CODEWITHSUNDEEP
cc-preprocessor
user1806687
user1806687

Reputation: 920

Array in #define

I am trying to achieve something like this:

#define MACRO(x) {PORTB=0,PORTC=0,PORTD=0}

MACRO(0); //This would get replaced by PORTB=0;
MACRO(1); //PORTC=0;
MACRO(2); //PORTD=0;

I want to create a "macro array". I would pass in an index to it and it would return the right code.

Is this possible?

EDIT:

If it helps, PORTB, PORTC and PORTD are all #defines.

Upvotes: 3

Views: 1085

Answers (2)

5gon12eder
5gon12eder

Reputation: 25459

It can be done using the preprocessor, but it is arguably ugly.

#define MACRO_CASE0 PORTB = 0
#define MACRO_CASE1 PORTC = 0
#define MACRO_CASE2 PORTD = 0

#define MACRO(X) MACRO_CASE##X

Also have a look at the Boost.Preprocessor library. (It works for both C and C++.)

Update: After the discussion with Jonathan Leffler (see below) I feel an obligation to update the answer with an exhortation for new C programmers not to abuse the (powerful but dirty) technique shown above.

If you – as the OP requested – want to pass in an index to it and it would return the right code, then you'll need to resort to preprocessor programming. However, if all you want to do is execute different code based on some condition and want this to have no run-time overhead if the condition is a compile-time constant, then the following approach is not only much cleaner but also more flexible as it allows passing run-time values too.

/* '#include' this definition in any file where you want to use it. */
static inline void
do_the_right_thing(const int selector)
{
  switch (selector)
    {
   case 0:
      PORTB = 0;
      break;
   case 1:
      PORTC = 0;
      break;
   case 2:
      PORTD = 0;
      break;
   default:
      assert(!"cannot do the right thing: invalid selector");
    }
}

Now, in your code, if you write

do_the_right_thing(1);  /* selector is a compile-time constant */

a decent compiler with appropriate optimizations enabled will generate no overhead compared to using a macro. However, you can also write

do_the_right_thing(rand() % 3);  /* selector is a run-time expression */

and the compiler will insert some fast switching code to select the appropriate operation at run-time.

Upvotes: 5

Jonathan Leffler
Jonathan Leffler

Reputation: 755104

This would do the task, but it doesn't extend or generalize very gracefully:

#define MACRO(x) (((x) == 0) ? PORTB=0 : ((x) == 1) ? PORTC=0 : PORTD=0)

Upvotes: 3

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