std:: swappable, std:: swappable_with

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C++
Concepts library
Exposition-only concepts
 (C++20)
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 .

Equality preservation

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

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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