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qtbase/src/corelib/text/qanystringview.cpp
Marc Mutz b7a67b46e6 Long live \constraints! 
We have divergence in the way we document function, operator and
constructor constraints. About half use \note, while the other
doesn't. Some say "if and only if" while others say just "participates
only if".

So add a qdoc macro, \constraints, to semantically mark up these
constraints. It expands to a section titled `Constraints`, and
uses a predefined sentence (prefix) for constraints.

Documentation for constraints is moved to the end of the comment
blocks to separate them from the rest of the text.

Apply them to all the standard-ish constraint documentation blocks
(grepped for "participate"). I didn't look for other, one-off, ways
documentation authors may have documented constraints, but I'm also
not aware of any.

Re-wrap lines only if the result fits into a single line.

As a drive-by, drop additional "if"s, as in "only if X and -if- Y" to
make the texts work with the `Constraints` section.

Fixes: QTBUG-106871
Pick-to: 6.9 6.8 6.5
Change-Id: I18c2f9f734474017264e49165389f8c9c7f34030
Reviewed-by: Kai Köhne <kai.koehne@qt.io>
Reviewed-by: Paul Wicking <paul.wicking@qt.io>
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
2025-02-28 13:51:18 +00:00

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// Copyright (C) 2020 Klarälvdalens Datakonsult AB, a KDAB Group company, info@kdab.com, author Marc Mutz <marc.mutz@kdab.com>
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR LGPL-3.0-only OR GPL-2.0-only OR GPL-3.0-only
#include "qanystringview.h"
#include "qdebug.h"
#include "qttypetraits.h"
QT_BEGIN_NAMESPACE
/*!
\class QAnyStringView
\inmodule QtCore
\since 6.0
\brief 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.
\reentrant
\ingroup tools
\ingroup string-processing
\compares strong
\compareswith strong char16_t QChar {const char16_t *} {const char *} \
QByteArray QByteArrayView QString QStringView QUtf8StringView \
QLatin1StringView
\endcompareswith
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 \c
char, \c char8_t, QChar, \c ushort, \c char16_t or (on platforms,
such as Windows, where it is a 16-bit type) \c 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, \c{(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 \c{u8"Hello World"}, a \c char8_t
string literal.
Like elsewhere in Qt, QAnyStringView assumes \c 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:
\snippet code/src_corelib_text_qanystringview.cpp 0
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 \e{Literal Type}.
\section2 Compatible Character Types
QAnyStringView accepts strings over a variety of character types:
\list
\li \c char (both signed and unsigned)
\li \c char8_t (C++20 only)
\li \c char16_t
\li \c wchar_t (where it's a 16-bit type, e.g. Windows)
\li \c ushort
\li \c QChar
\endlist
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).
\section2 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).
\sa QUtf8StringView, QStringView
*/
/*!
\typedef QAnyStringView::difference_type
Alias for \c{std::ptrdiff_t}. Provided for compatibility with the STL.
*/
/*!
\typedef QAnyStringView::size_type
Alias for qsizetype. Provided for compatibility with the STL.
*/
/*!
\fn QAnyStringView::QAnyStringView()
Constructs a null string view.
\sa isNull()
*/
/*!
\fn QAnyStringView::QAnyStringView(std::nullptr_t)
Constructs a null string view.
\sa isNull()
*/
/*!
\fn template <typename Char, QAnyStringView::if_compatible_char<Char> = true> QAnyStringView::QAnyStringView(const Char *str, qsizetype len)
Constructs a string view on \a str with length \a len.
The range \c{[str,len)} must remain valid for the lifetime of this string view object.
Passing \nullptr as \a str is safe if \a len is 0, too, and results in a null string view.
The behavior is undefined if \a len is negative or, when positive, if \a str is \nullptr.
\constraints \c Char is a compatible character type.
\sa isNull(), {Compatible Character Types}
*/
/*!
\fn template <typename Char, QAnyStringView::if_compatible_char<Char> = true> QAnyStringView::QAnyStringView(const Char *first, const Char *last)
Constructs a string view on \a first with length (\a last - \a first).
The range \c{[first,last)} must remain valid for the lifetime of
