qspan.qdoc

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// Copyright (C) 2023 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
 
/*!
    \class QSpan
    \inmodule QtCore
    \since 6.7
    \brief A non-owning container over contiguous data.
    \ingroup tools
    \reentrant
 
    QSpan<T, E> is a template class where \a T is the element type and \a E
    is the extent (number of elements, or \c{std::dynamic_extent} for a
    variable-sized span).
 
    A QSpan references a contiguous portion of another contiguous container.
    It acts as an interface type for all kinds of contiguous containers,
    without the need to construct an owning container such as QList or
    std::vector first.
 
    The data referenced by a QSpan may be represented as an array (or
    array-compatible data-structure such as QList, std::vector,
    QVarLengthArray, etc.). QSpan itself merely stores a pointer to the data,
    so users must ensure that QSpan objects do not outlive the data they
    reference.
 
    Unlike views such as QStringView, QLatin1StringView and QUtf8StringView,
    referenced data can be modified through a QSpan object. To prevent this,
    construct a QSpan over a \c{const T} (see \l{Const and Mutable Spans}):
 
    \code
    int numbers[] = {0, 1, 2};
    QSpan<int> span = numbers;
    span[0] = 42;
    // numbers == {42, 1, 2};
    QSpan<const int> cspan = numbers;
    cspan[0] = 0; // ERROR: cspan[0] is read-only
    \endcode
 
    \target variable-fixed-spans
    \section2 Variable-Size and Fixed-Size Spans
 
    A QSpan can be \e{fixed-size} or \e{variable-sized}.
 
    A variable-sized span is formed by omitting the second template argument
    (or setting it to \c{std::dynamic_extent}, which is, however, only
    available in C++20 builds), as seen in the example above.
 
    A fixed-size span is formed by passing a number as the second template
    argument:
 
    \code
    int numbers[] = {0, 1, 2};
    QSpan<int, 3> span = numbers;
    QSpan<const int, 3> = numbers; // also OK
    \endcode
 
    As the name suggests, a fixed-size span's size() is fixed at compile-time
    whereas the size() of a variable-sized span is determined only at run-time.
 
    A fixed-size span is not default-constructible (unless its \l extent is zero
    (0)). A variable-sized span \e{is} default-constructible and will have
    \c{data() == nullptr} and \c{size() == 0}.
 
    A fixed-size span can be implicitly converted into a variable-sized one.
    The opposite direction (variable-length into fixed-length) has the
    precondition that both span's sizes must match.
 
    \target const-mutable-spans
    \section2 Const and Mutable Spans
 
    Unlike with owning containers, \c{const} is \e{shallow} in QSpan: you can
    still modify the data through a const QSpan (but not through a
    \c{QSpan<const T>}), and begin() and end() are not overloaded on
    \c{const}/non-\c{const}. There are cbegin() and cend(), though, that return
    const_iterators which prevent modification of the data even though \c{T} is
    not const:
    \code
    int numbers[] = {0, 1, 2};
    const QSpan<int> span = numbers;
    span.front() = 42;   // OK, numbers[0] == 42 now
    *span.begin() = 31;  // OK, numbers[0] == 31 now
    *span.cbegin() = -1; // ERROR: cannot assign through a const_iterator
    \endcode
 
    \target other-span-properties
    \section2 Other Properties
 
    QSpan should be passed by value, not by reference-to-const:
 
    \code
    void consume(QSpan<const int> data); // OK
    void consume(const QSpan<const int> &data); // works, but is non-idiomatic and less efficient
    \endcode
 
    \c{QSpan<T,N>} is a \e{Literal Type}, regardless of whether \c{T} is a
    Literal Type or not.
 
    \target span-STL
    \section2 QSpan vs. std::span
 
    QSpan is closely modelled after
    \l{https://en.cppreference.com/w/cpp/container/span}{std::span}, but has a
    few differences which we'll discuss here. Since they both implicitly
    convert into each other, you're free to choose whichever one you like best
    in your own code.
 
