18 Concepts library [concepts]

18.4 Language-related concepts [concepts.lang]

18.4.9 Concept swappable [concept.swappable]

Let t1 and t2 be equality-preserving expressions that denote distinct equal objects of type T, and let u1 and u2 similarly denote distinct equal objects of type U.
[Note 1: 
t1 and u1 can denote distinct objects, or the same object.
— end note]
An operation exchanges the values denoted by t1 and u1 if and only if the operation modifies neither t2 nor u2 and:
  • If T and U are the same type, the result of the operation is that t1 equals u2 and u1 equals t2.
  • If T and U are different types and common_reference_with<decltype((t1)), decltype((u1))> is modeled, the result of the operation is that C(t1) equals C(u2) and C(u1) equals C(t2) where C is common_reference_t<decltype((t1)), decltype((u1))>.
The name ranges​::​swap denotes a customization point object ([customization.point.object]).
The expression ranges​::​swap(E1, E2) for subexpressions E1 and E2 is expression-equivalent to an expression S determined as follows:
  • S is (void)swap(E1, E2)198 if E1 or E2 has class or enumeration type ([basic.compound]) and that expression is valid, with overload resolution performed in a context that includes the declaration template<class T> void swap(T&, T&) = delete; and does not include a declaration of ranges​::​swap.
    If the function selected by overload resolution does not exchange the values denoted by E1 and E2, the program is ill-formed, no diagnostic required.
    [Note 2: 
    This precludes calling unconstrained program-defined overloads of swap.
    When the deleted overload is viable, program-defined overloads need to be more specialized ([temp.func.order]) to be selected.
    — end note]
  • Otherwise, if E1 and E2 are lvalues of array types ([basic.compound]) with equal extent and ranges​::​swap(*E1, *E2) is a valid expression, S is (void)ranges​::​swap_ranges(E1, E2), except that noexcept(S) is equal to noexcept(​ranges​::​swap(*E1, *E2)).
  • Otherwise, if E1 and E2 are lvalues of the same type T that models move_constructible<T> and assignable_from<T&, T>, S is an expression that exchanges the denoted values.
    S is a constant expression if noexcept(S) is equal to is_nothrow_move_constructible_v<T> && is_nothrow_move_assignable_v<T>.
  • Otherwise, ranges​::​swap(E1, E2) is ill-formed.
    [Note 3: 
    This case can result in substitution failure when ranges​::​swap(E1, E2) appears in the immediate context of a template instantiation.
    — end note]
[Note 4: 
Whenever ranges​::​swap(E1, E2) is a valid expression, it exchanges the values denoted by E1 and E2 and has type void.
— end note]
template<class T> concept swappable = requires(T& a, T& b) { ranges::swap(a, b); };
template<class T, class U> concept swappable_with = 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)); };
[Note 5: 
The semantics of the swappable and swappable_with concepts are fully defined by the ranges​::​swap customization point object.
— end note]
[Example 1: 
User code can ensure that the evaluation of swap calls is performed in an appropriate context under the various conditions as follows: #include <cassert> #include <concepts> #include <utility> namespace ranges = std::ranges; template<class T, std::swappable_with<T> U> void value_swap(T&& t, U&& u) { ranges::swap(std::forward<T>(t), std::forward<U>(u)); } template<std::swappable T> void lv_swap(T& t1, T& t2) { ranges::swap(t1, t2); } namespace N { struct A { int m; }; struct Proxy { A* a; Proxy(A& a) : a{&a} {} friend void swap(Proxy x, Proxy y) { ranges::swap(*x.a, *y.a); } }; Proxy proxy(A& a) { return Proxy{ a }; } } int main() { int i = 1, j = 2; lv_swap(i, j); assert(i == 2 && j == 1); N::A a1 = { 5 }, a2 = { -5 }; value_swap(a1, proxy(a2)); assert(a1.m == -5 && a2.m == 5); }
— end example]
The name swap is used here unqualified.