QUtf8StringView Class▲

  • Header: QUtf8StringView

  • Since: Qt 6.0

  • CMake:

    find_package(Qt6 REQUIRED COMPONENTS Core)

    target_link_libraries(mytarget PRIVATE Qt6::Core)

  • qmake: QT += core

  • Group: QUtf8StringView is part of tools, string-processing

Detailed Description▲

A QUtf8StringView references a contiguous portion of a UTF-8 string it does not own. It acts as an interface type to all kinds of UTF-8 string, without the need to construct a QString or QByteArray first.

The UTF-8 string may be represented as an array (or an array-compatible data-structure such as std::basic_string, etc.) of char8_t, char, signed char or unsigned char.

QUtf8StringView is designed as an interface type; its main use-case is as a function parameter type. When QUtf8StringViews are used as automatic variables or data members, care must be taken to ensure that the referenced string data (for example, owned by a std::u8string) outlives the QUtf8StringView on all code paths, lest the string view ends up referencing deleted data.

When used as an interface type, QUtf8StringView allows a single function to accept a wide variety of UTF-8 string data sources. One function accepting QUtf8StringView thus replaces several function overloads (taking e.g. QByteArray), 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 (C++20) or char (C++17) string literal. The char8_t incompatibility between C++17 and C++20 goes away when using QUtf8StringView.

Like all views, QUtf8StringViews should be passed by value, not by reference-to-const:

 
Sélectionnez
    void myfun1(QUtf8StringView sv);        // preferred
    void myfun2(const QUtf8StringView &sv); // compiles and works, but slower

If you want to give your users maximum freedom in what strings they can pass to your function, consider using QAnyStringView instead.

QUtf8StringView can also be used as the return value of a function. If you call a function returning QUtf8StringView, take extra care to not keep the QUtf8StringView 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 QUtf8StringView into a QString.

QUtf8StringView is a Literal Type.

Compatible Character Types▲

QUtf8StringView accepts strings over a variety of character types:

  • char (both signed and unsigned)

  • char8_t (C++20 only)

Sizes and Sub-Strings▲

All sizes and positions in QUtf8StringView functions are in UTF-8 code points (that is, UTF-8 multibyte sequences count as two, three or four, depending on their length). QUtf8StringView does not an attempt to detect or prevent slicing right through UTF-8 multibyte sequences. This is similar to the situation with QStringView and surrogate pairs.

C++20, char8_t, and QUtf8StringView▲

In C++20, u8"" string literals changed their type from const char[] to const char8_t[]. If Qt 6 could have depended on C++20, QUtf8StringView would store char8_t natively, and the following functions and aliases would use (pointers to) char8_t:

This is what QUtf8StringView is expected to look like in Qt 7, but for Qt 6, this was not possible. Instead of locking users into a C++17-era interface for the next decade, Qt provides two QUtf8StringView classes, in different (inline) namespaces. The first, in namespace q_no_char8_t, has a value_type of const char and is universally available. The second, in namespace q_has_char8_t, has a value_type of const char8_t and is only available when compiling in C++20 mode.

q_no_char8_t is an inline namespace regardless of C++ edition, to avoid accidental binary incompatibilities. To use the char8_t version, you need to name it explicitly with q_has_char8_t::QUtf8StringView.

Internally, both are instantiations of the same template class, QBasicUtf8StringView. Please do not use the template class's name in your source code.

See Also▲

See also QAnyStringView, QUtf8StringView, QString

Member Type Documentation▲

 

QUtf8StringView::const_iterator▲

This typedef provides an STL-style const iterator for QUtf8StringView.

See Also▲

See also iterator, const_reverse_iterator

QUtf8StringView::const_pointer▲

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

QUtf8StringView::const_reference▲

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

QUtf8StringView::const_reverse_iterator▲

This typedef provides an STL-style const reverse iterator for QUtf8StringView.

See Also▲

See also reverse_iterator, const_iterator

QUtf8StringView::difference_type▲

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

QUtf8StringView::iterator▲

This typedef provides an STL-style const iterator for QUtf8StringView.

QUtf8StringView does not support mutable iterators, so this is the same as const_iterator.

See Also▲

See also const_iterator, reverse_iterator

QUtf8StringView::pointer▲

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

QUtf8StringView does not support mutable pointers, so this is the same as const_pointer.

QUtf8StringView::reference▲

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

QUtf8StringView does not support mutable references, so this is the same as const_reference.

QUtf8StringView::reverse_iterator▲

This typedef provides an STL-style const reverse iterator for QUtf8StringView.

QUtf8StringView does not support mutable reverse iterators, so this is the same as const_reverse_iterator.

See Also▲

See also const_reverse_iterator, iterator

QUtf8StringView::size_type▲

Alias for qsizetype. Provided for compatibility with the STL.

[alias] QUtf8StringView::storage_type▲

Alias for char.

QUtf8StringView::value_type▲

Alias for const char. Provided for compatibility with the STL.

Member Function Documentation▲

 

[constexpr] QUtf8StringView::QUtf8StringView()▲

Constructs a null string view.

See Also▲

See also isNull()

[constexpr] QUtf8StringView::QUtf8StringView(std::nullptr_t)▲

Constructs a null string view.

See Also▲

See also isNull()

[constexpr] QUtf8StringView::QUtf8StringView(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. The compatible character types are: char8_t, char, signed char and unsigned char.

[constexpr] QUtf8StringView::QUtf8StringView(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. The compatible character types are: char8_t, char, signed char and unsigned char.

[constexpr] QUtf8StringView::QUtf8StringView(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 if Char is a compatible character type. The compatible character types are: char8_t, char, signed char and unsigned char.

See Also▲

See also fromArray()

[constexpr] QUtf8StringView::QUtf8StringView(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: char8_t, char, signed char and unsigned char.

[constexpr] QUtf8StringView::QUtf8StringView(const Container &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 Container is an instantiation of std::basic_string with a compatible character type. The compatible character types are: char8_t, char, signed char and unsigned char.

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▲

See also isNull(), isEmpty()

[constexpr] QUtf8StringView::storage_type QUtf8StringView::at(qsizetype n) const▲

Returns the code point at position n in this string view.

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

See Also▲

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

[constexpr] QUtf8StringView::storage_type QUtf8StringView::back() const▲

Returns the last code point in the string view. Same as last().

This function is provided for STL compatibility.

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

See Also▲

See also front()

QUtf8StringView::const_iterator QUtf8StringView::begin() const▲

Returns a const STL-style iterator pointing to the first code point in the string view.

This function is provided for STL compatibility.

See Also▲

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

QUtf8StringView::const_iterator QUtf8StringView::cbegin() const▲

Same as begin().

This function is provided for STL compatibility.