Nontype template parameter

Question

I\'m having trouble with nontype(int variable) template parameter.
Why can\'t I pass a constant int variable to a function and let the function instantiate the template?

Answer 1

Basically, C++ has two kinds of constants:

const int a = 5;
MyTemplate foo; // OK

const int b = rand();
MyTemplate foo; // Not OK.

The first example is a compile-time constant. In C++ standard speak, it's an Integral Constant Expression (ICE). The second example is a run-time constant. It has the same C++ type (const int) but it's not an ICE.

Your function void run(const int j) is a run-time constant. You could even pass in user input. Therefore it's not a valid template argument.

The reason for the rule is that the compiler must generate code based on the template argument value. It can't do so if it doesn't have a compile-time constant.