25 Iterators library [iterators]

25.3 Iterator requirements [iterator.requirements]

25.3.4 Iterator concepts [iterator.concepts]

25.3.4.3 Concept indirectly_­writable [iterator.concept.writable]

The indirectly_­writable concept specifies the requirements for writing a value into an iterator's referenced object.
template<class Out, class T> concept indirectly_­writable = requires(Out&& o, T&& t) { *o = std::forward<T>(t); // not required to be equality-preserving *std::forward<Out>(o) = std::forward<T>(t); // not required to be equality-preserving const_cast<const iter_reference_t<Out>&&>(*o) = std::forward<T>(t); // not required to be equality-preserving const_cast<const iter_reference_t<Out>&&>(*std::forward<Out>(o)) = std::forward<T>(t); // not required to be equality-preserving };
Let E be an expression such that decltype((E)) is T, and let o be a dereferenceable object of type Out.
Out and T model indirectly_­writable<Out, T> only if
  • If Out and T model indirectly_­readable<Out> && same_­as<iter_­value_­t<Out>, decay_­t<T>>, then *o after any above assignment is equal to the value of E before the assignment.
After evaluating any above assignment expression, o is not required to be dereferenceable.
If E is an xvalue ([basic.lval]), the resulting state of the object it denotes is valid but unspecified ([lib.types.movedfrom]).
[Note 1:
The only valid use of an operator* is on the left side of the assignment statement.
Assignment through the same value of the indirectly writable type happens only once.
— end note]
[Note 2:
indirectly_­writable has the awkward const_­cast expressions to reject iterators with prvalue non-proxy reference types that permit rvalue assignment but do not also permit const rvalue assignment.
Consequently, an iterator type I that returns std​::​string by value does not model indirectly_­writable<I, std​::​string>.
— end note]