std::basic_const_iterator

From cppreference.com


`template< std::input_iterator Iter > class basic_const_iterator;`		(since C++23)

std::basic_const_iterator is an iterator adaptor which behaves exactly like the underlying iterator (which must be at least an LegacyInputIterator or model input_iterator), except that dereferencing converts the value returned by the underlying iterator as immutable. Specializations of std::basic_const_iterator are constant iterators, that is, the iterator can never be used as an output iterator because modifying elements is not allowed.

Member types

Member type	Definition
`iterator_category` (conditionally present)	If `Iter` models `forward_iterator`: member `iterator_category` is the same type as `std::iterator_traits<Iter>::iterator_category`. Otherwise, there is no member `iterator_category`.
`iterator_concept`	std::contiguous_iterator_tag, if `Iter` models `contiguous_iterator`; std::random_access_iterator_tag, if `Iter` models `random_access_iterator`; std::bidirectional_iterator_tag, if `Iter` models `bidirectional_iterator`; std::forward_iterator_tag, if `Iter` models `forward_iterator`; std::input_iterator_tag otherwise.
`value_type`	`std::iter_value_t<Iter>`
`difference_type`	`std::iter_difference_t<Iter>`
`reference` (private)	`std::iter_const_reference_t<Iter>` (exposition-only member type*)

Member objects

Member name	Definition
`current` (private)	the underlying iterator from which `base()` copies or moves (exposition-only member object*)

Member functions

(constructor)	constructs a new `basic_const_iterator` (public member function) [edit]
base	accesses the underlying iterator (public member function) [edit]
operator*operator->	accesses the pointed-to element (public member function) [edit]
operator[]	accesses an element by index (public member function) [edit]
operator++operator++(int)operator+=operator--operator--(int)operator-=	advances or decrements the iterator (public member function) [edit]
operator constant-iterator	converts into any constant iterator to which an underlying iterator can be convertible (public member function) [edit]
operator==operator<operator>operator<=operator>=operator<=>	compares the underlying iterators (public member function) [edit]

Non-member functions

Helper classes

Helper alias templates

`template< std::input_iterator I > using const_iterator = /* see description */;`		(since C++23)

If I models constant-iterator (an exposition-only concept), then const_iterator<I> denotes a type I. Otherwise, basic_const_iterator<I>.

`template< std::semiregular S > using const_sentinel = /* see description */;`		(since C++23)

If S models input_iterator, then const_sentinel<S> denotes a type const_iterator<S>. Otherwise, S.

Helper function templates

`template< std::input_iterator T > constexpr const_iterator<T> make_const_iterator( I it ) { return it; }`		(since C++23)
`template< std::semiregular S > constexpr const_sentinel<S> make_const_sentinel( S s ) { return s; }`		(since C++23)

Notes

Feature-test macro	Value	Std	Feature
`__cpp_lib_ranges_as_const`	`202207L`	(C++23)	`std::basic_const_iterator`
`__cpp_lib_ranges_as_const`	`202311L`	(C++23) (DR)	`std::basic_const_iterator` should follow its underlying type's convertibility

Example

#include <cassert>
#include <iterator>
#include <vector>

int main()
{
    std::vector v{1, 2, 3};
    std::vector<int>::iterator i = v.begin();
    *i = 4;   // OK, v[0] == 4 now
    i[1] = 4; // OK, the same as *(i + 1) = 4;

    auto ci = std::make_const_iterator(i);
    assert(*ci == 4);   // OK, can read the underlying object
    assert(ci[0] == 4); // OK, ditto
    // *ci = 13;        // Error: location is read-only
    // ci[0] = 13;      // Error: ditto
    ci.base()[0] = 42;  // OK, underlying iterator is writable
    assert(*ci == 42);  // OK, underlying location v[0] was modified
}

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
P2836R1	C++23	`basic_const_iterator` doesn't follow its underlying type's convertibility	conversion operator provided