#include<conio.h>
#include <stdio.h>
#define small 0
#define big 1
#define dummy( _x_ ) \
( small > big ) ? ( printf _x_ ) : ( void( 0 ) )
int main() {
dummy( ( "Four is %d", 4 ) );
getch();
return 0;
}
When I compiled above program in gcc, it is giving the error below:
error : expected ')' before numeric constant.
I am not understanding why I am getting it? To me it seems everything is correct. Please help.
The void(0) part is a syntax error. You try to call a function named void with a 0 argument. This will work:
( ( small > big ) ? printf _x_ : (void) 0 )
The problem seems to come from your void(0). I can't even compile it as a valid C expression/statement.
int main()
{
void(0);
return 0;
}
Gives:
error: expected identifier or ‘(’ before numeric constant
Just replace void(0) with 0. Ternary operator alternatives are supposed to have the same type anyway.
Jens gives you the solution, but seems that you need (void)printf in order to skip warning:
warning: ISO C forbids conditional expr with only one void side [-pedantic]
This works without warnings:
#define dummy( _x_ ) \
(small > big) ? (void)printf _x_ : (void)0
On the other hand dummy( ( "Four is %d", 4 ) ); looks ugly to me, I suggest to use __VA_ARGS__ and skip double parenthesis in your function call:
#include <stdio.h>
#define small 0
#define big 1
#define dummy(...) \
(small > big) ? (void)printf(__VA_ARGS__) : (void)0
int main(void)
{
dummy("Four is %d", 4);
return 0;
}
Or don't pass params:
#include <stdio.h>
#define small 0
#define big 1
#define dummy (!(small > big)) ? (void)0 : (void)printf
int main(void)
{
dummy("Four is %d", 4);
return 0;
}
After the macro replacements your code changed like this-
int main() {
( 0 > 1 ) ? ( printf ( "Four is %d", 4 ) ) : ( void( 0 ) );
return 0;
}
Here the compiler doesn't know what is void( 0 ). It is not a valid syntax in C. So try to replace it by some other statement.
Try the following code
#define dummy( _x_ ) \
( small > big ) ? ( printf _x_ ) : (printf("Conditon fails!\n"))
or
#define dummy( _x_ ) \
( small > big ) ? ( printf _x_ ) : ((void) 0)
