Introduction / Context:
The C/C++ preprocessor runs before compilation and handles directives like #define, #include, and conditional compilation. #ifndef is commonly used in header guards to ensure a file is processed only once even if included multiple times. Understanding what #ifndef actually tests prevents subtle multiple-definition errors.
Given Data / Assumptions:
- #ifndef stands for “if not defined”.
- Defined/undefined status refers to preprocessor macros created with #define or via compiler options.
- Preprocessor activity occurs before type checking or object instantiation.
Concept / Approach:
- #ifndef SYMBOL checks whether the macro SYMBOL is not currently defined.
- Typical pattern: #ifndef HEADER_NAME
#define HEADER_NAME
... header content ...
#endif - It does not test runtime values, class existence, object creation, or namespace declarations.
Step-by-Step Solution:
Identify the entity #ifndef operates on → macros/preprocessor symbols.Relate to header guards that prevent multiple inclusion.Therefore, #ifndef tests whether a macro has been defined; if not, the guarded code is included.
Verification / Alternative check:
Compile a project with and without a macro defined via -DMYFLAG; #ifndef MYFLAG will include code only when MYFLAG is not set.
Why Other Options Are Wrong:
- Header included: Inclusion is managed via macros in guards, but #ifndef itself checks the macro, not history of inclusion.
- Runtime variable value: Preprocessor cannot see runtime state.
- Class instantiation or namespace declaration: These are compile-time language constructs after preprocessing.
Common Pitfalls:
- Misspelling guard macros, leading to ineffective guards.
- Confusing #ifdef with #ifndef; the former checks if defined, the latter if not defined.
Final Answer:
a macro (preprocessor symbol) has been defined
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