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:
  • 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 that model CommonReference<const T&, const U&>, the result of the operation is that C(t1) equals C(u2) and C(u1) equals C(t2) where C is common_­reference_­t<const T&, const U&>.
The name ranges::swap denotes a customization point object ([customization.point.object]).
The expression ranges::swap(E1, E2) for some subexpressions E1 and E2 is expression-equivalent to an expression S determined as follows:
  • S is (void)swap(E1, E2)222 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 declarations
    template<class T>
      void swap(T&, T&) = delete;
    template<class T, size_t N>
      void swap(T(&)[N], T(&)[N]) = 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 with no diagnostic required.
  • 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 MoveConstructible<T> and Assignable<T&, T>, S is an expression that exchanges the denoted values.
    S is a constant expression if
    • T is a literal type ([basic.types]),
    • both E1 = std::move(E2) and E2 = std::move(E1) are constant subexpressions ([defns.const.subexpr]), and
    • 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>.
  • 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 SwappableWith = CommonReference<const remove_reference_t<T>&, const remove_reference_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 SwappableWith concepts are fully defined by the ranges::swap customization point.
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::SwappableWith<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 proxy(A& a) { return Proxy{ &a }; }

  void swap(A& x, Proxy p) {
    ranges::swap(x.m, p.a->m);
  }
  void swap(Proxy p, A& x) { swap(x, p); }      // satisfy symmetry requirement
}

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.