how to redefine preprocessor directives of c++ libraries in an c++ application program -

i give following example illustrate question. suppose creating c++ library, , head file , implementation file follows:

// head.h class __declspec(dllexport) myclass {      public:           int obtain_value();  };  // head.cpp  int myclass::obtain_value() {   #ifndef define_five      return 6;   #else      return 5;   #endif  } 

in library, if define define_five when compiling library, obtain 5 when invoking obtain_value function; otherwise, obtain 6. let's assume when compiled library, did not define define_five. in c++ program call library, when voking obtain_value function, expect have 6. question is, can in c++ program invokes library output of calling function 5. thanks.

the preprocessor directives evaluated @ compile time, cannot change code in runtime. if compile library , define_five not defined, preprocessor in first pass replace code following:

// head.cpp int myclass::obtain_value() {      return 6;     } 

as can see, parse , remove preprocessor directives. then, real compiling stage starts machine code generation.

if want dynamic values in library, have change interface, example:

// head.h class __declspec(dllexport) myclass {      public:           int obtain_value(bool give_me_five=false);  };  // head.cpp  int myclass::obtain_value(bool give_me_five) {      if (!give_me_five)         return 6;      else return 5; }