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std:: indirectly_writable

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C++
Iterator library
Iterator concepts
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Algorithm concepts and utilities
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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 's referenced object.

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 > is modeled only if:

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 (except where stated otherwise).

Notes

The only valid use of operator * is on the left side of an assignment expression. Assignment through the same value of an indirectly writable type may happen only once.

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 . 

struct Object
{
    Object& operator=(const Object& other) = default;
    int x;
};
struct ProxyReference
{
    ProxyReference& operator=(const ProxyReference& other) = default;
    const ProxyReference& operator=(const Object& o) const
    {
        *p = o;
        return *this;
    }
    Object* p;
};
struct I1 { Object& operator*(); };
struct I2 { Object operator*(); };
struct I3 { ProxyReference operator*(); };
static_assert(std::indirectly_writable<I1, Object>);
static_assert(!std::indirectly_writable<I2, Object>);
static_assert(std::indirectly_writable<I3, Object>);
static_assert(!std::indirectly_writable<I3, ProxyReference>);
void f(I1 i1, I2 i2, I3 i3, Object o)
{
    *i1 = o;  // OK, assigns to the value referenced by *i1
    *i2 = o;  // OK, but meaningless: This assigns to the temporary returned by *i2
    *i3 = o;  // OK, calls ProxyReference::operator=(const Object& o) const
              // which performs *ptr = o, where ptr is (*i3).p
}