Thread: How the hell do you write macros?

How do you write macros, and how do they work?

Like a normal function, sort of:

Code:

#define PowerOfTwo(n) (n * n)

They have to be written in one single line, or use \ at the endof a line to break it into multiple lines.

Would it be possible to write most of a program using only pre-processing directives and marcos?

Macros directly modify your code, and hence, can be dangerous if not used properly. For example, let's take a look at a use of the macro provided above:

Code:

#define SQUARE(x) (x * x)

int main( )
{
   int y = 3;
   int z = SQUARE(y + 1);
}

Now, clearly, the intended meaning is to assign z = (y + 1) * (y + 1) = 4 * 4 = 16. However, there is a direct substitution done, so what happens is: z = y + 1 * y + 1 = 2 * y + 1 = 7. This could be fixed as follows:

Code:

#define SQUARE(x) ((x) * (x))

... but it demonstrates the danger. As to writing a whole program using them... well, you have to have actual code in the macros themselves, and all you are going to do is make a mess, so unless you are writing code for say, the International Obfuscated C Code Contest, there is absolutely no point.

Originally Posted by Finchie_88

Would it be possible to write most of a program using only pre-processing directives and marcos?

Yes, but why would you want to?

A note... If you do write macros, enclose all uses of your parameters in parentheses:

Code:

#define GREATER(x,y)((x)>(y)?(x):(y))

As I pointed out above, operator precedence can be a real pain dealing with macros, so make sure that you don't get any side effects.