    \list
    \li QSpan is using the signed qsizetype as \c{size_type}
        whereas \c{std::span} uses \c{size_t}.
    \li (since Qt 6.9) \c{QSpan<const T>} doesn't detach Qt containers, \c{std::span} does.
    \li All QSpan constructors are implicit;
        many \c{std::span} ones are \c{explicit}.
    \li QSpan can be constructed from rvalue owning containers, \c{std::span} can not.
    \endlist
 
    The last two are required for source-compatibility when functions that took
    owning containers are converted to take QSpan instead, which is a
    vitally-important use-case in Qt. The use of qsizetype is for consistency
    with the rest of Qt containers. QSpan template arguments still use size_t
    to avoid introducing unnecessary error conditions (negative sizes).
 
    \target span-compatible-iterators
    \section2 Compatible Iterators
 
    QSpan can be constructed from an iterator and size or from an
    iterator pair, provided the iterators are \e{compatible} ones.
    Eventually, this should mean C++20 \c{std::contiguous_iterator} and
    \c{std::sentinel_for}, but while Qt still supports C++17, only raw pointers
    are considered contiguous iterators.
 
    \target span-compatible-ranges
    \section2 Compatible Ranges
 
    QSpan can also be constructed from a \e{compatible} range. A range is
    compatible if it has \l{span-compatible-iterators}{compatible iterators}.
 
    \sa QList, QStringView, QLatin1StringView, QUtf8StringView
*/
 
//
// Nested types and constants
//
 
/*!
    \typedef QSpan::element_type
 
    An alias for \c{T}. Includes the \c{const}, if any.
 
    This alias is provided for compatbility with the STL.
 
    \sa value_type, pointer, {Const and Mutable Spans}
*/
 
/*!
    \typedef QSpan::value_type
 
    An alias for \c{T}. Excludes the \c{const}, if any.
 
    This alias is provided for compatbility with the STL.
 
    \sa element_type, {Const and Mutable Spans}
*/
 
/*!
    \typedef QSpan::size_type
 
    An alias for qsizetype. This \l{span-STL}{differs from \c{std::span}}.
 
    This alias is provided for compatbility with the STL.
*/
 
/*!
    \typedef QSpan::difference_type
 
    An alias for qptrdiff. This \l{span-STL}{differs from \c{std::span}}.
 
    This alias is provided for compatbility with the STL.
*/
 
/*!
    \typedef QSpan::pointer
 
    An alias for \c{T*} and \c{element_type*}, respectively. Includes the \c{const}, if any.
 
    This alias is provided for compatbility with the STL.
 
    \sa element_type, const_pointer, reference, iterator, {Const and Mutable Spans}
*/
 
/*!
    \typedef QSpan::const_pointer
 
    An alias for \c{const T*} and \c{const element_type*}, respectively.
 
    This alias is provided for compatbility with the STL.
 
    \sa element_type, pointer, const_reference, const_iterator, {Const and Mutable Spans}
*/
 
/*!
    \typedef QSpan::reference
 
    An alias for \c{T&} and \c{element_type&}, respectively. Includes the \c{const}, if any.
 
    This alias is provided for compatbility with the STL.
 
    \sa element_type, const_reference, pointer, {Const and Mutable Spans}
*/
 
/*!
    \typedef QSpan::const_reference
 
    An alias for \c{const T&} and \c{const element_type&}, respectively.
 
    This alias is provided for compatbility with the STL.
 
    \sa element_type, reference, const_pointer, {Const and Mutable Spans}
*/
 
/*!
    \typedef QSpan::iterator
 
    An alias for \c{T*} and \c{pointer}, respectively. Includes the \c{const}, if any.
 
    \sa pointer, const_iterator, reverse_iterator, {Const and Mutable Spans}
*/
 
/*!
    \typedef QSpan::const_iterator
 
    An alias for \c{const T*} and \c{const_pointer}, respectively.
 
