Semantics check: Can #undef be applied only to a macro that was previously #define’d?

Introduction / Context:Maintaining large codebases often requires ensuring a macro is not defined, regardless of prior state. The #undef directive provides that guarantee, and its exact semantics matter for portable build scripts and headers.

Given Data / Assumptions:

• We consider standard-preprocessor behavior for #undef.
• The statement claims #undef is valid only if a prior #define existed.

Concept / Approach:The directive #undef removes any existing definition of a macro name. If the macro is not currently defined, #undef simply ensures the name is undefined; this is not an error. This allows defensive coding patterns such as #undef NDEBUG before redefining it based on build settings.

Step-by-Step Solution:Start with macro possibly defined or not.Apply #undef NAME.Result: NAME is guaranteed to be not defined, with no diagnostic required.Optionally follow with a new #define NAME … to reset the value as needed.

Verification / Alternative check:Preprocess a file containing only #undef FOO and observe that no error is produced. Add #ifdef FOO checks to confirm it is undefined afterward.

Why Other Options Are Wrong:“Correct” contradicts the allowed behavior; restrictions to function-like macros or same-file use are not in the standard; “compiler dependent” is misleading—conforming preprocessors accept #undef on names whether previously defined or not.

Common Pitfalls:Confusing #undef with language-level deletion (there is none); forgetting that macro scope is translation-unit wide after preprocessing; attempting to #undef keywords or identifiers that are not macros (which has no effect).

Final Answer:Incorrect