Compiler returning an error when passing a const variable: template argument is not a constant expression

Question

So I wrote this code ->

#include <iostream>
#include <bitset>

int main(){

    int num, temp, digits = 0;
    std::cin >> num;
    temp = num;

    while(temp){
        temp /= 10;
        ++digits;
    }

    const int size = digits;

    std::bitset<size> a(num);
    std::cout << a << std::endl;

    return 0;
}

The bitset container isnt accepting the const integer size as a parameter and throwing an error -Non-type template argument is not a constant expression. I want to know why this is happening as size has been declared as a constant and it's value isnt gonna change during the run-time of my program ?

Answer 1

A const variable can be interpreted differently, depending on what is assigned to it.

1. When assigned a compile time constant: it will be a compile time constant. That means that during compilation the constant value can be directly used in-place.

2. When assigned from another variable (which is not a compile time constant) : the new variable is unmodifilable. In that sense the variable is not a compile time constant. It cannot be modified in that block of code.

A template requires a compile time constant.