    \sa const_pointer, iterator, const_reverse_iterator, {Const and Mutable Spans}
*/
 
/*!
    \typedef QSpan::reverse_iterator
 
    An alias for \c{std::reverse_iterator<iterator>}. Includes the \c{const}, if any.
 
    \sa iterator, const_reverse_iterator, {Const and Mutable Spans}
*/
 
/*!
    \typedef QSpan::const_reverse_iterator
 
    An alias for \c{std::reverse_iterator<const_iterator>}.
 
    \sa const_iterator, reverse_iterator, {Const and Mutable Spans}
*/
 
/*!
    \variable QSpan::extent
 
    The second template argument of \c{QSpan<T, E>}, that is, \c{E}. This is
    \c{std::dynamic_extent} for \l{variable-fixed-spans}{variable-sized spans}.
 
    \note While all other sizes and indexes in QSpan use qsizetype, this
    variable, like \c{E}, is actually of type \c{size_t}, for compatibility with
    \c{std::span} and \c{std::dynamic_extent}.
 
    \sa size()
*/
 
//
// Constructors and SMFs
//
 
/*!
    \fn template <typename T, size_t E> QSpan<T,E>::QSpan()
 
    Default constructor.
 
    This constructor is only present if \c{E} is either zero (0) or
    \c{std::dynamic_extent}. In other words: only fixed-zero-sized or variable-sized spans
    are default-constructible.
 
    \sa extent, {Variable-Size and Fixed-Size Spans}
*/
 
/*!
    \fn template <typename T, size_t E> QSpan<T,E>::QSpan(const QSpan &other)
    \fn template <typename T, size_t E> QSpan<T,E>::QSpan(QSpan &&other)
    \fn template <typename T, size_t E> QSpan<T,E> &QSpan<T,E>::operator=(const QSpan &other)
    \fn template <typename T, size_t E> QSpan<T,E> &QSpan<T,E>::operator=(QSpan &&other)
    \fn template <typename T, size_t E> QSpan<T,E>::~QSpan()
 
    These Special Member Functions are implicitly-defined.
 
    \note Moves are equivalent to copies. Only data() and size() are copied
    from span to span, not the referenced data.
*/
 
/*!
    \fn template <typename T, size_t E> template <typename It, QSpan<T, E>::if_compatible_iterator<It>> QSpan<T,E>::QSpan(It first, qsizetype count)
 
    Constructs a QSpan referencing the data starting at \a first and having length
    \a count.
 
    \c{[first, count)} must be a valid range.
 
    \constraints \c{It} is \l{span-compatible-iterators}{a compatible iterator}.
*/
 
/*!
    \fn template <typename T, size_t E> template <typename It, QSpan<T, E>::if_compatible_iterator<It>> QSpan<T,E>::QSpan(It first, It last)
 
    Constructs a QSpan referencing the data starting at \a first and having length
    (\a last - \a first).
 
    \c{[first, last)} must be a valid range.
 
    \constraints \c{It} is \l{span-compatible-iterators}{a compatible iterator}.
*/
 
/*!
    \fn template <typename T, size_t E> template <size_t N> QSpan<T,E>::QSpan(q20::type_identity_t<T> (&arr)[N]);
    \fn template <typename T, size_t E> template <typename S, size_t N, QSpan<T, E>::if_qualification_conversion<S> = true> QSpan<T,E>::QSpan(std::array<S, N> &arr);
    \fn template <typename T, size_t E> template <typename S, size_t N, QSpan<T, E>::if_qualification_conversion<S> = true> QSpan<T,E>::QSpan(const std::array<S, N> &arr);
 
    Constructs a QSpan referencing the data in the supplied array \a arr.
 
    \note \c{q20::type_identity_t} is a C++17 backport of C++20's
    \l{https://en.cppreference.com/w/cpp/types/type_identity}{\c{std::type_identity_t}}.
 
