std::basic_const_iterator

 

C++

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Feature test macros (C++20)

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Iterator library

Iterator concepts
indirectly_readable (C++20) indirectly_writable (C++20) weakly_incrementable (C++20) incrementable (C++20) is-integer-like is-signed-integer-like (C++20)(C++20)	sentinel_for (C++20) sized_sentinel_for (C++20) input_iterator (C++20) output_iterator (C++20) input_or_output_iterator (C++20)	forward_iterator (C++20) bidirectional_iterator (C++20) random_access_iterator (C++20) contiguous_iterator (C++20)
Iterator primitives
input_iterator_tagoutput_iterator_tagforward_iterator_tagbidirectional_iterator_tagrandom_access_iterator_tagcontiguous_iterator_tag (C++20)	iter_value_titer_difference_titer_reference_titer_const_reference_titer_rvalue_reference_titer_common_reference_t (C++20)(C++20)(C++20)(C++23)(C++20)(C++20)	iterator (deprecated in C++17) iterator_traits incrementable_traits (C++20) indirectly_readable_traits (C++20)
Algorithm concepts and utilities
Indirect callable concepts
indirectly_unary_invocableindirectly_regular_unary_invocable (C++20)(C++20)	indirect_unary_predicate (C++20) indirect_binary_predicate (C++20)	indirect_equivalence_relation (C++20) indirect_strict_weak_order (C++20)
Common algorithm requirements
indirectly_movable (C++20) indirectly_movable_storable (C++20) indirectly_copyable (C++20)	indirectly_copyable_storable (C++20)   indirectly_swappable (C++20) indirectly_comparable (C++20)	permutable (C++20) mergeable (C++20) sortable (C++20)
Utilities
indirect_result_t (C++20)	projected (C++20)	projected_value_t (C++26)
Iterator adaptors
reverse_iterator make_reverse_iterator (C++14) move_iterator (C++11) make_move_iterator (C++11) default_sentinel_tdefault_sentinel (C++20)(C++20) unreachable_sentinel_tunreachable_sentinel (C++20)(C++20)	front_insert_iterator back_insert_iterator inserter insert_iterator front_inserter back_inserter move_sentinel (C++20)	common_iterator (C++20) counted_iterator (C++20) basic_const_iterator (C++23) const_iterator (C++23) const_sentinel (C++23) make_const_iterator (C++23) make_const_sentinel (C++23)
Stream iterators istream_iterator    ostream_iterator istreambuf_iterator ostreambuf_iterator	Iterator customization points ranges::iter_move (C++20) ranges::iter_swap (C++20)
Iterator operations advance distance prev (C++11)   next (C++11) ranges::advance (C++20) ranges::distance (C++20) ranges::prev (C++20) ranges::next (C++20)	Range access begincbegin (C++11)(C++14) rbegincrbegin (C++14)(C++14)   endcend (C++11)(C++14) rendcrend (C++14)(C++14)   sizessize (C++17)(C++20) empty (C++17) data (C++17)

 

std::basic_const_iterator

Member functions

basic_const_iterator::basic_const_iterator

basic_const_iterator::base

basic_const_iterator::operator*basic_const_iterator::operator->

basic_const_iterator::operator[]

basic_const_iterator::operator++basic_const_iterator::operator+=basic_const_iterator::operator--basic_const_iterator::operator-=

basic_const_iterator::operator==basic_const_iterator::operator<basic_const_iterator::operator<=basic_const_iterator::operator>basic_const_iterator::operator>=basic_const_iterator::operator<=>

basic_const_iterator::operator constant-iterator

Non-member functions

operator<operator<=operator>operator>= (C++23)(C++23)(C++23)(C++23)

operator+ (C++23)

operator- (C++23)

iter_move(std::basic_const_iterator) (C++23)

Helper classes

common_type<std::basic_const_iterator> (C++23)

 

Defined in header <iterator>
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 iterator adaptor (public member function)
base	accesses the underlying iterator (public member function)
operator*operator->	accesses the pointed-to element (public member function)
operator[]	accesses an element by index (public member function)
operator++operator++(int)operator+=operator--operator--(int)operator-=	advances or decrements the iterator (public member function)
operator constant-iterator	converts into any constant iterator to which an underlying iterator can be convertible (public member function)
operator==operator<operator>operator<=operator>=operator<=>	compares the underlying iterators (public member function)

Non-member functions

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

Helper classes

std::common_type<std::basic_const_iterator> (C++23)

determines the common type of an iterator and an adapted basic_const_iterator type (class template specialization)

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
	202311L	(C++23) (DR)	std::basic_const_iterator should follow 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 underlying type's convertibility	conversion operator provided