How can I calculate at compiletime in c++

Question

How to calculate some mathematical expression i.e. finding factorial of n; during compiletime in c++ ?

http://stackoverflow.com/questions/3082113/calculating-factorial-using-template-meta-programming — Alexander Tobias Bockstaller, Oct 19 '12 at 07:30
Besides template metaprogramming, you could probably use c++11 [`constexpr`](http://en.wikipedia.org/wiki/C%2B%2B11#constexpr_-_Generalized_constant_expressions) and recursion. Unfortunately I cannot check this right now. — juanchopanza, Oct 19 '12 at 07:33
This is too much of a homework-style question, plus the reader was rude enough to downvote those who gave links as answers, he doesn't want do any research, he wanted it down for him.. — CashCow, Oct 19 '12 at 07:40

score 2 · Accepted Answer · edited May 23 '17 at 12:04

2

Here is a wiki article about template meta-programing in C++.

Here is another wiki article about compile-time function execution.

Here is a SO question regarding the factorial.

Let's take the wiki xample of computing factorial in compile-time.

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

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

// Factorial<4>::value == 24
// Factorial<0>::value == 1
const int x = Factorial<4>::value; // == 24
const int y = Factorial<0>::value; // == 1

Since all of the arguments needed are known in the compile time (they are explicitly mentioned in Factorial<4>, for example), the compiler is able to generate all the needed code. After that, the value of the Factorial<4> struct will be 24 which can be later used as if you've hardcoded it yourself.

edited May 23 '17 at 12:04

Community

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answered Oct 19 '12 at 07:29

SingerOfTheFall

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Mind explaining the downvotes please? – SingerOfTheFall Oct 19 '12 at 07:31
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I guess link only answers are discouraged.. – matthias krull Oct 19 '12 at 07:33
Why? They explain the question. – Oct 19 '12 at 07:34
@mugen well the answer isn't really detailed too, and the OP doesn't show any research attempt. Not to mention this is partially a duplicate. – SingerOfTheFall Oct 19 '12 at 07:34
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[See here](http://meta.stackexchange.com/questions/8231/are-answers-that-just-contain-links-elsewhere-really-good-answers) (I didn't downvote though). – Jesse Good Oct 19 '12 at 07:35
I think it is better to flag questions accordingly than to answer with half a sentence. But that is just my opinion. – matthias krull Oct 19 '12 at 07:38

score -1 · Answer 2 · answered Oct 19 '12 at 07:30

-1

Welcome to the so-called template meta-programming.

This page describes what it is. It has a specific example to calculate the factorial of an integer in compile-time.

answered Oct 19 '12 at 07:30

valdo

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How can I calculate at compiletime in c++

2 Answers2