    \constraints
    \list
    \li either \c{N} or \l{extent} are \c{std::dynamic_extent} or otherwise \l{extent} \c{==} \c{N}
    \li and either \c{S} or \c{const S} are the same as \c{T}.
    \endlist
*/
 
/*!
    \fn template <typename T, size_t E> template <typename Range, QSpan<T, E>::if_compatible_range<Range> = true> QSpan<T,E>::QSpan(Range &&r)
 
    Constructs a QSpan referencing the data in the supplied range \a r.
 
    \constraints \c{Range} is \l{span-compatible-ranges}{a compatible range}.
*/
 
/*!
    \fn template <typename T, size_t E> template <typename S, size_t N, QSpan<T, E>::if_qualification_conversion<S> = true> QSpan<T,E>::QSpan(QSpan<S, N> other);
    \fn template <typename T, size_t E> template <typename S, size_t N, QSpan<T, E>::if_qualification_conversion<S> = true> QSpan<T,E>::QSpan(std::span<S, N> other);
 
    Constructs a QSpan referencing the data in the supplied span \a other.
 
    \constraints
    \list
    \li either \c{N} or \l{extent} are \c{std::dynamic_extent} or \l{extent} \c{==} \c{N}
    \li and either \c{S} or \c{const S} are the same as \c{T}.
    \endlist
*/
 
/*!
    \fn template <typename T, size_t E> QSpan<T, E>::QSpan(std::initializer_list<value_type> il);
 
    Constructs a QSpan referencing the data in the supplied initializer list \a il.
 
    \note This constructor is \c{noexcept} only if \c{E} is \c{std::dynamic_extent}.
 
    \note If \c{E} is not \c{std::dynamic_extent} and the size of \a il is not \c{E}, the behavior is undefined.
 
    \constraints \c{T} is \c{const}-qualified.
 
    \sa {Const and Mutable Spans}
*/
 
//
// Member functions: sizes
//
 
/*!
    \fn template <typename T, size_t E> auto QSpan<T, E>::size() const
 
    Returns the size of the span, that is, the number of elements it references.
 
    \sa size_bytes(), empty(), isEmpty()
*/
 
/*!
    \fn template <typename T, size_t E> auto QSpan<T, E>::size_bytes() const
 
    Returns the size of the span in bytes, that is, the number of elements
    multiplied by \c{sizeof(T)}.
 
    \sa size(), empty(), isEmpty()
*/
 
/*!
    \fn template <typename T, size_t E> auto QSpan<T, E>::empty() const
    \fn template <typename T, size_t E> auto QSpan<T, E>::isEmpty() const
 
    Returns whether the span is empty, that is, whether \c{size() == 0}.
 
    These functions do the same thing: empty() is provided for STL
    compatibility and isEmpty() is provided for Qt compatibility.
 
    \sa size(), size_bytes()
*/
 
//
// element access
//
 
/*!
    \fn template <typename T, size_t E> QSpan<T, E>::operator[](size_type idx) const
 
    Returns a reference to the element at index \a idx in the span.
 
    The index must be in range, that is, \a idx >= 0 and \a idx < size(),
    otherwise the behavior is undefined.
 
    \sa front(), back(), size(), empty(), {Const and Mutable Spans}
*/
 
/*!
    \fn template <typename T, size_t E> auto QSpan<T, E>::front() const
 
    Returns a reference to the first element in the span.
 
    The span must not be empty, otherwise the behavior is undefined.
 
    \sa operator[](), back(), size(), empty(), {Const and Mutable Spans}
*/
 
/*!
    \fn template <typename T, size_t E> auto QSpan<T, E>::back() const
 
    Returns a reference to the last element in the span.
 
    The span must not be empty, otherwise the behavior is undefined.
 
    \sa operator[](), front(), size(), empty(), {Const and Mutable Spans}
*/
 
/*!
    \fn template <typename T, size_t E> auto QSpan<T, E>::data() const
 
    Returns a pointer to the beginning of the span.
 
