23 Iterators library [iterators]

23.3 Iterator requirements [iterator.requirements]

23.3.4 Iterator concepts [iterator.concepts]

23.3.4.2 Concept indirectly_­readable [iterator.concept.readable]

1
#
Types that are indirectly readable by applying operator* model the indirectly_­readable concept, including pointers, smart pointers, and iterators.
template<class In>
  concept indirectly-readable-impl =
    requires(const In in) {
      typename iter_value_t<In>;
      typename iter_reference_t<In>;
      typename iter_rvalue_reference_t<In>;
      { *in } -> same_as<iter_reference_t<In>>;
      { ranges::iter_move(in) } -> same_as<iter_rvalue_reference_t<In>>;
    } &&
    common_reference_with<iter_reference_t<In>&&, iter_value_t<In>&> &&
    common_reference_with<iter_reference_t<In>&&, iter_rvalue_reference_t<In>&&> &&
    common_reference_with<iter_rvalue_reference_t<In>&&, const iter_value_t<In>&>;

template<class In>
  concept indirectly_readable =
    indirectly-readable-impl<remove_cvref_t<In>>;
2
#
Given a value i of type I, I models indirectly_­readable only if the expression *i is equality-preserving.
[Note
:
The expression *i is indirectly required to be valid via the exposition-only dereferenceable concept ([iterator.synopsis]).
— end note
]