PRE01-A. Use parentheses within macros around variable names

Parenthesize all variable names in macro definitions. See also [[PRE02]].

Non-Compliant Code Example

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

#define CUBE(I) (I * I * I)
int a = 81 / CUBE(2 + 1);

As a result, the invocation

int a = 81 / CUBE(2 + 1);

expands to

int a = 81 / (2 + 1 * 2 + 1 * 2 + 1);  /* evalutaes to 11 */

while the desired behavior is

int a = 81 / ( (2 + 1) * (2 + 1) * (2 + 1)); /* evaluates to 3 */

Compliant Solution

Parenthesizing all variable names the in the CUBE() macro allows it to expand correctly (when invoked in this manner).

#define CUBE(I) ( (I) * (I) * (I) )
int a = 81 / CUBE(2 + 1); 

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]]).

Priority: P3 Level: L3

References

• Summit 05 Question 10.1
• ISO/IEC 9899-1999 Section 6.10 Preprocessing directives