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[docs] Add three-way comparison overview

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Task-number: QTBUG-128837
Pick-to: 6.8
Change-Id: I64ae12994f8a48ee5f4e48ca6ed7f5980476f2aa
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
(cherry picked from commit 737c87b2b21e30abf5e51126fd8ff4e45521144e)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
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Ivan Solovev 2024-11-22 15:35:36 +01:00 committed by Qt Cherry-pick Bot
parent 7328f90e64
commit 624784a729
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src/corelib/doc/src/cpp20-overview.qdoc
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@ -57,6 +57,44 @@
between the classes. See the corresponding section in the between the classes. See the corresponding section in the
\l {span-STL} {QSpan class documentation} for more details. \l {span-STL} {QSpan class documentation} for more details.
\section1 Three-way comparison operator
C++20 introduced \c {operator<=>()}, also known as the
\l {https://en.cppreference.com/w/cpp/language/operator_comparison#Three-way_comparison}
{three-way comparison operator}, and three ordering types to represent
the results of the comparison:
\list
\li \l {https://en.cppreference.com/w/cpp/utility/compare/strong_ordering}
{std::strong_ordering};
\li \l {https://en.cppreference.com/w/cpp/utility/compare/weak_ordering}
{std::weak_ordering};
\li \l {https://en.cppreference.com/w/cpp/utility/compare/partial_ordering}
{std::partial_ordering}.
\endlist
In Qt 6.8, many of the QtCore classes got support for \c {operator<=>()}.
To use the new operator, the user project must be compiled in C++20 mode.
However, starting from Qt 6.7, C++17 users can use
\l {Comparison types overview}{our own equivalents} of the std ordering
types.
As a C++20 language feature, backporting \c {operator<=>()} to C++17 is not
possible, but you can use the \l Qt::compareThreeWay() function, which acts
like a C++17 version of \c {operator<=>()} for built-in C++ types (such as
integers, floating-point, and enumeration types).
Qt also defines helper functions \c {compareThreeWay()} for various classes
in QtCore. These are all implemented as \l {https://wg21.link/p1601}
{hidden friends}. Users can implement their own \c {compareThreeWay()}
functions for custom types.
Finally, Qt provides a \l qCompareThreeWay() function template, which serves
as a generic three-way comparison implementation. It relies on
\l {Qt::compareThreeWay()} and the above-mentioned free
\c {compareThreeWay()} functions in its implementation.
\section1 Additional important features \section1 Additional important features
Qt has also adopted a few more features from C++20, such as: Qt has also adopted a few more features from C++20, such as: