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 parenthesize 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]]).

Risk Assessment

References

[[Summit 05]] Question 10.1
[[ISO/IEC 9899-1999]] Sections 6.10, "Preprocessing directives," and 5.1.1, "Translation environment"