9 Iterators library [iterators]

9.3 Iterator requirements [iterator.requirements]

9.3.5 Concept Writable [iterators.writable]

1

The Writable concept specifies the requirements for writing a value into an iterator's referenced object.

  template <class Out, class T>
  concept bool 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 reference_t<Out>&&>(*o) =
        std::forward<T>(t); // not required to be equality preserving
      const_cast<const reference_t<Out>&&>(*std::forward<Out>(o)) =
        std::forward<T>(t); // not required to be equality preserving
    };
2

Let E be an an expression such that decltype((E)) is T, and let o be a dereferenceable object of type Out. Writable<Out, T> is satisfied only if

  • (2.1)

    If Readable<Out> && Same<value_type_t<Out>, decay_t<T>> is satisfied, 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 ( ISO/IEC 14882:2014 §[basic.lval]), the resulting state of the object it denotes is valid but unspecified ( ISO/IEC 14882:2014 §[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 writable type happens only once.  — end note ]