[concept.swappable]

# 18 Concepts library [[concepts]](./#concepts)

## 18.4 Language-related concepts [[concepts.lang]](concepts.lang#concept.swappable)

### 18.4.9 Concept swappable [concept.swappable]

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/concepts.tex#L566)

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[.](#1.sentence-1)

[*Note [1](#note-1)*:

t1 and u1 can denote distinct objects, or the same object[.](#1.sentence-2)

â *end note*]

An operation[*exchanges the values*](#def:exchanges_the_values) denoted by t1 and u1 if and only
if the operation modifies neither t2 nor u2 and:

- [(1.1)](#1.1)

  If T and U are the same type, the result of the operation
 is that t1 equals u2 and u1 equals t2[.](#1.1.sentence-1)

- [(1.2)](#1.2)

  If T and U are different types and [common_reference_with](concept.commonref#concept:common_reference_with "18.4.5 Concept common_­reference_­with [concept.commonref]")<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))>[.](#1.2.sentence-1)

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/concepts.tex#L590)

The name ranges​::​swap denotes a customization point
object ([[customization.point.object]](customization.point.object "16.3.3.3.5 Customization Point Object types"))[.](#2.sentence-1)

The expressionranges​::​swap(E1, E2) for subexpressions E1 and E2 is expression-equivalent to an expressionS determined as follows:

- [(2.1)](#2.1)

  S is (void)swap(E1, E2)[192](#footnote-192 "The name swap is used here unqualified.") if E1 or E2 has class or enumeration type ([[basic.compound]](basic.compound "6.9.4 Compound types")) and that expression is valid, with
 overload resolution performed in a context that includes the declarationtemplate<class T>void swap(T&, T&) = delete; and does not include a declaration of ranges​::​swap[.](#2.1.sentence-1)
  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[.](#2.1.sentence-2)
  [*Note [2](#note-2)*:
  This precludes calling unconstrained program-defined overloads of swap[.](#2.1.sentence-3)
  When the deleted overload is viable, program-defined overloads
 need to be more specialized ([[temp.func.order]](temp.func.order "13.7.7.3 Partial ordering of function templates")) to be selected[.](#2.1.sentence-4)
 â *end note*]

- [(2.2)](#2.2)

  Otherwise, if E1 and E2 are lvalues of array types ([[basic.compound]](basic.compound "6.9.4 Compound types"))
 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))[.](#2.2.sentence-1)

- [(2.3)](#2.3)

  Otherwise, if E1 and E2 are lvalues of the
 same type T that models [move_constructible](concept.moveconstructible#concept:move_constructible "18.4.13 Concept move_­constructible [concept.moveconstructible]")<T> and [assignable_from](concept.assignable#concept:assignable_from "18.4.8 Concept assignable_­from [concept.assignable]")<T&, T>, S is an expression that exchanges the denoted values[.](#2.3.sentence-1)
  S is a constant expression if
  * [(2.3.1)](#2.3.1)

T is a literal type ([[basic.types.general]](basic.types.general#term.literal.type "6.9.1 General")),

  * [(2.3.2)](#2.3.2)

both E1 = std​::​move(E2) and E2 = std​::​move(E1) are
 constant subexpressions ([[defns.const.subexpr]](defns.const.subexpr "3.15 constant subexpression")), and

  * [(2.3.3)](#2.3.3)

the full-expressions of the initializers in the declarationsT 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>[.](#2.3.sentence-2)

- [(2.4)](#2.4)

  Otherwise, ranges​::​swap(E1, E2) is ill-formed[.](#2.4.sentence-1)
  [*Note [3](#note-3)*:
  This case can result in substitution failure when ranges​::​swap(E1, E2) appears in the immediate context of a template instantiation[.](#2.4.sentence-2)
 â *end note*]

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/concepts.tex#L660)

[*Note [4](#note-4)*:

Whenever ranges​::​swap(E1, E2) is a valid expression, it
exchanges the values denoted byE1 and E2 and has type void[.](#3.sentence-1)

â *end note*]

[ð](#concept:swappable)

`template<class T>
  concept [swappable](#concept:swappable "18.4.9 Concept swappable [concept.swappable]") = requires(T& a, T& b) { ranges::swap(a, b); };
`

[ð](#concept:swappable_with)

`template<class T, class U>
  concept [swappable_with](#concept:swappable_with "18.4.9 Concept swappable [concept.swappable]") =
    [common_reference_with](concept.commonref#concept:common_reference_with "18.4.5 Concept common_­reference_­with [concept.commonref]")<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));
    };
`

[4](#4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/concepts.tex#L684)

[*Note [5](#note-5)*:

The semantics of the [swappable](#concept:swappable "18.4.9 Concept swappable [concept.swappable]") and [swappable_with](#concept:swappable_with "18.4.9 Concept swappable [concept.swappable]") concepts are fully defined by the ranges​::​swap customization point object[.](#4.sentence-1)

â *end note*]

[5](#5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/concepts.tex#L690)

[*Example [1](#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](#concept:swappable_with "18.4.9 Concept swappable [concept.swappable]")<T> U>void value_swap(T&& t, U&& u) { ranges::swap(std::forward<T>(t), std::forward<U>(u));}template<std::[swappable](#concept:swappable "18.4.9 Concept swappable [concept.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);}

â *end example*]

[192)](#footnote-192)[192)](#footnoteref-192)

The name swap is used
 here unqualified[.](#footnote-192.sentence-1)
