Difference between revisions of "cpp/iterator/ranges/iter swap"

Latest revision as of 09:41, 1 August 2023

Swaps values denoted by two iterators.

The effect of the exposition-only helper function iter-exchange-move is equivalent to

std::iter_value_t<X> old(std::ranges::iter_move(x));
*x = std::ranges::iter_move(y);
return old;

ranges::iter_swap(i1, i2) is expression-equivalent to:

1. (void)iter_swap(i1, i2), if i1 or i2 has a class or enumeration type and the expression is well-formed, where the overload resolution of iter_swap is performed with the additional candidate void iter_swap(auto, auto) = delete;^[1], excluding std::ranges::iter_swap itself.
   • If the selected overload does not exchange the value denoted by i1 and i2, the program is ill-formed, no diagnostic required.
2. Otherwise, ranges::swap(*i1, *i2) if both I1 and I2 model indirectly_readable and if std::iter_reference_t<I1> and std::iter_reference_t<I2> model swappable_with.
3. Otherwise, (void)(*i1 = iter-exchange-move(i2, i1)), if std::indirectly_movable_storable<I1, I2> and std::indirectly_movable_storable<I2, I1> are both modeled, except that i1 is only evaluated once.
4. Otherwise, ranges::iter_swap(i1, i2) is ill-formed, which can result in substitution failure when ranges::iter_swap(i1, i2) appears in the immediate context of a template instantiation.

1. ↑ This precludes calling unconstrained std::iter_swap.

Customization point objects

The name ranges::iter_swap denotes a customization point object, which is a const function object of a literal semiregular class type. For exposition purposes, the cv-unqualified version of its type is denoted as __iter_swap_fn.

All instances of __iter_swap_fn are equal. The effects of invoking different instances of type __iter_swap_fn on the same arguments are equivalent, regardless of whether the expression denoting the instance is an lvalue or rvalue, and is const-qualified or not (however, a volatile-qualified instance is not required to be invocable). Thus, ranges::iter_swap can be copied freely and its copies can be used interchangeably.

Given a set of types Args..., if std::declval<Args>()... meet the requirements for arguments to ranges::iter_swap above, __iter_swap_fn models

• std::invocable<__iter_swap_fn, Args...>,
• std::invocable<const __iter_swap_fn, Args...>,
• std::invocable<__iter_swap_fn&, Args...>, and
• std::invocable<const __iter_swap_fn&, Args...>.
  

Otherwise, no function call operator of __iter_swap_fn participates in overload resolution.

[edit] Example

[edit] See also