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cconditional-compilationifndef

Can #ifndef ignore method or variable duplications?

Consider the code.

#ifndef FOO_H
#define FOO_H
//Code
#endif

Code can be following cases

// Case 1: 
#define foo 0

// Case 2:
void foo_method(){};

// Case 3:
int foo;

foo.h is included in many C files. When I compile only the case 1 is without errors, other cases throw errors of duplication.

Why is so when the foo.h is not concatenated to C files except the one while compiling?

Solution

  • About case 2:
    you should only declare the function signature, not the body. it has nothing to do with the pre-processor commands.

    in the header file (decleration only)

    #if <Condition>
void foo();
#endif

    in the C file 

    #if <Condition>
void foo(){
   //body
}

#endif

    about case 3:
    it's similar to case 2, with the addition that you should declare variables in the header file if they are extern , there is no need to declare them in a header file otherwise. if they are declared as extern, they also need to be declared in a C file without the extern keyword:

    in the header file:

    #if <Condition>
extern int bar;
#endif

    in the C file:

    #if <Condition>
int bar;
#endif