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Feb 19th, 2008

macro that defines functions

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hello,

does anyone know if I can write a macro that takes a number as argument and according to that number declares a variable number of functions ?
ex:

DEFINE_FUNC( 4 );

is replaced with

void func_1();
void func_2();
void func_3();
void func_4();

thx

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kux

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Feb 19th, 2008

Re: macro that defines functions

As far as I know, this isn't posssible. You could use a class with a memberfunction and then delcare an array of classes. Example:

C++ Syntax (Toggle Plain Text)
#include <iostream>
using namespace std;
 
class foo
{
public:
    int func(int a)
    {
       return a;
     }
};
 
int main(void)
{
    const int SIZE = 5;
    foo bar[SIZE];
 
    for (int i = 0; i < SIZE; i++)
       cout << bar[i].func(i) << endl;
 
    cin.get();
   return 0;
}
#include <iostream>
using namespace std;

class foo
{
public:
    int func(int a)
    {
       return a;
     }
};

int main(void)
{
    const int SIZE = 5;
    foo bar[SIZE];
	
    for (int i = 0; i < SIZE; i++)
       cout << bar[i].func(i) << endl;

    cin.get();
   return 0;
}

Why would you want to do that anyway? Do the functions have the same implementation?
Perhaps you can clarify yourself with an example or code?

Niek

Last edited by Nick Evan; Feb 19th, 2008 at 6:04 am. Reason: Example code

Nick Evan

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Feb 19th, 2008

Re: macro that defines functions

ok, thx 4 the help
managed to work arround without the help of that macro

kux

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Feb 19th, 2008

Re: macro that defines functions

Actually, it is possible. You need to use token concatenation.
#define DEFINE_FUNC( n ) void func_ ## n()

You would use it in the normal way:
DEFINE_FUNC( 12 );

The preprocessor would turn that into:
void func_12();

While I can't imagine why you want to do this, there are, in fact, legitimate reasons to do token concatenation (which goes a long way into explaining why the preprocessor can do it at all).

Hope this helps.

Duoas

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Feb 19th, 2008

Re: macro that defines functions

Click to Expand / Collapse Quote originally posted by Duoas ...

Actually, it is possible. You need to use token concatenation.
#define DEFINE_FUNC( n ) void func_ ## n()

How do you propose to do the precompiler looping?

Dave Sinkula

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Feb 19th, 2008

Re: macro that defines functions

Click to Expand / Collapse Quote originally posted by Duoas ...

Actually, it is possible. You need to use token concatenation.
#define DEFINE_FUNC( n ) void func_ ## n()

You would use it in the normal way:
DEFINE_FUNC( 12 );

The preprocessor would turn that into:
void func_12();

While I can't imagine why you want to do this, there are, in fact, legitimate reasons to do token concatenation (which goes a long way into explaining why the preprocessor can do it at all).

Hope this helps.

true, what i meant to say is that if u have

DEFINE_FUNC (12)
the preprocessor should replace with 12 function declarations named func_1 func_2 ... func_12
so as Dave said, the looping is the problem

anyway as I said, I managed to carry out my stuff without using this, but if someone can figure out how to do the macro thingie I would be thankfull. I heard something that boost's preprocessor library can do that. didn't get to try it out : http://boost.org/libs/preprocessor/doc/ref/repeat.html.

cheers

kux

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Feb 19th, 2008

Re: macro that defines functions

Ah, I misunderstood the original post.

Duoas

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Feb 20th, 2008

Re: macro that defines functions

> I heard something that boost's preprocessor library can do that.
> didn't get to try it out : http://boost.org/libs/preprocessor/doc/ref/repeat.html.
BOOST_PP_REPEAT gives a fast horizontal repetition from 0 to N-1
since you need a repetition starting at 1, using BOOST_PP_REPEAT_FROM_TO would be easier.

c++ Syntax (Toggle Plain Text)
#include <boost/preprocessor/cat.hpp>
#include <boost/preprocessor/repetition/repeat_from_to.hpp>
#include <iostream>
 
#define DEFUN( z, N, arg ) int BOOST_PP_CAT( arg, N )() \
{ return N * N ; }
 
#define DEFINE_FUNC(N) \
BOOST_PP_REPEAT_FROM_TO( 1, N, DEFUN, square_of_ )
 
DEFINE_FUNC(15)
 
int main()
{
  std::cout << square_of_11() << '\n' ;
}
#include <boost/preprocessor/cat.hpp>
#include <boost/preprocessor/repetition/repeat_from_to.hpp>
#include <iostream>

#define DEFUN( z, N, arg ) int BOOST_PP_CAT( arg, N )() \
{ return N * N ; }

#define DEFINE_FUNC(N) \
BOOST_PP_REPEAT_FROM_TO( 1, N, DEFUN, square_of_ )

DEFINE_FUNC(15)
 
int main()
{
  std::cout << square_of_11() << '\n' ;
}

Last edited by vijayan121; Feb 20th, 2008 at 8:53 am.

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