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std::swappable,std::swappable_with(3) C++ Standard Libary std::swappable,std::swappable_with(3)


std::swappable,std::swappable_with - std::swappable,std::swappable_with


Defined in header <concepts>
template< class T >

concept swappable =
requires(T& a, T& b) { (1) (since C++20)
ranges::swap(a, b);

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)); (2) (since C++20)
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));


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

An expression is equality preserving if it results in equal outputs given equal

* The inputs to an expression consist of its operands.
* The outputs of an expression consist of its result and all operands modified by
the expression (if any).

In specification of standard concepts, operands are defined as the largest
subexpressions that include only:

* an id-expression, and
* invocations of std::move, std::forward, and std::declval.

The cv-qualification and value category of each operand is determined by assuming
that each template type parameter denotes a cv-unqualified complete non-array object

Every expression required to be equality preserving is further required to be
stable: two evaluations of such an expression with the same input objects must have
equal outputs absent any explicit intervening modification of those input objects.

Unless noted otherwise, every expression used in a requires-expression is required
to be equality preserving and stable, and the evaluation of the expression may
modify only its non-constant operands. Operands that are constant must not be