std::input_iterator(3)

NAME
std::input_iterator - std::input_iterator

Synopsis
   Defined in header <iterator>
   template< class I >
       concept input_iterator =
           std::input_or_output_iterator<I> &&
           std::indirectly_readable<I> && (since C++20)
           requires { typename /*ITER_CONCEPT*/<I>; } &&
           std::derived_from</*ITER_CONCEPT*/<I>,
   std::input_iterator_tag>;
   The input_iterator concept is a refinement of input_or_output_iterator, adding the
   requirement that the referenced values can be read (via indirectly_readable) and the
   requirement that the iterator concept tag be present.

Notes
   Unlike the LegacyInputIterator requirements, the input_iterator concept does not
   require equality_comparable, since input iterators are typically compared with
   sentinels.

Example
   A minimum input iterator.
 #include <cstddef>
 #include <iterator>
 struct SimpleInputIterator
 {
     using difference_type = std::ptrdiff_t;
     using value_type = int;
     int operator*() const;
     SimpleInputIterator& operator++();
     void operator++(int) { ++*this; }
 };
 static_assert(std::input_iterator<SimpleInputIterator>);

See also
   input_or_output_iterator specifies that objects of a type can be incremented and
   (C++20) dereferenced
                        (concept)
   forward_iterator specifies that an input_iterator is a forward iterator,
   (C++20) supporting equality comparison and multi-pass
                        (concept)