this string view object.
Passing \nullptr as \a first is safe if \a last is \nullptr, too,
and results in a null string view.
The behavior is undefined if \a last precedes \a first, or \a first
is \nullptr and \a last is not.
\constraints \c Char is a compatible character type.
\sa isNull(), {Compatible Character Types}
*/
/*!
\fn template <typename Char> QAnyStringView::QAnyStringView(const Char *str)
Constructs a string view on \a str. The length is determined
by scanning for the first \c{Char(0)}.
\a str must remain valid for the lifetime of this string view object.
Passing \nullptr as \a str is safe and results in a null string view.
\constraints \a str is not an array and \c Char is a
compatible character type.
\sa isNull(), {Compatible Character Types}
*/
/*!
\fn template <typename Char, size_t N> QAnyStringView::QAnyStringView(const Char (&string)[N])
Constructs a string view on the character string literal \a string.
The view covers the array until the first \c{Char(0)} is encountered,
or \c N, whichever comes first.
If you need the full array, use fromArray() instead.
\a string must remain valid for the lifetime of this string view
object.
\constraints \a
string is an actual array and \c Char is a compatible character
type.
\sa {Compatible Character Types}
*/
/*!
\fn QAnyStringView::QAnyStringView(const QString &str)
Constructs a string view on \a str.
\c{str.data()} must remain valid for the lifetime of this string view object.
The string view will be null if and only if \c{str.isNull()}.
*/
/*!
\fn QAnyStringView::QAnyStringView(const QByteArray &str)
Constructs a string view on \a str. The data in \a str is interpreted as UTF-8.
\c{str.data()} must remain valid for the lifetime of this string view object.
The string view will be null if and only if \c{str.isNull()}.
*/
/*!
\fn template <typename Container, QAnyStringView::if_compatible_container<Container>> QAnyStringView::QAnyStringView(const Container &str)
Constructs a string view on \a str. The length is taken from \c{std::size(str)}.
\c{std::data(str)} must remain valid for the lifetime of this string view object.
The string view will be empty if and only if \c{std::size(str) == 0}. It is unspecified
whether this constructor can result in a null string view (\c{std::data(str)} would
have to return \nullptr for this).
\constraints \c Container is a
container with a compatible character type as \c{value_type}.
\sa isNull(), isEmpty()
*/
/*!
\fn template <typename Char, size_t Size> static QAnyStringView fromArray(const Char (&string)[Size]) noexcept
Constructs a string view on the full character string literal \a string,
including any trailing \c{Char(0)}. If you don't want the
null-terminator included in the view then you can use the constructor
overload taking a pointer and a size:
\snippet code/src_corelib_text_qanystringview.cpp 2
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.
\a string must remain valid for the lifetime of this string view
object.
This function will work with any array literal if \c Char is a
compatible character type.
*/
/*!
\fn 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.
*/
/*!
\fn 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 \e not null-terminated.
\sa size_bytes()
*/
/*!
\fn bool QAnyStringView::empty() const
Returns whether this string view is empty - that is, whether \c{size() == 0}.
This function is provided for STL compatibility.
\sa isEmpty(), isNull(), size()
*/
/*!
\fn bool QAnyStringView::isEmpty() const
Returns whether this string view is empty - that is, whether \c{size() == 0}.
This function is provided for compatibility with other Qt containers.
\sa empty(), isNull(), size()
*/
/*!
\fn bool QAnyStringView::isNull() const
Returns whether this string view is null - that is, whether \c{data() == nullptr}.
This functions is provided for compatibility with other Qt containers.
\sa empty(), isEmpty(), size()
*/
/*!
\fn qsizetype QAnyStringView::size() const
Returns the size of this string view, in the encoding's code points.
\sa empty(), isEmpty(), isNull(), size_bytes(), {Sizes and Sub-Strings}
*/
/*!
\fn 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.
\sa size(), data()
*/
/*!
\fn QAnyStringView::length() const
Same as size().
This function is provided for compatibility with other Qt containers.
