How to define macro function which support no input parameter and support also input parametr in the same time

Question

I want to define a macro function which support at the same time:

1) No input parameter

2) Input parameters

some thing like that:

#define MACRO_TEST(X)\
    printf("this is a test\n");\
    printf("%d\n",x) // the last printf should not executed if there is no input parameter when calling the macro

In the main:

int main()
{
    MACRO_TEST(); // This should display only the first printf in the macro
    MACRO_TEST(5); // This should display both printf in the macro
}

You can't do that. In which context do you need such polymorphism on Macro? — greydet, Dec 20 '12 at 17:05
Can't you just use a variadic function? Or at the very least put an `if` in the macro? — netcoder, Dec 20 '12 at 17:13

geocar · Accepted Answer · 2016-10-13T20:02:38.167

5

You can use sizeof for this purpose.

Consider something like this:

#define MACRO_TEST(X) { \
  int args[] = {X}; \
  printf("this is a test\n");\
  if(sizeof(args) > 0) \
    printf("%d\n",*args); \
}

edited Oct 13 '16 at 20:02

answered Dec 20 '12 at 17:23

geocar

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Oh, I had not thought about a temporary array. That's smart! – cmc Dec 20 '12 at 17:25
And please, put it in `do { } while (0)`! – Shahbaz Dec 20 '12 at 17:27
Keep in mind that this is not portable. ISO C (e.g.: with `-pedantic` compiler flag) forbids both empty initializer lists and zero-size arrays (both will occur if no argument is passed to the macro), although it is supported by GNU extensions. – netcoder Dec 20 '12 at 18:21
I updated your answer in order to support another kind of input behaviour – MOHAMED Dec 21 '12 at 08:50

score 1 · Answer 2 · answered Dec 20 '12 at 17:26

gcc and recent versions of MS compilers support variadic macros - that is macros that work similar to printf.

gcc documentation here: http://gcc.gnu.org/onlinedocs/gcc/Variadic-Macros.html

Microsoft documentation here: http://msdn.microsoft.com/en-us/library/ms177415(v=vs.80).aspx

David Ranieri · Answer 3 · 2012-12-21T10:32:11.543

Not exactly that but ...

#include <stdio.h>

#define MTEST_
#define MTEST__(x) printf("%d\n",x)
#define MACRO_TEST(x)\
    printf("this is a test\n");\
    MTEST_##x    

int main(void)
{
    MACRO_TEST();
    MACRO_TEST(_(5));
    return 0;
}

EDIT

And if 0 can be used as skip:

#include <stdio.h>

#define MACRO_TEST(x) \
    do { \
        printf("this is a test\n"); \
        if (x +0) printf("%d\n", x +0); \
    } while(0)

int main(void)
{
    MACRO_TEST();
    MACRO_TEST(5);
    return 0;
}

score 0 · Answer 4 · answered Dec 20 '12 at 17:30

The C99 standard says,

An identifier currently defined as an object-like macro shall not be redefined by another #define reprocessing directive unless the second definition is an object-like macro definition and the two replacement lists are identical. Likewise, an identifier currently defined as a function-like macro shall not be redefined by another #define preprocessing directive unless the second definition is a function-like macro definition that has the same number and spelling of parameters, and the two replacement lists are identical.

I think compiler prompts a warning of redefined MACRO. Hence it is not possible.

How to define macro function which support no input parameter and support also input parametr in the same time

4 Answers4

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