std::indirectly_writable

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Defined in header <iterator>
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 std::iter_reference_t <Out> && > ( *o) = std::forward <T> (t) ;
const_cast < const std::iter_reference_t <Out> && > ( * std::forward <Out> (o) ) =
std::forward <T> (t) ;
} ;

/* none of the four expressions above are required to be equality-preserving */
(since C++20)

The concept indirectly_writable<Out, T> specifies the requirements for writing a value whose type and value category are encoded by T into an iterator Out

Semantic requirements

Let e be an expression such that decltype((e)) is T, and o be a dereferenceable object of type Out, then indirectly_writable<Out, T>

o is not required to be dereferenceable after evaluating any of the assignment expressions above. If e is an xvalue, the resulting state of the object it denotes is valid but unspecified.

Equality preservation

Expressions declared in requires expressions of the standard library concepts are required to be equality-preserving

Notes

The only valid use of operator*

The required expressions with const_cast prevent indirectly_readable objects with prvalue reference types from satisfying the syntactic requirements of indirectly_writable by accident, while permitting proxy references to continue to work as long as their constness is shallow. See Ranges TS issue 381

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