How Are Undefined Constants Treated in #if Conditions?

The Significance of Undefined Constants in #if Conditions

In the world of programming, the #if preprocessor directive plays a crucial role in conditional compilation. It allows the selective inclusion or exclusion of code based on specified conditions. A common question that arises in this context is: What happens when an undefined constant is used in an #if condition?

The C99 and C standards provide a clear answer to this question. According to §6.10.1 ¶3 of the C99 standard, "After all replacements due to macro expansion and the defined unary operator have been performed, all remaining identifiers are replaced with the pp-number 0." Similarly, the C standard states in §16.1 ¶4 that "After all replacements due to macro expansion and the defined unary operator have been performed, all remaining identifiers and keywords, except for true and false, are replaced with the pp-number 0."

This means that you can rely on the assumption that undefined constants are treated as zero in the evaluation of #if conditions. Consequently, the code snippet below:

... will result in the "Code that will be executed if MY_CONSTANT is undefined" being executed. This behavior is guaranteed by the C and C standards and provides a predictable and consistent way to handle undefined constants in #if conditions.

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