16 Library introduction [library]

16.5 Library-wide requirements [requirements]

16.5.3 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:
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
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 meeting any of the iterator requirements ([iterator.requirements]) meets the Cpp17ValueSwappable requirements if, for any dereferenceable object x of type X, *x is swappable.
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);
— end example