Computing the factorial of a small integer at compile time

Question

I just implemented (once again) a recursive template for computing the factorial of an integer at compile time (who would had thought that some day I'll actually need it!). Still, instead of rolling my own, I went to Boost looking for an answer. However, the factorial function in special math specifically forbids its use with integer types, so I just wrote my own.

Still, is there another function in Boost that I should use? Should I cast my integer to double and use the boost::factorial function? Is the computation performed at compile time?

Answer 1

You don't need Boost, this is just 1-liner if you have C++11:

constexpr uint64_t factorial(uint64_t n) { 
    return n == 0 ? 1  :  n * factorial(n-1); 
}

And it will work even if your arg is not compile time constant too. uint64_t will work with n < 21.

If you are doing it in compile time and multiply with floating point value - there will be no conversion overhead (conversion will be at compile time too).