Why can I have a const pair<const T, T> reference to a pair<T, T>?

Question

I found out by chance that I can have a const std::pair<const int, int>& reference to a std::pair<int,int>:

#include <utility>
int main()
{
    std::pair<int, int> p{1,2};
    const std::pair<const int, int>& ref_ok{p}; // why does this work?
    std::pair<const int, int>& ref_invalid{p}; // then why does this not work?
}

Why is this possible, considering that const std::pair<const int, int> and std::pair<int, int> are different types that are not related by inheritance?

Answer 1

const std::pair<const int, int>& ref_ok{p}; is actually materializing a temporary std::pair<const int, int> initialized to the same values as p and the reference initialization is extending the lifetime of the temporary to that of the lifetime of the reference.

std::pair<const int, int>& ref_invalid{p}; is not allowed because non-const references can't bind to a temporary.

The following code example shows that ref_ok is not actually a reference to p. Changes to p have no effect on ref_ok.

#include <iostream>
#include <utility>

int main()
{
    std::pair<int,int> p{1,2};
    const std::pair<const int, int>& ref_ok{p}; // why does this work?

    std::cout << p.first << ' ' << ref_ok.first << '\n';

    p.first = 5;

    std::cout << p.first << ' ' << ref_ok.first << '\n';
}

Output :

1 1
5 1

Live example : https://godbolt.org/z/8bfM7fYbx