[iterator.concept.writable] (original) (raw)

24 Iterators library [iterators]

24.3 Iterator requirements [iterator.requirements]

24.3.4 Iterator concepts [iterator.concepts]

24.3.4.3 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); *std::forward<Out>(o) = std::forward<T>(t); const_cast<const iter_reference_t<Out>&&>(*o) = std::forward<T>(t); const_cast<const iter_reference_t<Out>&&>(*std::forward<Out>(o)) = std::forward<T>(t); };

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 modelindirectly_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.

[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​::​stringby value does not model indirectly_writable<I, std​::​string>.

— _end note_]