23 Iterators library [iterators]

23.3 Iterator requirements [iterator.requirements]

23.3.4 Iterator concepts [iterator.concepts]

23.3.4.3 Concept indirectly_­writable [iterator.concept.writable]

1
#
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
    };
2
#
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
  • (2.1)
    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.
3
#
After evaluating any above assignment expression, o is not required to be dereferenceable.
4
#
If E is an xvalue ([basic.lval]), the resulting state of the object it denotes is valid but unspecified ([lib.types.movedfrom]).
5
#
[ Note
:
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
 ]
6
#
[ Note
:
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
 ]