[Objective] C Preprocessor macro possible

[nickbits]

I have a bunch of code that relies on a preprocessor constant.

I'd like to have a macro that conceptually works like this:

#define A_OR_B(a,b) \
#ifdef A \
(a) \
#else \
(b) \
#endif

And I'd use it like abc(A_OR_B(1, 2));

Right now I'm doing:
#ifdef A
abc(1)
#else
abc(2)
#endif

And I don't like that. Any suggestions? I'm using objective C. I thought I might be able to do something with a C++ template but objective C isn't a superset of C++, only C.

I could have a function that that does the check but I'd rather not have the function call overhead... but right now I think that is a better idea than all my #ifdefs...

 

[mosco]

What's your concern about function call overhead? Is this going to be called a lot?

Can you give some more detail about what functionality you trying to switch between?

 

[Ken g6]

Well, this works in C:

#ifdef A
#define A_OR_B(a,b) (a)
#else
#define A_OR_B(a,b) (b)
#endif

 

[uclabachelor]

I have a bunch of code that relies on a preprocessor constant.

I'd like to have a macro that conceptually works like this:

#define A_OR_B(a,b) \
#ifdef A \
(a) \
#else \
(b) \
#endif

And I'd use it like abc(A_OR_B(1, 2));

Right now I'm doing:
#ifdef A
abc(1)
#else
abc(2)
#endif

And I don't like that. Any suggestions? I'm using objective C. I thought I might be able to do something with a C++ template but objective C isn't a superset of C++, only C.

I could have a function that that does the check but I'd rather not have the function call overhead... but right now I think that is a better idea than all my #ifdefs...

I'd go with Keng6 wrote or, something like:

#ifdef A
#define A_OR_B a
#else
#define A_OR_B b
#endif

Your original code is too hard to read. Keep it clean!

 

[nickbits]

What's your concern about function call overhead? Is this going to be called a lot?

Can you give some more detail about what functionality you trying to switch between?

Not really concerned with the overhead as much as I think there should be a better way.

I have 2 targets in my project that have different features enabled and I have some coordinates that depend on the target.

 

[nickbits]

Well, this works in C:

#ifdef A
#define A_OR_B(a,b) (a)
#else
#define A_OR_B(a,b) (b)
#endif

I don't think that works. It would require the pre processor to make 2 passes which I don't think it does. I think that was the first thing I tried but maybe I'm wrong.

 

[Leros]

I don't think that works. It would require the pre processor to make 2 passes which I don't think it does. I think that was the first thing I tried but maybe I'm wrong.

I think it will work. That's basically how include guards work.

#ifndef HEADER_H
#define HEADER_H
 
struct foo {
    int member;
};
 
#endif /* HEADER_H */

See http://en.wikipedia.org/wiki/Include_guard#Use_of_.23include_guards

 

[Ken g6]

I don't think that works. It would require the pre processor to make 2 passes which I don't think it does. I think that was the first thing I tried but maybe I'm wrong.

It works. I tested it.

#include <stdio.h>
//#define A
#ifdef A
#define A_OR_B(a,b) (a)
#else
#define A_OR_B(a,b) (b)
#endif

int main(void) {
  printf("A OR B is %s\n", A_OR_B("A", "B"));

  return 0;
}

Uncomment #define A to change the output of A_OR_B.