Both the #ifndef and #ifdef directives permit conditional compilation in that the statements immediately following these directives, up to either the #else or #endif directives, are compiled only if the condition is true, whereas the statements following the #else are compiled only if the condition is false.

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Quizplus · Accepted Answer

The #ifndef directive checks if a macro is not defined and compiles the following statements if the condition is true, while #ifdef checks if a macro is defined, compiling the subsequent statements if true. The #else directive allows for alternative compilation when the initial condition is false.

Both the #Ifndef and #Ifdef Directives Permit Conditional Compilation in That