在编译时计算小整数的阶乘 [英] Computing the factorial of a small integer at compile time

问题描述

我刚刚(再次)实现了一个递归模板，用于在编译时计算整数的阶乘(谁曾以为有一天我实际上会需要它！).尽管如此，我还是没有自己动手，而是去 Boost 寻找答案.但是，特殊数学中的阶乘函数特别禁止将其与整数类型一起使用，因此我只是编写了自己的函数.

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.

还是，Boost中我应该使用另一个功能吗?我应该将整数转换为double并使用boost::factorial函数吗?计算是否在编译时执行?

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?

推荐答案

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).

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