std::indirectly_writable

 

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Iterator library

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indirectly_readable (C++20) indirectly_writable (C++20) weakly_incrementable (C++20) incrementable (C++20) is-integer-like is-signed-integer-like (C++20)(C++20)	sentinel_for (C++20) sized_sentinel_for (C++20) input_iterator (C++20) output_iterator (C++20) input_or_output_iterator (C++20)	forward_iterator (C++20) bidirectional_iterator (C++20) random_access_iterator (C++20) contiguous_iterator (C++20)
Iterator primitives
input_iterator_tagoutput_iterator_tagforward_iterator_tagbidirectional_iterator_tagrandom_access_iterator_tagcontiguous_iterator_tag (C++20)	iter_value_titer_difference_titer_reference_titer_const_reference_titer_rvalue_reference_titer_common_reference_t (C++20)(C++20)(C++20)(C++23)(C++20)(C++20)	iterator (deprecated in C++17) iterator_traits incrementable_traits (C++20) indirectly_readable_traits (C++20)
Algorithm concepts and utilities
Indirect callable concepts
indirectly_unary_invocableindirectly_regular_unary_invocable (C++20)(C++20)	indirect_unary_predicate (C++20) indirect_binary_predicate (C++20)	indirect_equivalence_relation (C++20) indirect_strict_weak_order (C++20)
Common algorithm requirements
indirectly_movable (C++20) indirectly_movable_storable (C++20) indirectly_copyable (C++20)	indirectly_copyable_storable (C++20)   indirectly_swappable (C++20) indirectly_comparable (C++20)	permutable (C++20) mergeable (C++20) sortable (C++20)
Utilities
indirect_result_t (C++20)	projected (C++20)	projected_value_t (C++26)
Iterator adaptors
reverse_iterator make_reverse_iterator (C++14) move_iterator (C++11) make_move_iterator (C++11) default_sentinel_tdefault_sentinel (C++20)(C++20) unreachable_sentinel_tunreachable_sentinel (C++20)(C++20)	front_insert_iterator back_insert_iterator inserter insert_iterator front_inserter back_inserter move_sentinel (C++20)	common_iterator (C++20) counted_iterator (C++20) basic_const_iterator (C++23) const_iterator (C++23) const_sentinel (C++23) make_const_iterator (C++23) make_const_sentinel (C++23)
Stream iterators istream_iterator    ostream_iterator istreambuf_iterator ostreambuf_iterator	Iterator customization points ranges::iter_move (C++20) ranges::iter_swap (C++20)
Iterator operations advance distance prev (C++11)   next (C++11) ranges::advance (C++20) ranges::distance (C++20) ranges::prev (C++20) ranges::next (C++20)	Range access begincbegin (C++11)(C++14) rbegincrbegin (C++14)(C++14)   endcend (C++11)(C++14) rendcrend (C++14)(C++14)   sizessize (C++17)(C++20) empty (C++17) data (C++17)

 

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

• If std::indirectly_readable<Out> is modeled and std::iter_value_t<Out> is the same type as std::decay_t<T>, then *o after any above assignment is equal to the value of e before the assignment.

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}