Why are ifndef and define used in C header files

Question

I have been seeing code like this usually in the start of header files    ifndef HEADERFILE H  define HEADERFILE H   And at the end of the file is   endif   What is the purpose of this

User · Answer

They are called ifdef or include guards   If writing a small program it might seems that it is not needed  but as the project grows you could intentionally or unintentionally include one file many times  which can result in compilation warning like variable already declared    ifndef checks whether HEADERFILE H is not declared   define will declare HEADERFILE H once  ifndef generates true   endif is to know the scope of  ifndef i e end of  ifndef   If it is not declared which means  ifndef generates true then only the part between  ifndef and  endif executed otherwise not  This will prevent from again declaring the identifiers  enums  structure  etc

User · Answer

Those are called  include guards   Once the header is included  it checks if a unique value  in this case HEADERFILE H  is defined  Then if it s not defined  it defines it and continues to the rest of the page   When the code is included again  the first ifndef fails  resulting in a blank file   That prevents double declaration of any identifiers such as types  enums and static variables

User · Answer

This prevent from the multiple inclusion of same header file multiple time    ifndef   COMMON H    define   COMMON H     header file content  endif   Suppose you have included this header file in multiple files  So first time    COMMON H   is not defined  it will get defined and header file included   Next time   COMMON H   is defined  so it will not include again

User · Answer

ifndef  lt token gt     code     else    code to include if the token is defined     endif    ifndef checks whether the given token has been  defined earlier in the file or in an included file  if not  it includes the code between it and the closing  else or  if no  else is present   endif statement   ifndef is often used to make header files idempotent by defining a token once the file has been included and checking that the token was not set at the top of that file    ifndef  INCL GUARD  define  INCL GUARD  endif

[c++] Why are #ifndef and #define used in C++ header files?

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