This subclause provides definitions for swappable types and expressions.

In these
definitions, let t denote an expression of type T, and let u
denote an expression of type U.

An object t is *swappable with* an object u if and only if:

- the expressions swap(t, u) and swap(u, t) are valid when evaluated in the context described below, and
- these expressions have the following effects:
- the object referred to by t has the value originally held by u and
- the object referred to by u has the value originally held by t.

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 on a candidate set that includes:

- the two swap function templates defined in <utility> ([utility.syn]) and
- the lookup set produced by argument-dependent lookup.

[Note

: *end note*

]If T and U are both fundamental types or arrays of
fundamental types and the declarations from the header <utility> 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) or
std::swap(u, t) as appropriate.

— A type X meeting any of the iterator requirements ([iterator.requirements])
meets the Cpp17ValueSwappable requirements if,
for any dereferenceable object
x of type X,
*x is swappable.

[Example

: *end example*

]User code can ensure that the evaluation of swap calls
is performed in an appropriate context under the various conditions as follows:

— #include <utility> // Requires: std::forward<T>(t) shall be 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 } // Requires: lvalues of T shall be swappable. 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); }