How to ensure constexpr function never called at runtime?

Question

Lets say that you have a function which generates some security token for your application, such as some hash salt, or maybe a symetric or asymetric key.

Now lets sa

Answer 1

Since now we have C++17, there is an easier solution:

template 
struct constant {
    constexpr static decltype(V) value = V;
};

The key is that non-type arguments can be declared as auto. If you are using standards before C++17 you may have to use std::integral_constant. There is also a proposal about the constant helper class.

An example:

template 
struct constant {
    constexpr static decltype(V) value = V;
};

constexpr uint64_t factorial(int n) {
    if (n <= 0) {
        return 1;
    }
    return n * factorial(n - 1);
}

int main() {
    std::cout << "20! = " << constant::value << std::endl;
    return 0;
}