Why does std::is_const<const int&>::value evaluate to false?

Question

This is a spin off of the question How to check if object is const or not?.

I was surprised to see the following program

#include <iostream>
#include <type_traits>

int main() 
{
   std::cout << std::boolalpha;
   std::cout << std::is_const<const int&>::value << "\n";
}

produced this output

false

In what context does it make sense to think of const int& as a non-const type?

Answer 1

Perhaps it'll be easier to understand with this example

std::cout << std::is_const<int const *>::value << "\n";  // pointer to const int
std::cout << std::is_const<int * const>::value << "\n";  // const pointer to int

Output:

false
true

The first type is a pointer to a const int, while in the second the int * itself is const. Hence it results in true while the former is false. Similarly, what you have a reference to a const int. If int& const were valid it'd result in true.

Answer 2

A const qualifier on a reference just means that the value can't be modified via the reference. It can still be modified by other means. For example:

int a = 1;
const int &b = a;

std::cout << b << std::endl;  // Prints 1

a = 2;

std::cout << b << std::endl;  // Prints 2

Thus, you can't assume that the value of a const reference is actually constant.