Page History

Wiki MarkupMacro replacement lists should be parenthesized to protect any lower - precedence operators from the surrounding expression. See also \[[PRE00-A. Prefer inline functions to macros]\] and \[[PRE01-A. Use parentheses within macros around parameter names]\].

Non-Compliant Code Example

...

#define EOF -1
/* ... */
if (c EOF) {
   /* ... */
}

...

In this example, the programmer has mistakenly omitted the comparison operator (see \[[MSC02-A. Avoid errors of omission]\]) from the conditional statement, which should be {{c \ != EOF}}. After macro expansion, the conditional expression is incorrectly evaluated as a binary operation: {{c-1}}. This is syntactically correct, even though it is certainly not what the programmer intended.

Parenthesizing the -1 in the declaration of EOF ensures that the macro expansion is evaluated correctly.

...

Once this modification is made, the non-compliant code example no longer compiles as because the macro expansion results in the conditional expression c (-1), which is no longer syntactically valid.

...

Note that there must be a space after EOF, because otherwise it becomes a function-like macro (and one that is incorrectly formed, since -1 cannot be a formal parameter).

...