您不需要Boost，如果您拥有C ++ 11，这只是1-liner:

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); 
}

即使您的arg也不是编译时间常数，它也将起作用. uint64_t将与n< 21.

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

如果您在编译时执行此操作并乘以浮点值-则不会有转换开销(转换也将在编译时进行).

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).