25 Iterators library [iterators]

25.3 Iterator requirements [iterator.requirements]

25.3.3 Customization point objects [iterator.cust] ranges​::​iter_swap [iterator.cust.swap]

The name ranges​::​iter_swap denotes a customization point object ([customization.point.object]) that exchanges the values ([concept.swappable]) denoted by its arguments.
Let iter-exchange-move be the exposition-only function template:
template<class X, class Y> constexpr iter_value_t<X> iter-exchange-move(X&& x, Y&& y) noexcept(noexcept(iter_value_t<X>(iter_move(x))) && noexcept(*x = iter_move(y)));
Effects: Equivalent to: iter_value_t<X> old_value(iter_move(x)); *x = iter_move(y); return old_value;
The expression ranges​::​iter_swap(E1, E2) for subexpressions E1 and E2 is expression-equivalent to:
  • (void)iter_swap(E1, E2), if either E1 or E2 has class or enumeration type and iter_swap(E1, E2) is a well-formed expression with overload resolution performed in a context that includes the declaration template<class I1, class I2> void iter_swap(I1, I2) = delete; and does not include a declaration of ranges​::​iter_swap.
    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.
    [Note 1: 
    This precludes calling unconstrained std​::​iter_swap.
    When the deleted overload is viable, program-defined overloads need to be more specialized ([temp.func.order]) to be selected.
    — end note]
  • Otherwise, if the types of E1 and E2 each model indirectly_readable, and if the reference types of E1 and E2 model swappable_with ([concept.swappable]), then ranges​::​swap(*E1, *E2).
  • Otherwise, if the types T1 and T2 of E1 and E2 model indirectly_movable_storable<T1, T2> and indirectly_movable_storable<T2, T1>, then (void)(*E1 = iter-exchange-move(E2, E1)), except that E1 is evaluated only once.
  • Otherwise, ranges​::​iter_swap(E1, E2) is ill-formed.
    [Note 2: 
    This case can result in substitution failure when ranges​::​iter_swap(E1, E2) appears in the immediate context of a template instantiation.
    — end note]