    The same as calling begin().
 
    \sa begin(), front(), {Const and Mutable Spans}
*/
 
//
// iterators
//
 
/*!
    \fn template <typename T, size_t E> auto QSpan<T, E>::begin() const
 
    Returns an interator pointing at the beginning of the span.
 
    Because QSpan iterators are just pointers, this is the same as calling
    data().
 
    \sa end(), cbegin(), rbegin(), crbegin(), data(), {Const and Mutable Spans}
*/
 
/*!
    \fn template <typename T, size_t E> auto QSpan<T, E>::end() const
 
    Returns an iterator pointing to one past the end of the span.
 
    Because QSpan iterators are just pointers, this it the same as calling
    \c{data() + size()}.
 
    \sa begin(), cend(), rend(), crend(), data(), size(), {Const and Mutable Spans}
*/
 
/*!
    \fn template <typename T, size_t E> auto QSpan<T, E>::cbegin() const
 
    Returns a const_iterator pointing to the beginning of the span.
 
    This will return a read-only iterator even if \c{T} is not \c{const}:
    \code
    QSpan<int> span = ~~~;
    *span.begin() = 42; // OK
    *span.cbegin() = 42; // ERROR: cannot assign through a const_iterator
    \endcode
 
    \sa cend(), begin(), crbegin(), rbegin(), data(), {Const and Mutable Spans}
*/
 
/*!
    \fn template <typename T, size_t E> auto QSpan<T, E>::cend() const
 
    Returns a const_iterator pointing to one past the end of the span.
 
    \sa cbegin(), end(), crend(), rend(), data(), size(), {Const and Mutable Spans}
*/
 
/*!
    \fn template <typename T, size_t E> auto QSpan<T, E>::rbegin() const
 
    Returns a reverse_iterator pointing to the beginning of the reversed span.
 
    \sa rend(), crbegin(), begin(), cbegin(), {Const and Mutable Spans}
*/
 
/*!
    \fn template <typename T, size_t E> auto QSpan<T, E>::rend() const
 
    Returns a reverse_iterator pointing to one past the end of the reversed span.
 
    \sa rbegin(), crend(), end(), cend(), {Const and Mutable Spans}
*/
 
/*!
    \fn template <typename T, size_t E> auto QSpan<T, E>::crbegin() const
 
    Returns a const_reverse_iterator pointing to the beginning of the reversed span.
 
    \sa crend(), rbegin(), cbegin(), begin(), {Const and Mutable Spans}
*/
 
/*!
    \fn template <typename T, size_t E> auto QSpan<T, E>::crend() const
 
    Returns a const_reverse_iterator pointing to one past the end of the reversed span.
 
    \sa crbegin(), rend(), cend(), end(), {Const and Mutable Spans}
*/
 
//
// compile-time subspans:
//
 
/*!
    \fn template <typename T, size_t E> template <std::size_t Count> auto QSpan<T, E>::first() const
    \keyword first-t
 
    Returns a \l{variable-fixed-spans}{fixed-sized} span of size \c{Count}
    referencing the first \c{Count} elements of \c{*this}.
 
    The span must hold at least \c{Count} elements (\c{E} >= \c{Count} \e{and}
    size() >= \c{Count}), otherwise the behavior is undefined.
 
    \sa first(QSpan<T,E>::size_type), last(), subspan()
*/
 
/*!
    \fn template <typename T, size_t E> template <std::size_t Count> auto QSpan<T, E>::last() const
    \keyword last-t
 
    Returns a \l{variable-fixed-spans}{fixed-sized} span of size \c{Count}
    referencing the last \c{Count} elements of \c{*this}.
 
    The span must hold at least \c{Count} elements (\c{E} >= \c{Count} \e{and}
    size() >= \c{Count}), otherwise the behavior is undefined.
 
