std:: basic_const_iterator

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C++
Iterator library
Iterator concepts
Iterator primitives
Algorithm concepts and utilities
Indirect callable concepts
Common algorithm requirements
(C++20)
(C++20)
(C++20)
Utilities
(C++20)
Iterator adaptors
(C++20)
(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)

Range access
(C++11) (C++14)
(C++14) (C++14)
(C++11) (C++14)
(C++14) (C++14)
(C++17) (C++20)
(C++17)
(C++17)
std::basic_const_iterator
Member functions
Non-member functions
(C++23) (C++23) (C++23) (C++23)
(C++23)
(C++23)
(C++23)
Helper classes
(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 :

Otherwise, there is no member iterator_category .

iterator_concept
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

constructs a new iterator adaptor
(public member function)
accesses the underlying iterator
(public member function)
accesses the pointed-to element
(public member function)
accesses an element by index
(public member function)
advances or decrements the iterator
(public member function)
converts into any constant iterator to which an underlying iterator can be convertible
(public member function)
compares the underlying iterators
(public member function)

Non-member functions

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

Helper classes

(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
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