Question

For example, never define a macro like this:

#define DANGER 60 + 2 

This can potentially be dangerous when we do an operation like this:

int wrong_value = DANGER * 2; // Expecting 124 

Instead, define like this because you don’t know how the user of the macro may use it:

#define HARMLESS (60 + 2) 

The example is trivial, but that pretty much explains my question. Are there any set of guidelines or best practices that you would recommend when writing a macro?

Thanks for your time!

Answer 1

Not only should you put parens around the arguments, you should put parens around the expression returned.

#define MIN(a,b)  a < b ? a : b     // WRONG    int i = MIN(1,2); // works int i = MIN(1,1+1); // breaks  #define MIN(a,b)  (a) < (b) ? (a) : (b)   // STILL WRONG  int i = MIN(1,2); // works int i = MIN(1,1+1); // now works int i = MIN(1,2) + 1; // breaks  #define MIN(a,b)  ((a) < (b) ? (a) : (b))   // GOOD  int i = MIN(1,2); // works int i = MIN(1,1+1); // now works int i = MIN(1,2) + 1; // works 

However, MIN(3,i++) is still broken…

The best rule is only to use #defines only when NO OTHER APPROACH WILL WORK! I know you’re asking about C instead of C++, but still bear his in mind.