std::basic_const_iterator<Iter>::operator constant-iterator

Since C++23

Returns the converted constant iterator to which an underlying iterator current can be explicitly or implicitly convertible.

# Declarations

template< /*not-a-const-iterator*/ CI >
requires /*constant-iterator*/<CI> &&
std::convertible_to<Iter const&, CI>
constexpr operator CI() const &;

(since C++23)

template< /*not-a-const-iterator*/ CI >
requires /*constant-iterator*/<CI> &&
std::convertible_to<Iter, CI>
constexpr operator CI() &&;

(since C++23)

# Example

#include <iterator>
#include <ranges>
#include <vector>
 
void foo(std::vector<int>::const_iterator) {}
 
int main()
{
    auto v = std::vector<int>();
    {
        // ranges::cbegin below returns vector<int>::const_iterator
        auto i1 = std::ranges::cbegin(v);
        foo(i1); // okay
    }
 
    auto t = v | std::views::take_while([](int const x) { return x < 100; });
    {
        // ranges::cbegin below returns basic_const_iterator<vector<int>::iterator>
        auto i2 = std::ranges::cbegin(t);
        foo(i2); // error until P2836R1
    }
}

# 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