you must assume that a QAnyStringView thus created will start to reference stale data at the end of the full-expression, when temporaries are deleted. That means that using it to pass a single character to a QAnyStringView-taking function is ok and safe (as long as the function documentation doesn't ask for a lifetime longer than the initial call):

     int to_int(QAnyStringView);
     int res = to_int(u'9'); // OK, data stays around for the duration of the call

But keeping the object around longer is undefined behavior:

     QAnyStringView ch = u'9';
     int res = to_int(ch); // (silent) ERROR: ch references deleted data

If you need this, prefer

     const auto nine = u'9';
     QAnyStringView ch(nine); // ok, references `nine`, which outlives `ch`
     int res = to_int(ch); // 9

The above is true for all directly supported Compatible Character Types.

If ch is not one of these types, but merely converts to QChar, e.g. QChar::SpecialCharacter or QLatin1Char, the QAnyStringView will bind to a temporary object that will have been deleted at the end of the full expression, just like in the second example.

If ch cannot be represented in a single UTF-16 code unit (e.g. because it's a `char32_t` value), this constructor decomposes ch into two UFT-16 code units. The resulting QAnyStringView will have a size() of `2` in that case, and the temporary buffer in which the decomposition is stored is deleted at the end of the full-expression, similar to

     [](char32_t ch, auto &&tmp = QChar::fromUcs4(ch)) {
         return QAnyStringView(tmp);
     }

The equivalent safe version in this case would be

     const auto decomposed = QChar::fromUcs4(ch);
     QAnyStringView ch(decomposed);

See also QChar::fromUcs4() and Compatible Character Types.

### `[constexpr noexcept]` template <typename Char, size_t N> QAnyStringView::QAnyStringView(const Char (&)[N] string)

Constructs a string view on the character string literal string. The view covers the array until the first `Char(0)` is encountered, or `N`, whichever comes first. If you need the full array, use fromArray() instead.

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

#### Constraints

Participates in overload resolution only if string is an actual array and `Char` is a compatible character type.

See also Compatible Character Types.

### `[constexpr noexcept]` template <typename Char> QAnyStringView::QAnyStringView(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.

#### Constraints

Participates in overload resolution only if str is not an array and `Char` is a compatible character type.

See also isNull() and Compatible Character Types.

### `[constexpr noexcept]` template <typename Container, QAnyStringView::if_compatible_container<Container> = true> QAnyStringView::QAnyStringView(const Container &str)

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

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

The string view will be empty if and only if `std::size(str) == 0`. It is unspecified whether this constructor can result in a null string view (`std::data(str)` would have to return `nullptr` for this).

#### Constraints

Participates in overload resolution only if `Container` is a container with a compatible character type as `value_type`.

See also isNull() and isEmpty().

### `[noexcept]` QAnyStringView::QAnyStringView(const QByteArray &str)

Constructs a string view on str. The data in str is interpreted as UTF-8.

`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()`.

### `[noexcept]` QAnyStringView::QAnyStringView(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()`.

### `[constexpr noexcept]` QAnyStringView::QAnyStringView(std::nullptr_t)

Constructs a null string view.

See also isNull().

### `[constexpr]` template <typename Char, QAnyStringView::if_compatible_char<Char> = true> QAnyStringView::QAnyStringView(const Char *first, const Char *last)

Constructs a string view on first with length (last - first).

The range `[first,last)` must remain valid for the lifetime of this string view object.

Passing `nullptr` as first is safe if last is `nullptr`, too, and results in a null string view.

The behavior is undefined if last precedes first, or first is `nullptr` and last is not.

#### Constraints

Participates in overload resolution only if `Char` is a compatible character type.

See also isNull() and Compatible Character Types.

### `[constexpr]` template <typename Char, QAnyStringView::if_compatible_char<Char> = true> QAnyStringView::QAnyStringView(const Char *str, qsizetype len)

Constructs a string view on str with length len.

The range `[str,len)` must remain valid for the lifetime of this string view object.

Passing `nullptr` as str is safe if len is 0, too, and results in a null string view.

The behavior is undefined if len is negative or, when positive, if str is `nullptr`.

#### Constraints

Participates in overload resolution only if `Char` is a compatible character type.

See also isNull() and Compatible Character Types.

### `[since 6.9]` template <typename... Args> QString QAnyStringView::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 QAnyStringView.

