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macro that defines functions

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Join Date: Jan 2008

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kux

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Junior Poster

macro that defines functions

Feb 19th, 2008

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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niek_e

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Cenosillicaphobiac

Re: macro that defines functions

Feb 19th, 2008

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:

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 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 niek_e; Feb 19th, 2008 at 6:04 am. Reason: Example code

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kux

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Re: macro that defines functions

Feb 19th, 2008

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

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Duoas

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Posting Virtuoso

Re: macro that defines functions

Feb 19th, 2008

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.

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Dave Sinkula Dave Sinkula is offline

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long time no c

Re: macro that defines functions

Feb 19th, 2008

•

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?

"One of the methods used by statists to destroy capitalism consists in establishing controls that tie a given industry hand and foot, making it unable to solve its problems, then declaring that freedom has failed and stronger controls are necessary." --Ayn Rand

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Join Date: Jan 2008

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kux

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Junior Poster

Re: macro that defines functions

Feb 19th, 2008

•

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

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Duoas

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Posting Virtuoso

Re: macro that defines functions

Feb 19th, 2008

Ah, I misunderstood the original post.

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Veteran Poster

Re: macro that defines functions

Feb 20th, 2008

> 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.

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 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' ;
}