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.
current (private) the underlying iterator from which base() copies or moves
If I models constant-iterator (an exposition-only concept), then const_iterator<I> denotes a type I. Otherwise, basic_const_iterator<I>.
If S models input_iterator, then const_sentinel<S> denotes a type const_iterator<S>. Otherwise, S.
#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
}[edit] 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++23basic_const_iterator doesn't follow its underlying type's convertibility conversion operator provided
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