QUtf8StringView Class

The QUtf8StringView class provides a unified view on UTF-8 strings with a read-only subset of the QString API. More...

Header: #include <QUtf8StringView>
CMake: find_package(Qt6 COMPONENTS Core REQUIRED)
target_link_libraries(mytarget PRIVATE Qt6::Core)
qmake: QT += core
Since: Qt 6.0

Note: All functions in this class are reentrant.

Public Types

Public Functions

QUtf8StringView(const Container &str)
QUtf8StringView(const Char *str)
QUtf8StringView(const Char (&)[N] string = N)
QUtf8StringView(const Char *first, const Char *last)
QUtf8StringView(const Char *str, qsizetype len)
QUtf8StringView(std::nullptr_t)
QUtf8StringView()
QUtf8StringView::storage_type at(qsizetype n) const
QUtf8StringView::storage_type back() const
QUtf8StringView::const_iterator begin() const
QUtf8StringView::const_iterator cbegin() const
QUtf8StringView::const_iterator cend() const
void chop(qsizetype n)
QUtf8StringView chopped(qsizetype n) const
QUtf8StringView::const_reverse_iterator crbegin() const
QUtf8StringView::const_reverse_iterator crend() const
QUtf8StringView::const_pointer data() const
bool empty() const
QUtf8StringView::const_iterator end() const
QUtf8StringView first(qsizetype n) const
QUtf8StringView::storage_type front() const
bool isEmpty() const
bool isNull() const
QUtf8StringView last(qsizetype n) const
qsizetype length() const
QUtf8StringView::const_reverse_iterator rbegin() const
QUtf8StringView::const_reverse_iterator rend() const
qsizetype size() const
QUtf8StringView sliced(qsizetype pos) const
QUtf8StringView sliced(qsizetype pos, qsizetype n) const
QString toString() const
void truncate(qsizetype n)
const char8_t *utf8() const
QUtf8StringView::storage_type operator[](qsizetype n) const

Static Public Members

QUtf8StringView fromArray(const Char (&)[Size] string = Size)

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:

    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 QAnyStringView, QUtf8StringView, and QString.

Member Type Documentation

QUtf8StringView::const_iterator

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

See also iterator and 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 reverse_iterator and 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 const_iterator and 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 const_reverse_iterator and 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

template <typename Container, if_compatible_container<Container>> 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 isNull() and isEmpty().

template <typename Char> 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.

template <typename Char, size_t N> 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 fromArray().

template <typename Char, if_compatible_char<Char>> 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.

template <typename Char, if_compatible_char<Char>> 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.

QUtf8StringView::QUtf8StringView(std::nullptr_t)

Constructs a null string view.

See also isNull().

QUtf8StringView::QUtf8StringView()

Constructs a null string view.

See also isNull().

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

QUtf8StringView::storage_type QUtf8StringView::back() const

Returns the last code point 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().

QUtf8StringView::const_iterator QUtf8StringView::begin() const

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

This function is provided for STL compatibility.

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

QUtf8StringView::const_iterator QUtf8StringView::cbegin() const

Same as begin().

This function is provided for STL compatibility.

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

QUtf8StringView::const_iterator QUtf8StringView::cend() const

Same as end().

This function is provided for STL compatibility.

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

void QUtf8StringView::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().

See also sliced(), first(), last(), chopped(), and truncate().

QUtf8StringView QUtf8StringView::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().

See also sliced(), first(), last(), chop(), and truncate().

QUtf8StringView::const_reverse_iterator QUtf8StringView::crbegin() const

Same as rbegin().

This function is provided for STL compatibility.

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

QUtf8StringView::const_reverse_iterator QUtf8StringView::crend() const

Same as rend().

This function is provided for STL compatibility.

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

QUtf8StringView::const_pointer QUtf8StringView::data() const

Returns a const pointer to the first code point in the string.

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

See also begin(), end(), and utf8().

bool QUtf8StringView::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().

QUtf8StringView::const_iterator QUtf8StringView::end() const

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

This function is provided for STL compatibility.

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

QUtf8StringView QUtf8StringView::first(qsizetype n) const

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

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

See also last(), sliced(), chopped(), chop(), and truncate().

[static] template <typename Char, size_t Size, if_compatible_char<Char>> QUtf8StringView QUtf8StringView::fromArray(const Char (&)[Size] string = Size)

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: char8_t, char, signed char and unsigned char.

QUtf8StringView::storage_type QUtf8StringView::front() const

Returns the first code point 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().

bool QUtf8StringView::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 QUtf8StringView::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().

QUtf8StringView QUtf8StringView::last(qsizetype n) const

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

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

See also first(), sliced(), chopped(), chop(), and truncate().

qsizetype QUtf8StringView::length() const

Same as size().

This function is provided for compatibility with other Qt containers.

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

QUtf8StringView::const_reverse_iterator QUtf8StringView::rbegin() const

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

This function is provided for STL compatibility.

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

QUtf8StringView::const_reverse_iterator QUtf8StringView::rend() const

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

This function is provided for STL compatibility.

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

qsizetype QUtf8StringView::size() const

Returns the size of this string view, in UTF-8 code points (that is, multi-byte sequences count as more than one for the purposes of this function, the same as surrogate pairs in QString and QStringView).

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

QUtf8StringView QUtf8StringView::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().

See also first(), last(), chopped(), chop(), and truncate().

QUtf8StringView QUtf8StringView::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().

See also first(), last(), chopped(), chop(), and truncate().

QString QUtf8StringView::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.

void QUtf8StringView::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().

See also sliced(), first(), last(), chopped(), and chop().

const char8_t *QUtf8StringView::utf8() const

Returns a const pointer to the first code point in the string.

The result is returned as a const char8_t*, so this function is only available when compiling in C++20 mode.

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

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

QUtf8StringView::storage_type QUtf8StringView::operator[](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 at(), front(), and back().

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