This function was introduced in Qt 6.9.

See also QString::arg(Args&&...).

### `[constexpr]` QChar QAnyStringView::back() const

Returns the last character in the string view.

This function is provided for STL compatibility.

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

See also front() and Sizes and Sub-Strings.

### `[constexpr, since 6.5]` void QAnyStringView::chop(qsizetype n)

Truncates this string view by n code points.

Same as `*this = first(size() - n)`.

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

This function was introduced in Qt 6.5.

See also sliced(), first(), last(), chopped(), truncate(), slice(), and Sizes and Sub-Strings.

### `[constexpr, since 6.5]` QAnyStringView QAnyStringView::chopped(qsizetype n) const

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

Same as `first(size() - n)`.

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

This function was introduced in Qt 6.5.

See also sliced(), first(), last(), chop(), truncate(), slice(), and Sizes and Sub-Strings.

### `[static noexcept]` int QAnyStringView::compare(QAnyStringView lhs, QAnyStringView rhs, Qt::CaseSensitivity cs = Qt::CaseSensitive)

Compares the string view lhs with the string view rhs and returns a negative integer if lhs is less than rhs, a positive integer if it is greater than rhs, and zero if they are equal.

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

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

### `[constexpr noexcept]` const void *QAnyStringView::data() const

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

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

See also size_bytes().

### `[constexpr noexcept]` bool QAnyStringView::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(), and size().

### `[constexpr, since 6.5]` QAnyStringView QAnyStringView::first(qsizetype n) const

Returns a string view that contains the first n code points of this string view.

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

This function was introduced in Qt 6.5.

See also last(), sliced(), chopped(), chop(), truncate(), slice(), and Sizes and Sub-Strings.

### `[static constexpr noexcept]` template <typename Char, size_t Size, QAnyStringView::if_compatible_char<Char> = true> QAnyStringView QAnyStringView::fromArray(const Char (&)[Size] string)

Constructs a string view on the full character string literal string, including any trailing `Char(0)`. If you don't want the null-terminator included in the view then you can chop() it off when you are certain it is at the end. Alternatively you can use the constructor overload taking an array literal which will create a view up to, but not including, the first null-terminator in the data.

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

This function will work with any array literal 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`.

### `[constexpr]` QChar QAnyStringView::front() const

Returns the first character in the string view.

This function is provided for STL compatibility.

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

See also back() and Sizes and Sub-Strings.

### `[constexpr noexcept]` bool QAnyStringView::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(), and size().

### `[constexpr noexcept]` bool QAnyStringView::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(), and size().

### `[constexpr, since 6.5]` QAnyStringView QAnyStringView::last(qsizetype n) const

Returns a string view that contains the last n code points of this string view.

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

This function was introduced in Qt 6.5.

See also first(), sliced(), chopped(), chop(), truncate(), slice(), and Sizes and Sub-Strings.

### `[constexpr noexcept]` qsizetype QAnyStringView::length() const

Same as size().

This function is provided for compatibility with other Qt containers.

See also size().

### `[constexpr noexcept, since 6.8]` qsizetype QAnyStringView::max_size() const

This function is provided for STL compatibility.

It returns the maximum number of elements that the string view can theoretically represent. In practice, the number can be much smaller, limited by the amount of memory available to the system.

Note: The returned value is calculated based on the currently used character type, so calling this function on two different views may return different results.

This function was introduced in Qt 6.8.

### `[constexpr noexcept]` qsizetype QAnyStringView::size() const

Returns the size of this string view, in the encoding's code points.

See also empty(), isEmpty(), isNull(), size_bytes(), and Sizes and Sub-Strings.

### `[constexpr noexcept]` qsizetype QAnyStringView::size_bytes() const

Returns the size of this string view, but in bytes, not code-points.

You can use this function together with data() for hashing or serialization.

This function is provided for STL compatibility.

See also size() and data().

### `[constexpr, since 6.8]` QAnyStringView &QAnyStringView::slice(qsizetype pos, qsizetype n)

Modifies this string view to start at position pos, extending for n code points.

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

This function was introduced in Qt 6.8.

See also sliced(), first(), last(), chopped(), chop(), truncate(), and Sizes and Sub-Strings.

