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

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:

(1.1)
If T and U are the same type, the result of the operation is that t1 equals u2 and u1 equals t2.
(1.2)
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:

(2.1)
S is (void)swap(E1, E2)218 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.
(2.2)
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)).
(2.3)
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
- (2.3.1)
  T is a literal type ([basic.types]),
- (2.3.2)
  both E1 = std::move(E2) and E2 = std::move(E1) are constant subexpressions ([defns.const.subexpr]), and
- (2.3.3)
  the full-expressions of the initializers in the declarations
```
T t1(std::move(E1));
T t2(std::move(E2));
```
  are constant subexpressions.
noexcept(S) is equal to is_nothrow_move_constructible_v<T> && is_nothrow_move_assignable_v<T>.
(2.4)
Otherwise, ranges::swap(E1, E2) is ill-formed.
[Note
: This case can result in substitution failure when ranges::swap(E1, E2) appears in the immediate context of a template instantiation. — end note
]

[Note

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

The semantics of the swappable and swappable_with concepts are fully defined by the ranges::swap customization point object.

— end note

]

[Example

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

]

218)

The name swap is used here unqualified.

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