    \sa last(QSpan<T,E>::size_type), first(), subspan()
*/
 
/*!
    \fn template <typename T, size_t E> template <std::size_t Offset> auto QSpan<T, E>::subspan() const
    \keyword subspan-t1
 
    Returns a span of size \c{E - Offset} referencing the remainder of this span
    after dropping the first \c{Offset} elements.
 
    If \c{*this} is a variable-sized span, the return type is a variable-sized
    span, otherwise it is a fixed-sized span.
 
    This span must hold at least \c{Offset} elements (\c{E} >= \c{Offset} \e{and}
    size() >= \c{Offset}), otherwise the behavior is undefined.
 
    \sa subspan(QSpan<T,E>::size_type), subspan(), first(), last()
    \sa {Variable-Size and Fixed-Size Spans}
*/
 
/*!
    \fn template <typename T, size_t E> template <std::size_t Offset, std::size_t Count> auto QSpan<T, E>::subspan() const
    \keyword subspan-t2
 
    Returns a span of size \c{Count} referencing the \c{Count} elements of this
    span starting at \c{Offset}.
 
    If \c{*this} is a variable-sized span, the return type is a variable-sized
    span, otherwise it is a fixed-sized span.
 
    This span must hold at least \c{Offset + Count} elements (\c{E} >=
    \c{Offset + Count} \e{and} size() >= \c{Offset + Count}), otherwise the
    behavior is undefined.
 
    \sa subspan(QSpan<T,E>::size_type, QSpan<T,E>::size_type), first(), last()
    \sa {Variable-Size and Fixed-Size Spans}
*/
 
//
// runtime subspans:
//
 
/*!
    \fn template <typename T, size_t E> auto QSpan<T, E>::first(qsizetype n) const
    \keyword first-n
 
    Returns a \l{variable-fixed-spans}{variable-sized} span of size \a n
    referencing the first \a n elements of \c{*this}.
 
    \a n must be non-negative.
 
    The span must hold at least \a n elements (\c{E} >= \a n \e{and} size() >=
    \a n), otherwise the behavior is undefined.
 
    \sa {first-t}{first<N>()}, last(QSpan<T,E>::size_type), subspan(QSpan<T,E>::size_type),
        subspan(QSpan<T,E>::size_type, QSpan<T,E>::size_type)
    \sa sliced(), chopped()
*/
 
/*!
    \fn template <typename T, size_t E> auto QSpan<T, E>::last(qsizetype n) const
    \keyword last-n
 
    Returns a \l{variable-fixed-spans}{variable-sized} span of size \a n
    referencing the last \a n elements of \c{*this}.
 
    \a n must be non-negative.
 
    The span must hold at least \a n elements (\c{E} >= \a n \e{and}
    size() >= \a n), otherwise the behavior is undefined.
 
    \sa last(), first(QSpan<T,E>::size_type), subspan(QSpan<T,E>::size_type),
        subspan(QSpan<T,E>::size_type, QSpan<T,E>::size_type), sliced(), chopped()
*/
 
/*!
    \fn template <typename T, size_t E> auto QSpan<T, E>::subspan(qsizetype pos) const
    \fn template <typename T, size_t E> auto QSpan<T, E>::sliced(qsizetype pos) const
    \keyword subspan-n1
 
    Returns a \l{variable-fixed-spans}{variable-sized} span of size \c{size() -
    pos} referencing the remainder of this span after dropping the first \a pos
    elements.
 
    \a pos must be non-negative.
 
    This span must hold at least \a pos elements (\c{E} >= \a pos \e{and}
    size() >= \a pos), otherwise the behavior is undefined.
 
    These functions do the same thing: subspan() is provided for STL
    compatibility and sliced() is provided for Qt compatibility.
 
