Thread: Defining a function as macro - C

Hey all.

I have some definition of MACRO in C which is not compiled.

Code:

#define SWAP(a, b) int tmp = a; a = b; b = tmp

I have seen the warnings/errors of the compiler saying the arguments are not declared, expected expression before int, etc...

But I don't understand what is the intrinsic difference between this one and other similar MACROS like:

Code:

#define MAX(a, b) ((a)>(b)? (a) : (b))

Does someone can explain it?

Thanks in advance.

Well, both are function-style macros. The former has the problem in that it is trying to do a generic swap, but because it has an int as the type of the temporary, the swap isn't all that generic: but it's possible to include the type of the temporary as a macro argument. It's also multi-statement, in which a better style would be to wrap the statements in a do while (0) loop so that the macro as a whole would expand to just one (compound) statement.

The difference is that the second one consists of an expression whereas your version consists of multiple statements. To properly deal with a macro with multiple statements you should use the following trick:

Code:

#define SWAP(a, b) do{ int tmp = a; a = b; b = tmp; }while(0)

The do loop will only execute the code once, but it's presence allows the multiple statements to appear like a single statement.