Section
std::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.
# Declarations
template< std::input_iterator Iter >
class basic_const_iterator;
(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
#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
| 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 |