How do you implement the factorial function in C++?

Question

Possible Duplicates:
Calculating large factorials in C++
Howto compute the factorial of x

How do you implement the factorial function in C++? And by this I mean properly implement it using whatever argument checking and error handling logic is appropriate for a general purpose math library in C++.

Answer 1

Recursive:

unsigned int factorial(unsigned int n) 
{
    if (n == 0)
       return 1;
    return n * factorial(n - 1);
}

Iterative:

unsigned int iter_factorial(unsigned int n)
{
    unsigned int ret = 1;
    for(unsigned int i = 1; i <= n; ++i)
        ret *= i;
    return ret;
}

Compile time:

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

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

void foo()
{
    int x = Factorial<4>::value; // == 24
    int y = Factorial<0>::value; // == 1
}