Question

How to calculate some mathematical expression i.e. finding factorial of n; during compiletime in c++ ?

Answer 1

Here is a wiki article about template meta-programing in C++.

Here is another wiki article about compile-time function execution.

Here is a SO question regarding the factorial.

Let’s take the wiki xample of computing factorial in compile-time.

template <int N>
struct Factorial {
    enum { value = N * Factorial<N - 1>::value };
};

template <>
struct Factorial<0> {
    enum { value = 1 };
};

// Factorial<4>::value == 24
// Factorial<0>::value == 1
const int x = Factorial<4>::value; // == 24
const int y = Factorial<0>::value; // == 1

Since all of the arguments needed are known in the compile time (they are explicitly mentioned in Factorial<4>, for example), the compiler is able to generate all the needed code. After that, the value of the Factorial<4> struct will be 24 which can be later used as if you’ve hardcoded it yourself.