    \sa subspan(), first(QSpan<T,E>::size_type), last(QSpan<T,E>::size_type), chopped(), slice()
*/
 
/*!
    \fn template <typename T, size_t E> auto QSpan<T, E>::subspan(qsizetype pos, qsizetype n) const
    \fn template <typename T, size_t E> auto QSpan<T, E>::sliced(qsizetype pos, qsizetype n) const
    \keyword subspan-n2
 
    Returns a \l{variable-fixed-spans}{variable-sized} span of size \a n
    referencing the \a n elements of this span starting at \a pos.
 
    Both \a pos and \a n must be non-negative.
 
    This span must hold at least \c{pos + n} elements (\c{E} >=
    \c{pos + n} \e{and} size() >= \c{pos + n}), otherwise the
    behavior is undefined.
 
    These functions do the same thing: subspan() is provided for STL
    compatibility and sliced() is provided for Qt compatibility.
 
    \sa subspan(), first(QSpan<T,E>::size_type), last(QSpan<T,E>::size_type), chopped(), slice()
*/
 
/*!
    \fn template <typename T, size_t E> auto QSpan<T,E>::chopped(qsizetype n) const
    \since 6.9
 
    Returns a \l{variable-fixed-spans}{variable-sized} span of size size() - \a
    n referencing the first size() - \a n elements of this span.
 
    Same as \c{first(size() - n)}.
 
    \a n must be non-negative.
 
    This span must hold at least \a n elements (\c{E} >= \a n \e{and} size() >=
    \a n), otherwise the behavior is undefined.
 
    \sa subspan(), first(QSpan<T,E>::size_type), last(QSpan<T,E>::size_type), chop()
*/
 
/*!
    \fn template <typename T, size_t E> void QSpan<T,E>::chop(qsizetype n)
    \since 6.9
 
    Same as \c{*this = chopped(}\a{n}\c{)}.
 
    This function is only available on \l{variable-fixed-spans}{variable-sized spans}.
 
    \sa chopped()
*/
 
/*!
    \fn template <typename T, size_t E> void QSpan<T,E>::slice(qsizetype pos)
    \since 6.9
 
    Same as \c{*this = sliced(}\a{pos}\c{)}.
 
    This function is only available on \l{variable-fixed-spans}{variable-sized spans}.
 
    \sa sliced()
*/
 
/*!
    \fn template <typename T, size_t E> void QSpan<T,E>::slice(qsizetype pos, qsizetype n)
    \since 6.9
 
    Same as \c{*this = sliced(}\a{pos}\c{, }\a{n}\c{)}.
 
    This function is only available on \l{variable-fixed-spans}{variable-sized spans}.
 
    \sa sliced()
*/
 
/*!
    \fn template <typename T, size_t E> auto QSpan<T, E>::as_bytes(QSpan s)
    \since 6.8
 
    Returns \a s as a \c{QSpan<const std::byte, E'>} whose size() is equal to
    \c{s.size_bytes()}.
 
    If \c{E} is \c{std::dynamic_extent} then so is \c{E'}.
    Otherwise, \c{E' = E * sizeof(T)}.
 
    \note \c{q20::dynamic_extent} is a C++17 backport of C++20's
    \l{https://en.cppreference.com/w/cpp/container/span/dynamic_extent}{\c{std::dynamic_extent}}.
 
    \sa as_writable_bytes(), size_bytes(), {Const and Mutable Spans}
*/
 
/*!
    \fn template <typename T, size_t E> auto QSpan<T, E>::as_writable_bytes(QSpan s)
    \since 6.8
 
    Returns \a s as a \c{QSpan<std::byte, E'>} whose size() is equal to
    \c{s.size_bytes()}.
 
    If \c{E} is \c{std::dynamic_extent} then so is \c{E'}.
    Otherwise, \c{E' = E * sizeof(T)}.
 
    \note \c{q20::dynamic_extent} is a C++17 backport of C++20's
    \l{https://en.cppreference.com/w/cpp/container/span/dynamic_extent}{\c{std::dynamic_extent}}.
 
    \constraints \c{!std::is_const_v<T>}.
 
    \sa as_bytes(), size_bytes(), {Const and Mutable Spans}
*/