Question

I am creating a library (.lib) in c++ with Visual Studio 2008. I would like to set a variable to change the behaviour of the library depending on the variable. Simplifying a lot, something like this:

#ifdef OPTION1
i = 1;
#else
i = 0;
#endif

But the variable (in this case OPTION1) should not be defined in the library itself, but in the code that links to the library, so that just changing the definition of the variable I could obtain different behaviours from the program, but always linking to the same library.

Is this possible, and how? Or is there a more elegant way to achieve what I want?

Answer 1

To pull this off, the code which depends on the macro must be compiled as part of the code which links to the library, not as part of the library itself. The best you could do is something like this:

In your public .h file:

namespace LibraryPrivate {

void functionForOptionSet();
void functionForOptionUnset();

}

#ifdef OPTION1
inline void dependentBehaviour() {
  LibraryPrivate::functionForOptionSet();
}
#else
inline void dependentBehaviour() {
  LibraryPrivate::functionForOptionUnset();
}
#endif

In you library’s .cpp file:

namespace LibraryPrivate {

void functionForOptionSet()
{ i = 1; }

void functionForOptionUnset()
{ i = 0; }

}

That is, you have to implement both options in the library, but you can (partially) limit the interface based on the macro. Kind of something like what WinAPI does with char vs. wchar_t functions: if provides both SomeFunctionA(char*) and SomeFunctionW(wchar_t*) and then a macro SomeFunction which expands to one of those.