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 parenthesize 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);  /* evaluates 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 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++);
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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"