NAME¶

std::basic_const_iterator - std::basic_const_iterator

Synopsis¶

Defined in header <iterator>
template< std::input_iterator Iter > (since C++23)
class basic_const_iterator;

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
If Iter models forward_iterator:

iterator_category * member iterator_category is the same type as
std::iterator_traits<Iter>::iterator_category.

Otherwise, there is no member iterator_category.
* std::contiguous_iterator_tag, if Iter models
contiguous_iterator;
* std::random_access_iterator_tag, if Iter models
iterator_concept 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 iterator adaptor
(public member function)
base accesses the underlying iterator
(public member function)
operator* accesses the pointed-to element
operator-> (public member function)
operator[] accesses an element by index
(public member function)
operator++
operator++(int)
operator+= advances or decrements the iterator
operator-- (public member function)
operator--(int)
operator-=
converts into any constant iterator to which an
operator constant-iterator underlying iterator can be convertible
(public member function)
operator==
operator<
operator> compares the underlying iterators
operator<= (public member function)
operator>=
operator<=>

Non-member functions¶

operator<
operator>
operator<= compares basic_const_iterator with non-basic_const_iterator
operator>= (function template)
operator<=>
(C++23)
operator+ advances or decrements the iterator
operator- (function template)
(C++23)
operator- computes the distance between two iterator adaptors
(C++23) (function template)
iter_move casts the result of dereferencing the underlying iterator to its
(C++23) associated rvalue reference type
(function)

Helper classes¶

determines the common type of an
std::common_type<std::basic_const_iterator> iterator and an adapted
(C++23) basic_const_iterator type
(class template specialization)

Helper alias templates

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

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 > (since C++23)
using const_sentinel = /* see description */;

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 > (since C++23)
constexpr const_sentinel<S> make_const_sentinel( S s ) { return s; }

Notes¶

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

Example¶

// Run this code

#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 conversion operator
underlying type's convertibility provided