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, Y, Z) (X) * (Y) * (Z)
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 adding parentheses around each argument, this definition of the CUBE() macro expands correctly in this situation.

#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