PRE02-A. Macro expansion must always be parenthesized

The macro expansion must always be parenthesized to protect any lower-precedence operators from the surrounding expression. See also [[PRE01]].

Non-Compliant Code Example

This CUBE() macro definition is non-compliant because it fails to parethesize the macro expansion.

#define CUBE(X) (X) * (X) * (X)
int i = 3;
int a = 81 / CUBE(i);

As a result, the invocation

int a = 81 / CUBE(i);

expands to

int a = 81 / i * i * i;

which evaluates as

int a = ((81 / i) * i) * i);  /* evaluates to 243 */

while the desired behavior is

int a = 81 / ( i * i * i); /* evaluates to 3 */

Compliant Solution

By parenthesizing the macro expansion, the CUBE() macro expands correctly (when invoked in this manner).

#define CUBE(X) ((X) * (X) * (X))
int i = 3;
int a = 81 / CUBE(i);

However, if a parameter appears several times in the expansion, the macro may not work properly if the actual argument is an expression with side effects. Given the CUBE() macro above, the invocation:

int a = 81 / CUBE(i++);

expands to:

int a = 81 / (i++ * i++ * i++); 

which is undefined (see [[EXP30]]).

References

• Summit 05 Question 10.1