What is the purpose of# undef preprocessor?
In the C Programming Language, the #undef directive tells the preprocessor to remove all definitions for the specified macro. A macro can be redefined after it has been removed by the #undef directive. Once a macro is undefined, an #ifdef directive on that macro will evaluate to false.
How does undef work?
The #undef directive removes the current definition of identifier. Consequently, subsequent occurrences of identifier are ignored by the preprocessor. To remove a macro definition using #undef, give only the macro identifier, not a parameter list.
What is the job of undef in C?
Undef is a directive in C programming language that helps to remove all the definitions for the given macro name or any constant defined using #define directive. It is a part of the preprocessor directive since it is called by the compiler automatically before actual compilation starts.
What does #ifndef do in C++?
#ifndef and #define are known as header guards. Their primary purpose is to prevent C++ header files from being included multiple times.
What does Undefine mean?
Undefine meaning To make indefinite; to obliterate or confuse the definition or limitations of. verb. 4. (programming) To remove the definition of; to return to an undefined state.
What is #line in C?
The #line directive tells the preprocessor to set the compiler’s reported values for the line number and filename to a given line number and filename.
What is the purpose of #define in C++?
The #define directive causes the compiler to substitute token-string for each occurrence of identifier in the source file. The identifier is replaced only when it forms a token. That is, identifier is not replaced if it appears in a comment, in a string, or as part of a longer identifier.
What is the difference between Ifdef and Ifndef?
ifdef means “if the following is defined” while ifndef means “if the following is not defined”. since that specifies the intent more clearly in this particular situation. precompilers accept that.
What is difference between null and undefined?
Null: It is the intentional absence of the value. It is one of the primitive values of JavaScript. Undefined: It means the value does not exist in the compiler.