Re: Compile time executed functions

[Jean-Marc Bourguet <jbourguet@free.fr>]

Ken Garlington wrote:

> Could you post an example of such a template, and what in the C++ standard
> you rely upon to guarantee compile-time calculations? 

First feature, things like
   enum { foo = x+y*z; }
are valid as long as x, y and z value are known at compile time and have
to be evaluated at compile time.

Second feature it is possible to provide "specialization" for template,
that is provide a different implementation when some template parameter
have some constant value.

The first ensure that evaluation is made at compile time.  The second
that it is possible to make some test.  Looping is available with
recursion so finally you have a computationnally complete language at
compile time.  It is painfull to write and even more painfull to use but
some like abusing the language...

An example, how to compute factorial

#include <iostream>

template <int F>
struct factorial {
   enum { RET = factorial<F-1>::RET*F };
};

template <>
struct factorial<1> {
   enum { RET = 1 };
};

int main() {
   std::cout << factorial<10>::RET << std::endl;
}

> Also, does
> "compile-time" include "link-time", or are you not allowed to have anything
> in the table that uses addresses?

As some C++ compilers do template instanciation at "link-time", I think
the answer is yes.

-- Jean-Marc