\sa size()
*/
/*!
\fn 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.
\sa back(), {Sizes and Sub-Strings}
*/
/*!
\fn 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.
\sa front(), {Sizes and Sub-Strings}
*/
/*!
\fn QAnyStringView::mid(qsizetype pos, qsizetype n) const
\since 6.5
Returns the substring of length \a n starting at position
\a pos in this object.
\deprecated Use sliced() instead in new code.
Returns an empty string view if \a n exceeds the
length of the string view. If there are less than \a n code points
available in the string view starting at \a pos, or if
\a n is negative (default), the function returns all code points that
are available from \a pos.
\sa first(), last(), sliced(), chopped(), chop(), truncate(), slice(), {Sizes and Sub-Strings}
*/
/*!
\fn QAnyStringView::left(qsizetype n) const
\since 6.5
\deprecated Use first() instead in new code.
Returns the substring of length \a n starting at position
0 in this object.
The entire string view is returned if \a n is greater than or equal
to size(), or less than zero.
\sa first(), last(), sliced(), chopped(), chop(), truncate(), slice(), {Sizes and Sub-Strings}
*/
/*!
\fn QAnyStringView::right(qsizetype n) const
\since 6.5
\deprecated Use last() instead in new code.
Returns the substring of length \a n starting at position
size() - \a n in this object.
The entire string view is returned if \a n is greater than or equal
to size(), or less than zero.
\sa first(), last(), sliced(), chopped(), chop(), truncate(), slice(), {Sizes and Sub-Strings}
*/
/*!
\fn QAnyStringView::first(qsizetype n) const
\since 6.5
Returns a string view that contains the first \a n code points
of this string view.
\note The behavior is undefined when \a n < 0 or \a n > size().
\sa last(), sliced(), chopped(), chop(), truncate(), slice(), {Sizes and Sub-Strings}
*/
/*!
\fn QAnyStringView::last(qsizetype n) const
\since 6.5
Returns a string view that contains the last \a n code points of this string view.
\note The behavior is undefined when \a n < 0 or \a n > size().
\sa first(), sliced(), chopped(), chop(), truncate(), slice(), {Sizes and Sub-Strings}
*/
/*!
\fn QAnyStringView::sliced(qsizetype pos, qsizetype n) const
\since 6.5
Returns a string view containing \a n code points of this string view,
starting at position \a pos.
//! [UB-sliced-index-length]
\note The behavior is undefined when \a pos < 0, \a n < 0,
or \a pos + \a n > size().
//! [UB-sliced-index-length]
\sa first(), last(), chopped(), chop(), truncate(), slice(), {Sizes and Sub-Strings}
*/
/*!
\fn QAnyStringView::sliced(qsizetype pos) const
\since 6.5
Returns a string view starting at position \a pos in this object,
and extending to its end.
//! [UB-sliced-index-only]
\note The behavior is undefined when \a pos < 0 or \a pos > size().
//! [UB-sliced-index-only]
\sa first(), last(), chopped(), chop(), truncate(), slice(), {Sizes and Sub-Strings}
*/
/*!
\fn QAnyStringView &QAnyStringView::slice(qsizetype pos, qsizetype n)
\since 6.8
Modifies this string view to start at position \a pos, extending for
\a n code points.
\include qanystringview.cpp UB-sliced-index-length
\sa sliced(), first(), last(), chopped(), chop(), truncate(), {Sizes and Sub-Strings}
*/
/*!
\fn QAnyStringView &QAnyStringView::slice(qsizetype pos)
\since 6.8
\overload
Modifies this string view to start at position \a pos, extending to
its end.
\include qanystringview.cpp UB-sliced-index-only
\sa sliced(), first(), last(), chopped(), chop(), truncate(), {Sizes and Sub-Strings}
*/
/*!
\fn QAnyStringView::chopped(qsizetype n) const
\since 6.5
Returns the substring of length size() - \a n starting at the
beginning of this object.
Same as \c{first(size() - n)}.
\note The behavior is undefined when \a n < 0 or \a n > size().
\sa sliced(), first(), last(), chop(), truncate(), slice(), {Sizes and Sub-Strings}
*/
/*!
\fn QAnyStringView::truncate(qsizetype n)
\since 6.5
Truncates this string view to \a n code points.
Same as \c{*this = first(n)}.
\note The behavior is undefined when \a n < 0 or \a n > size().
\sa sliced(), first(), last(), chopped(), chop(), {Sizes and Sub-Strings}
*/
/*!
\fn QAnyStringView::chop(qsizetype n)
\since 6.5
Truncates this string view by \a n code points.
Same as \c{*this = first(size() - n)}.
