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Macro that can take variable qty of arguments

Posted on 2002-03-25
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Last Modified: 2012-05-04
How can I make a macro that can take variable qty of arguments.

#ifdef DEBUG
#define DEBUG_OUTPUT(Data) print_debug(Data)
#else
#define DEBUG_OUTPUT(Data)
#endif

void function_x(int xyz)
{
 DEBUG_OUTPUT("function_x(%i)", xyz);
}

void Widgit_func(int xyz, string somedat)
{
 DEBUG_OUTPUT("Widgit_func(%i, %s)", xyz, somedat.c_str());
}

I need DEBUG_OUTPUT marco that can take any number of arguments.


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Question by:LO19810527
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10 Comments
 
LVL 86

Expert Comment

by:jkr
ID: 6894945
If you're using gcc, you can simply

#define DEBUG_OUTPUT(...) print_debug(...)

This won't work with other compilers, though...
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LVL 6

Expert Comment

by:thienpnguyen
ID: 6894954
#ifdef DEBUG
#define DEBUG_OUTPUT MyPrintf
#else
#define DEBUG_OUTPUT EmptyPrint
#endif



#include <stdio.h>
#include <stdarg.h>

void MyPrintf( char *s, ... )
{
}

void MyPrintf( char *s, ... )
{
va_list argList;
char msg[3000];

va_start( argList, s );
vsprintf( msg, s, argList);
va_end( argList );

printf("%s\n", msg );

}

int main()
{

    MyPrintf("%s %d", "Hello", 123);
    return 1;
}
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LVL 86

Expert Comment

by:jkr
ID: 6894956
BTW, just in case you weren't already aware of that: No other compiuler supports the construct of a 'macro taking a variable number of arguments', so you will eventually either end up having to use a function to perform that or use multiple macros that take account of the number of arguments you're passing, e.g. like

#ifdef DEBUG
#define DEBUG_OUTPUT1(d1) print_debug(d1)
#define DEBUG_OUTPUT2(d1, d2) print_debug(d1, d2)
// etc.pp.
#else
//...
#endif
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LVL 86

Expert Comment

by:jkr
ID: 6894960
thienpnguyen, your suggestion still involves an 'unnecessary' function call - and, BTW, I'd personally make it read

void MyPrintf( char *s, ... )
{
#ifdef DEBUG
//...
#endif
}
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LVL 86

Expert Comment

by:jkr
ID: 6894963
>>your suggestion still involves an 'unnecessary' function call

Which isn't really too bad, actually :o)
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LVL 30

Accepted Solution

by:
Axter earned 300 total points
ID: 6894968
What compiler are you using?

You could try the following:

#if DEBUG
#ifdef __GNUC__
#define DEBUG_PRINT(args...) printf_debug(args)
#else
#define DEBUG_PRINT printf_debug
#endif //__GNUC__
#else  
#ifdef __GNUC__
#define DEBUG_PRINT(args...)
#else
#define DEBUG_PRINT /##/
#endif //__GNUC__
#endif

The above method should work for both VC and g++
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LVL 30

Expert Comment

by:Axter
ID: 6894982
FYI:
The VC method does have one bad side effect.
Example:

//This works OK
void function_x(int xyz)
{
  DEBUG_PRINT("function_x(%i)", xyz);
  printf("Hello World");
}

//This will NOT works OK
void function_x(int xyz)
{
  if (xyz == 123) DEBUG_PRINT("function_x(%i)", xyz);
  printf("Hello World");
}

In NON-Debug mode, the compiler will see the second code like this:
if (xyz == 123) printf("Hello World");

You don't have this problem with the GNUC method.
0
 

Author Comment

by:LO19810527
ID: 6896020
jkr,
I tried your gcc method, but it failed to compile.
It seems like the correct syntax is the one posted by Axter.

Axter,
I tried your macro, and much to my surprise, it works on all the compilers I tested.  I did modify it a little.

#ifdef DEBUG
#ifdef __GNUC__
#define DEBUG_OUTPUT(args...) print_debug(args)
#else
#define DEBUG_OUTPUT print_debug
#endif //__GNUC__
#else  
#ifdef __GNUC__
#define DEBUG_OUTPUT(args...)
#else
#define DEBUG_OUTPUT ;/##/
#endif //__GNUC__
#endif //DEBUG

I added ";" before /##/
This will remove the bad side effect you posted.  The only down side to this modification is that it produces a warning on some compilers.  But I rather get the warning then to have the side effects.
0
 
LVL 30

Expert Comment

by:Axter
ID: 6896026
Just out of curiousity, what compilers did you test it on?
0
 

Author Comment

by:LO19810527
ID: 6896048
I tested it in Visual C++ 6.0, Window/DOS GNU compiler, Borland (BC4), C++ Builder 5, Turbo C++ 3.0 and 4.5

With the modification I made, it gave a warning in Turbo C++ 3.0 and 4.5
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