QAnyStringView Class | Qt Core 6.4.3

- Qt 6.4
- Qt Core
- C++ Classes
- QAnyStringView
- Qt 6.4.3 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

Note: All functions in this class are reentrant.

## Public Types

| difference_type
| size_type

## Public Functions

| QAnyStringView(const QString &str)
| QAnyStringView(const QByteArray &str)
| QAnyStringView(const Char *str)
| QAnyStringView(const Char (&)[N] string = N)
| QAnyStringView(const Char *first, const Char *last)
| QAnyStringView(const Char *str, qsizetype len)
| QAnyStringView(std::nullptr_t)
| QAnyStringView()
QChar | back() const
const void * | data() const
bool | empty() const
QChar | front() const
bool | isEmpty() const
bool | isNull() const
qsizetype | length() const
qsizetype | size() const
qsizetype | size_bytes() const
QString | toString() const
decltype(auto) | visit(Visitor &&v) const

## Static Public Members

int | compare(QAnyStringView lhs, QAnyStringView rhs, Qt::CaseSensitivity cs = Qt::CaseSensitive)

## Related Non-Members

bool | operator!=(QAnyStringView lhs, QAnyStringView rhs)
bool | operator<(QAnyStringView lhs, QAnyStringView rhs)
bool | operator<=(QAnyStringView lhs, QAnyStringView rhs)
bool | operator==(QAnyStringView lhs, QAnyStringView rhs)
bool | operator>(QAnyStringView lhs, QAnyStringView rhs)
bool | operator>=(QAnyStringView lhs, 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.

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).

#### 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 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

### 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()`.

### 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()`.

### 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.

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

See also isNull() and Compatible Character Types.

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

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.

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

See also Compatible Character Types.

### template <typename Char, if_compatible_char<Char>> 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.

This constructor only participates in overload resolution if `Char` is a compatible character type.

See also isNull() and Compatible Character Types.

### template <typename Char, if_compatible_char<Char>> 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`.

This constructor only participates in overload resolution if `Char` is a compatible character type.

See also isNull() and Compatible Character Types.

### QAnyStringView::QAnyStringView(std::nullptr_t)

Constructs a null string view.

See also isNull().

### QAnyStringView::QAnyStringView()

Constructs a null string view.

See also isNull().

### 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.

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

Returns an integer that compares to zero as lhs compares to rhs.

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

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

### 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().

### 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().

### 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.

### 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().

### 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().

### qsizetype QAnyStringView::length() const

Same as size().

This function is provided for compatibility with other Qt containers.

See also size().

### 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.

### 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().

### 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.

### 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

### bool operator!=(QAnyStringView lhs, QAnyStringView rhs)

### bool operator<(QAnyStringView lhs, QAnyStringView rhs)

### bool operator<=(QAnyStringView lhs, QAnyStringView rhs)

### bool operator==(QAnyStringView lhs, QAnyStringView rhs)

### bool operator>(QAnyStringView lhs, QAnyStringView rhs)

### bool operator>=(QAnyStringView lhs, QAnyStringView rhs)

Operators that compare lhs to rhs.

See also compare().