[swappable.requirements]

# 16 Library introduction [[library]](./#library)

## 16.4 Library-wide requirements [[requirements]](requirements#swappable.requirements)

### 16.4.4 Requirements on types and expressions [[utility.requirements]](utility.requirements#swappable.requirements)

#### 16.4.4.3 Swappable requirements [swappable.requirements]

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/lib-intro.tex#L1913)

This subclause provides definitions for swappable types and expressions[.](#1.sentence-1)

In these
definitions, let t denote an expression of type T, and let u denote an expression of type U[.](#1.sentence-2)

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/lib-intro.tex#L1918)

An object t is [*swappable with*](#def:swappable_with "16.4.4.3 Swappable requirements [swappable.requirements]") an object u if and only if

- [(2.1)](#2.1)

the expressions swap(t, u) and swap(u, t) are valid when
evaluated in the context described below, and

- [(2.2)](#2.2)

these expressions have the following effects:
  * [(2.2.1)](#2.2.1)

the object referred to by t has the value originally held by u and

  * [(2.2.2)](#2.2.2)

the object referred to by u has the value originally held by t[.](#2.sentence-1)

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/lib-intro.tex#L1931)

The context in which swap(t, u) and swap(u, t) are evaluated shall
ensure that a binary non-member function named “swap” is selected via [overload
resolution](over.match "12.2 Overload resolution [over.match]") on a candidate set that includes:

- [(3.1)](#3.1)

the two swap function templates defined in[<utility>](utility.syn#header:%3cutility%3e "22.2.1 Header <utility> synopsis [utility.syn]") and

- [(3.2)](#3.2)

the lookup set produced by [argument-dependent lookup](basic.lookup.argdep "6.5.4 Argument-dependent name lookup [basic.lookup.argdep]")[.](#3.sentence-1)

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

If T and U are both fundamental types or arrays of
fundamental types and the declarations from the header [<utility>](utility.syn#header:%3cutility%3e "22.2.1 Header <utility> synopsis [utility.syn]") are in
scope, the overall lookup set described above is equivalent to that of the
qualified name lookup applied to the expression std​::​swap(t, u) orstd​::​swap(u, t) as appropriate[.](#3.sentence-2)

— *end note*]

[*Note [2](#note-2)*:

It is unspecified whether a library component that has a swappable
requirement includes the header [<utility>](utility.syn#header:%3cutility%3e "22.2.1 Header <utility> synopsis [utility.syn]") to ensure an appropriate
evaluation context[.](#3.sentence-3)

— *end note*]

[4](#4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/lib-intro.tex#L1954)

An rvalue or lvalue t is [*swappable*](#def:swappable "16.4.4.3 Swappable requirements [swappable.requirements]") if and only if t is
swappable with any rvalue or lvalue, respectively, of type T[.](#4.sentence-1)

[5](#5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/lib-intro.tex#L1958)

A type X meets the *Cpp17Swappable* requirements
if lvalues of type X are swappable[.](#5.sentence-1)

[6](#6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/lib-intro.tex#L1962)

A type X meeting any of the iterator requirements ([[iterator.requirements]](iterator.requirements "24.3 Iterator requirements"))
meets the *Cpp17ValueSwappable* requirements if,
for any dereferenceable objectx of type X,*x is swappable[.](#6.sentence-1)

[7](#7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/lib-intro.tex#L1969)

[*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 <utility>// Preconditions: std​::​forward<T>(t) is swappable with std​::​forward<U>(u).template<class T, class U>void value_swap(T&& t, U&& u) {using std::swap;
 swap(std::forward<T>(t), std::forward<U>(u)); // OK, uses “swappable with'' conditions// for rvalues and lvalues}// Preconditions: T meets the [*Cpp17Swappable*](#:Cpp17Swappable "16.4.4.3 Swappable requirements [swappable.requirements]") requirements.template<class T>void lv_swap(T& t1, T& t2) {using std::swap;
 swap(t1, t2); // OK, uses swappable conditions for lvalues of type T}namespace N {struct A { int m; }; struct Proxy { A* a; };
 Proxy proxy(A& a) { return Proxy{ &a }; }void swap(A& x, Proxy p) { std::swap(x.m, p.a->m); // OK, uses context equivalent to swappable// conditions for fundamental types}void swap(Proxy p, A& x) { swap(x, p); } // satisfy symmetry constraint}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*]