Re: [Boost-users] Automate call with n template parameters

From: Jeremy Pack (rostovpack_at_[hidden])
Date: 2008-07-22 01:05:32

Hi Kevin!

On Mon, Jul 21, 2008 at 9:03 PM, Kevin Jenkins <gameprogrammer_at_[hidden]>
wrote:
> Is there any way to automate this, to support from 0 to n parameters?
>
> template <class P1, class P2, class P3, class P4, class P5>
> bool DoIt(P1 p1, P2 p2, P3 p3, P4 p4, P5 p5)    {

Boost.Preprocessor would do that.

Create two files, which I'll call:

main.hpp
implementation.hpp

In main.hpp:

#include <boost/preprocessor/repetition.hpp>
#include <boost/preprocessor/iteration/iterate.hpp>

#define NUM_FUNCTION_PARAMS 8  // Or however many you need.

// Define the function for 1 to 8 arguments.
#define BOOST_PP_ITERATION_LIMITS(1, NUM_FUNCTION_PARAMS);
#define BOOST_PP_FILENAME_1 "implementation.hpp"
#include BOOST_PP_ITERATE()

And then in implementation.hpp:

// Note: If you want to support 0 arguments, you'll need to use BOOST_PP_IF
// to only optionally include the template declaration. I'd recommend just
putting the
// argless version into main.hpp, unless the function is quite complex.
template <BOOST_PP_ENUM_PARAMS(N, class Param)>
inline void MyFunc(BOST_PP_ENUM_BINARY_PARAMS(N, Param, p)) {
  #define DO_THAT(n) DoThat<BOOST_PP_CAT(P, n)>(BOOST_PP_CAT(p, n));
  BOOST_PP_REPEAT(N, DO_THAT,)
  #undef DO_THAT
}

Hope that helps - and can't promise it will immediately compile!

I do this in the proposed Extension and Reflection libraries a few times.
The best example is probably in the adaptable_factory class, where I wanted
to support calling a factory function without knowing its arguments:

http://svn.boost.org/trac/boost/browser/sandbox/boost/extension/impl/adaptable_factory_free_functions.hpp
http://svn.boost.org/trac/boost/browser/sandbox/boost/extension/adaptable_factory.hpp

Let me know if you have problems getting your function to compile. Also,
with Boost.Preprocessor, "gcc -E" is your best friend.

Good luck!

Jeremy Pack
http://boost-extension.blogspot.com