25 Iterators library [iterators]

There are several concepts that group requirements of algorithms that take callable objects ([func.def]) as arguments.

To implement algorithms taking projections, it is necessary to determine the projected type of an iterator's value type.

For the exposition-only alias template indirect-value-t, indirect-value-t<T> denotes

• (1.1)
  invoke_result_t<Proj&, indirect-value-t<I>> if T names projected<I, Proj>, and
• (1.2)
  iter_value_t<T>& otherwise.

The indirect callable concepts are used to constrain those algorithms that accept callable objects ([func.def]) as arguments.

Class template projected is used to constrain algorithms that accept callable objects and projections ([defns.projection]).

It combines an indirectly_readable type I and a callable object type Proj into a new indirectly_readable type whose reference type is the result of applying Proj to the iter_reference_t of I.