[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) — *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]") 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[.](#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 declarationtemplatevoid 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) — *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]") and [assignable_from](concept.assignable#concept:assignable_from "18.4.8 Concept assignable_­from [concept.assignable]"), 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 && is_nothrow_move_assignable_v[.](#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) — *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) — *end note*] [🔗](#concept:swappable) `template concept [swappable](#concept:swappable "18.4.9 Concept swappable [concept.swappable]") = requires(T& a, T& b) { ranges::swap(a, b); }; ` [🔗](#concept:swappable_with) `template 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]") && requires(T&& t, U&& u) { ranges::swap(std::forward(t), std::forward(t)); ranges::swap(std::forward(u), std::forward(u)); ranges::swap(std::forward(t), std::forward(u)); ranges::swap(std::forward(u), std::forward(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) — *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 #include #include namespace ranges = std::ranges; template U>void value_swap(T&& t, U&& u) { ranges::swap(std::forward(t), std::forward(u));}templatevoid 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*] [192)](#footnote-192)[192)](#footnoteref-192) The name swap is used here unqualified[.](#footnote-192.sentence-1)