### `[constexpr, since 6.8]` QAnyStringView &QAnyStringView::slice(qsizetype pos)

Modifies this string view to start at position pos, extending to its end.

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

This is an overloaded function.

This function was introduced in Qt 6.8.

See also sliced(), first(), last(), chopped(), chop(), truncate(), and Sizes and Sub-Strings.

### `[constexpr, since 6.5]` QAnyStringView QAnyStringView::sliced(qsizetype pos) const

Returns a string view starting at position pos in this object, and extending to its end.

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

This function was introduced in Qt 6.5.

See also first(), last(), chopped(), chop(), truncate(), slice(), and Sizes and Sub-Strings.

### `[constexpr, since 6.5]` QAnyStringView QAnyStringView::sliced(qsizetype pos, qsizetype n) const

Returns a string view containing n code points of this string view, starting at position pos.

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

This function was introduced in Qt 6.5.

See also first(), last(), chopped(), chop(), truncate(), slice(), and Sizes and Sub-Strings.

### QString QAnyStringView::toString() const

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

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

### `[constexpr, since 6.5]` void QAnyStringView::truncate(qsizetype n)

Truncates this string view to n code points.

Same as `*this = first(n)`.

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

This function was introduced in Qt 6.5.

See also sliced(), first(), last(), chopped(), chop(), and Sizes and Sub-Strings.

### `[constexpr]` template <typename Visitor> decltype(auto) QAnyStringView::visit(Visitor &&v) const

Calls v with either a QUtf8StringView, QLatin1String, or QStringView, depending on the encoding of the string data this string-view references.

This is how most functions taking QAnyStringView fork off into per-encoding functions:

     void processImpl(QLatin1String s) { ~~~ }
     void processImpl(QUtf8StringView s) { ~~~ }
     void processImpl(QStringView s) { ~~~ }

     void process(QAnyStringView s)
     {
         s.visit([](auto s) { processImpl(s); });
     }

Here, we're reusing the same name, `s`, for both the QAnyStringView object, as well as the lambda's parameter. This is idiomatic code and helps track the identity of the objects through visit() calls, for example in more complex situations such as

     bool equal(QAnyStringView lhs, QAnyStringView rhs)
     {
         // assuming operator==(QAnyStringView, QAnyStringView) didn't, yet, exist:
         return lhs.visit([rhs](auto lhs) {
             rhs.visit([lhs](auto rhs) {
                 return lhs == rhs;
             });
         });
     }

visit() requires that all lambda instantiations have the same return type. If they differ, you get a compile error, even if there is a common type. To fix, you can use explicit return types on the lambda, or cast in the return statements:

     // wrong:
     QAnyStringView firstHalf(QAnyStringView input)
     {
         return input.visit([](auto input) {   // ERROR: lambdas return different types
             return input.sliced(0, input.size() / 2);
         });
     }
     // correct:
     QAnyStringView firstHalf(QAnyStringView input)
     {
         return input.visit([](auto input) -> QAnyStringView { // OK, explicit return type
             return input.sliced(0, input.size() / 2);
         });
     }
     // also correct:
     QAnyStringView firstHalf(QAnyStringView input)
     {
         return input.visit([](auto input) {
             return QAnyStringView(input.sliced(0, input.size() / 2)); // OK, cast to common type
         });
     }

## Related Non-Members

### `[noexcept]` bool operator!=(const QAnyStringView &lhs, const QAnyStringView &rhs)

### `[noexcept]` bool operator<(const QAnyStringView &lhs, const QAnyStringView &rhs)

### `[noexcept]` bool operator<=(const QAnyStringView &lhs, const QAnyStringView &rhs)

### `[noexcept]` bool operator==(const QAnyStringView &lhs, const QAnyStringView &rhs)

### `[noexcept]` bool operator>(const QAnyStringView &lhs, const QAnyStringView &rhs)

### `[noexcept]` bool operator>=(const QAnyStringView &lhs, const QAnyStringView &rhs)

Operators that compare lhs to rhs.

See also compare().

### `[since 6.7]` QDebug operator<<(QDebug d, QAnyStringView s)

Outputs s to debug stream d.

If `d.quotedString()` is `true`, indicates which encoding the string is in. If you just want the string data, use visit() like this:

     s.visit([&d) (auto s) { d << s; });

This function was introduced in Qt 6.7.

See also QAnyStringView::visit().