Variable arguements in macro using ellipsis - c

Question

I want to define a macro accepting either 1 or 2 parameters. Both the parameters should be different type. How to use ellipsis and read the arguments passed?
Below is the sample:
void test(char *var2)
{
printf("%s\n",var2);
}
#define PRINT_STRING(...) ( if (!var1) test(var2) )
int main(int argc, _TCHAR argv[]) {
PRINT_STRING(TRUE);
PRINT_STRING(FALSE,"Hello, World!");
return 0;
}

Answer 1

This is known as a Variadic macro.

Answer 2

If your compiler supports __VA_ARGS__, you can do it like this:
#include <stdio.h>
#define NUM_ARGS__(X, \
N64,N63,N62,N61,N60, \
N59,N58,N57,N56,N55,N54,N53,N52,N51,N50, \
N49,N48,N47,N46,N45,N44,N43,N42,N41,N40, \
N39,N38,N37,N36,N35,N34,N33,N32,N31,N30, \
N29,N28,N27,N26,N25,N24,N23,N22,N21,N20, \
N19,N18,N17,N16,N15,N14,N13,N12,N11,N10, \
N09,N08,N07,N06,N05,N04,N03,N02,N01, N, ...) N
#define NUM_ARGS(...) \
NUM_ARGS__(0, __VA_ARGS__, \
64,63,62,61,60, \
59,58,57,56,55,54,53,52,51,50, \
49,48,47,46,45,44,43,42,41,40, \
39,38,37,36,35,34,33,32,31,30, \
29,28,27,26,25,24,23,22,21,20, \
19,18,17,16,15,14,13,12,11,10, \
9, 8, 7, 6, 5, 4, 3, 2, 1, 0)
#define PRINT_STRING_1(var) \
{ if (!(var)) {} }
#define PRINT_STRING_2(var, ...) \
{ if (!(var)) test(__VA_ARGS__); }
#define PRINT_STRINGN__(N, ...) \
PRINT_STRING_##N(__VA_ARGS__)
#define PRINT_STRINGN(N, ...) \
PRINT_STRINGN__(N, __VA_ARGS__)
#define PRINT_STRING(...) \
PRINT_STRINGN(NUM_ARGS(__VA_ARGS__), __VA_ARGS__)
void test(char* var2)
{
printf("%s\n", var2);
}
int main(void)
{
PRINT_STRING(1);
PRINT_STRING(0, "Hello, World!");
PRINT_STRING(1, "You can't see me!");
return 0;
}
Output:
Hello, World!

Answer 3

To do something like that you'd have to implement a series of macros, something like this
#include <stdbool.h>
#define PRINT_STRING0(X, Y) do { if (X && Y) test(Y); } while(false)
#define PRINT_STRING1(X, Y, ...) PRINT_STRING0(X, Y)
#define PRINT_STRING(...) PRINT_STRING1(__VA_ARGS__, 0, 0)
The last of them (user interface) adds a second or third argument of 0. PRINT_STRING1 then ignores all arguments that are more than 2. And PRINT_STRING0 then does the work.
Some more remarks:
be careful that a macro like you want to program it here can be place syntactically just as any other statement. In the example here the do { } while(false) does that trick
since C99 has Boolean type and constants, these are _Bool or bool and false and true

Answer 4

Don't use a macro to do that, use a variadic function!
void print_string( bool should_print, ... )
{
if( should_print )
{
va_list argp;
va_start( argp, should_print);
char *string = va_arg(argp, char *);
if( string) printf("%s", string );
va_end( argp );
}
}
But be very careful when using these sorts of things, because va_arg doesn't check if you really have a 2nd argument.
Also, to use a macro that accepts one or 2 arguments, use the GCC trick (at the very bottom of this page):
#define PRINT_STRING( should , args... ) print_string( should , ##args )
(note the spaces between the commas)