CODEWITHSUNDEEP
cc-preprocessor
Trần Kim Dự
Trần Kim Dự

Reputation: 6102

Why use define keyword to define a function

I'm reading some source code. And not only this source code, but also many others, use this kind of programming:

#define DO_BINOP(FN_NAME,TFLAG,OPER) \
   void FN_NAME (bigint_stack &stack) { \
      bigint right = stack.front(); \
      stack.pop_front(); \
      TRACE (TFLAG, "right = " << right); \
      bigint left = stack.front(); \
      stack.pop_front(); \
      TRACE (TFLAG, "left = " << left); \
      bigint result = left OPER (right); \
      TRACE (TFLAG, "result = " << result); \
      stack.push_front (result); \
   }

DO_BINOP(do_add, '+', +   )

They use define to define a long-long string that acts as a function. I don't know what the benefit of doing this is, because this way makes the program harder to read, and harder to debug. I'm new to C, so I think this way must have some benefit, right?

Thanks.

Upvotes: 8

Views: 407

Answers (1)

Santa
Santa

Reputation: 11547

Because otherwise you'd have a source code that looks like this:

void do_add (bigint_stack &stack) {
    bigint right = stack.front();
    stack.pop_front();
    TRACE ('+', "right = " << right);
    bigint left = stack.front();
    stack.pop_front();
    TRACE ('+', "left = " << left);
    bigint result = left + (right);
    TRACE ('+', "result = " << result);
    stack.push_front (result);
}

void do_subtract (bigint_stack &stack) {
    bigint right = stack.front();
    stack.pop_front();
    TRACE ('-', "right = " << right);
    bigint left = stack.front();
    stack.pop_front();
    TRACE ('-', "left = " << left);
    bigint result = left - (right);
    TRACE ('-', "result = " << result);
    stack.push_front (result);
}

Etcetera...

Now, if you want to add another TRACE, for example, you'd have to again copy-paste it to all of them.

What the author wanted, really, is to define a way to generate functions from a set of parameterized inputs, in such a way that the resulting functions are all similar but they behave in slightly different ways depending on the input given to generate them. It's called meta-programming. Very common in coding parsers, which I suspect is where this snippet came from.

Now, in other languages, a construct specific to that language might exist to do meta-programming like this in a cleaner way (templates, metaclass, etc). But for C, macro is it.

Upvotes: 12

Related Questions