提升：计算小整数在编译时的阶乘 [英] Boost: computing the factorial of a small integer at compile time

问题描述

我刚刚实施（再次）在编译时计算一个整数的阶乘递归模板（谁曾想过有一天我会真的需要它！）。尽管如此，而不是我自己的滚动，我去提振寻找一个答案。然而，在特殊的数学阶乘函数明确禁止其使用​​整数类型，所以我只是写我自己的。

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.

不过，有升压另一个函数，我应该使用？我应该投我整一倍，并使用了boost ::阶乘函数？被计算在编译时进行的？

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

您无需升压，这仅仅是1班轮，如果你有C ++ 11：

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); 
}

和它的工作，即使你的arg是不是编译时间常数也。 uint64_t中将以N'LT工作; 21.

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

如果你正在做它在编译时间和浮点值乘 - 也就没有转换开销（转换将在编译的时候也是如此）。

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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