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std::indirectly_readable

Defined in header <iterator> 
template< class In >
  concept __IndirectlyReadableImpl = // exposition only
    requires(const In in) {
      typename std::iter_value_t<In>;
      typename std::iter_reference_t<In>;
      typename std::iter_rvalue_reference_t<In>;
      { *in } -> std::same_as<std::iter_reference_t<In>>;
      { ranges::iter_move(in) } -> std::same_as<std::iter_rvalue_reference_t<In>>;
    } &&
    std::common_reference_with<
      std::iter_reference_t<In>&&, std::iter_value_t<In>&
    > &&
    std::common_reference_with<
      std::iter_reference_t<In>&&, std::iter_rvalue_reference_t<In>&&
    > &&
    std::common_reference_with<
      std::iter_rvalue_reference_t<In>&&, const std::iter_value_t<In>&
    >;
 (since C++20) 
template< class In >
  concept indirectly_readable =
    __IndirectlyReadableImpl<std::remove_cvref_t<In>>;
 (since C++20)

The concept indirectly_readable is modeled by types that are readable by applying operator*, such as pointers, smart pointers, and input iterators.

Semantic requirements

Given a value i of type I, I models indirectly_readable only if all concepts it subsumes are modeled and the expression *i is equality-preserving.

Equality preservation

Expressions declared in requires-expressions of the standard library concepts are required to be equality-preserving (except where stated otherwise).

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https://en.cppreference.com/w/cpp/iterator/indirectly_readable