\note The behavior is undefined when \a n < 0 or \a n > size().
\sa sliced(), first(), last(), chopped(), truncate(), slice(), {Sizes and Sub-Strings}
*/
/*! \fn template <typename Visitor> decltype(auto) QAnyStringView::visit(Visitor &&v) const
Calls \a 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:
\code
void processImpl(QLatin1String s) { ~~~ }
void processImpl(QUtf8StringView s) { ~~~ }
void processImpl(QStringView s) { ~~~ }
void process(QAnyStringView s)
{
s.visit([](auto s) { processImpl(s); });
}
\endcode
Here, we're reusing the same name, \c 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
\code
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;
});
});
}
\endcode
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:
\code
// 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
});
}
\endcode
*/
/*!
\fn QAnyStringView::compare(QAnyStringView lhs, QAnyStringView rhs, Qt::CaseSensitivity cs)
Compares the string view \a lhs with the string view \a rhs and returns a
negative integer if \a lhs is less than \a rhs, a positive integer if it is
greater than \a rhs, and zero if they are equal.
If \a cs is Qt::CaseSensitive (the default), the comparison is case sensitive;
otherwise the comparison is case-insensitive.
\sa operator==(), operator<(), operator>()
*/
/*!
\fn bool QAnyStringView::operator==(const QAnyStringView &lhs, const QAnyStringView & rhs)
\fn bool QAnyStringView::operator!=(const QAnyStringView & lhs, const QAnyStringView & rhs)
\fn bool QAnyStringView::operator<=(const QAnyStringView & lhs, const QAnyStringView & rhs)
\fn bool QAnyStringView::operator>=(const QAnyStringView & lhs, const QAnyStringView & rhs)
\fn bool QAnyStringView::operator<(const QAnyStringView & lhs, const QAnyStringView & rhs)
\fn bool QAnyStringView::operator>(const QAnyStringView & lhs, const QAnyStringView & rhs)
Operators that compare \a lhs to \a rhs.
\sa compare()
*/
/*!
\fn template <typename QStringLike> qToAnyStringViewIgnoringNull(const QStringLike &s);
\since 6.0
\internal
Convert \a s to a QAnyStringView ignoring \c{s.isNull()}.
Returns a string view that references \a{s}'s data, but is never null.
This is a faster way to convert a QString or QByteArray to a QAnyStringView,
if null QStrings or QByteArrays can legitimately be treated as empty ones.
\sa QString::isNull(), QAnyStringView
*/
/*!
\fn QAnyStringView::max_size() const
\since 6.8
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.
*/
/*!
\fn QAnyStringView::operator<<(QDebug d, QAnyStringView s)
\since 6.7
\relates QDebug
Outputs \a s to debug stream \a d.
If \c{d.quotedString()} is \c true, indicates which encoding the string is
in. If you just want the string data, use visit() like this:
\code
s.visit([&d) (auto s) { d << s; });
\endcode
\sa QAnyStringView::visit()
*/
QDebug operator<<(QDebug d, QAnyStringView s)
{
struct S { const char *prefix, *suffix; };
const auto affixes = s.visit([](auto s) {
using View = decltype(s);
if constexpr (std::is_same_v<View, QLatin1StringView>) {
return S{"", "_L1"};
} else if constexpr (std::is_same_v<View, QUtf8StringView>) {
return S{"u8", ""};
} else if constexpr (std::is_same_v<View, QStringView>) {
return S{"u", ""};
} else {
static_assert(QtPrivate::type_dependent_false<View>());
}
});
const QDebugStateSaver saver(d);
d.nospace();
if (d.quoteStrings())
d << affixes.prefix;
s.visit([&d](auto s) { d << s; });
if (d.quoteStrings())
d << affixes.suffix;
return d;
}
/*!
\fn template <typename...Args> QString QAnyStringView::arg(Args &&...args) const
\since 6.9
\include qstringview.cpp qstring-multi-arg
\sa QString::arg(Args&&...)
*/
/*!
\fn template <typename Char, size_t Size, QAnyStringView::if_compatible_char<Char>> QAnyStringView QAnyStringView::fromArray(const Char (&string)[Size])
Constructs a string view on the full character string literal \a string,
including any trailing \c{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.
\a string must remain valid for the lifetime of this string view
object.
This function will work with any array literal if \c Char is a
compatible character type. The compatible character types are: \c QChar, \c ushort, \c
char16_t and (on platforms, such as Windows, where it is a 16-bit
type) \c wchar_t.
*/
QT_END_NAMESPACE
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