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std::swappable, std::swappable_with

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Defined in header <concepts>

template< class T >

concept swappable =
requires(T& a, T& b) {
ranges::swap (a, b);

};

(1) (since C++20)

template< class T, class U >

concept swappable_with =
std::common_reference_with <T, U> &&
requires(T&& t, U&& u) {
ranges::swap (std::forward <T>(t), std::forward <T>(t));
ranges::swap (std::forward <U>(u), std::forward <U>(u));
ranges::swap (std::forward <T>(t), std::forward <U>(u));
ranges::swap (std::forward <U>(u), std::forward <T>(t));

};

(2) (since C++20)

The concept swappable<T> specifies that lvalues of type T are swappable.

The concept swappable_with<T, U> specifies that expressions of the type and value category encoded by T and U are swappable with each other. swappable_with<T, U> is satisfied only if a call to ranges::swap (t, u) exchanges the value of t and u, that is, given distinct objects t2 equal to t and u2 equal to u, after evaluating either ranges::swap (t, u) or ranges::swap (u, t), t2 is equal to u and u2 is equal to t.

[edit] Equality preservation

Expressions declared in requires expressions of the standard library concepts are required to be equality-preserving (except where stated otherwise).

[edit] References

C++23 standard (ISO/IEC 14882:2024):

18.4.9 Concept swappable [concept.swappable]

C++20 standard (ISO/IEC 14882:2020):

18.4.9 Concept swappable [concept.swappable]

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