Question

I dont understand why a template parameter can be initialized only with a const variable.
As in, why doesn’t the following code work:

#include <iostream>
template <class T,int dim>
class Vec
{
    T _vec[dim];
    int _dim;
    public:
    Vec () : _dim(dim) {};
    ~Vec () {};
    // other operators and stuff
};
int main () {
   int dim = 3;
   Vec < int, dim> vecInt3;
}

If I add a const to the definition of dim in the main, everything is fine. Why is that?

Answer 1

Integer types parameters must be compile-time constants. You have to either use an integer literal or make your variable const. The reason is the template is instantiated before runtime and if there’s a chance you can later change a variable name the program will behave inconsistent with the template.