Page History

When defining macros, put parentheses around Parenthesize all variable names . This ensures that the macro is evaluated in a predictable mannerin 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 PRODUCTCUBE(A,B) AI) (I * I * BI)
int ac = 81 / PRODUCTCUBE0(32 +4, 51);

PRODUCT(3+4, 5) is converted to 3+4 * 5 by the preprocessor, which the compiler intreprets as 3+(4*5) = 23. Presumably, this does not match the intended behavior of the macro, which is (3+4) * 5 = 35.

Compliant Solution

As a result, the invocation

int c = 81 / CUBE0(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 namesthe in the{{CUBE()}} macro allows it to expand correctly (when invoked in this manner)By adding parentheses around each argument, this macro (correctly) evaluates to 35.

#define PRODUCTCUBE(A,BI) ( (I) * (AI) * (BI) )
int ac = PRODUCT(3+4, 5)

References

81 / CUBE(2 + 1); 

Priority: P3 Level: L3

References

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

...