16 Library introduction [library]

16.4 Library-wide requirements [requirements]

16.4.4 Requirements on types and expressions [utility.requirements] Swappable requirements [swappable.requirements]

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:
[Note 1: 
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.
— end note]
[Note 2: 
It is unspecified whether a library component that has a swappable requirement includes the header <utility> to ensure an appropriate evaluation context.
— end note]
An rvalue or lvalue t is swappable if and only if t is swappable with any rvalue or lvalue, respectively, of type T.
A type X meets the Cpp17Swappable requirements if lvalues of type X are swappable.
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 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 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]