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c++windowsdlldynamic-library

When you instantiate a subclass from a DLL, is the vtable correct?

When you have a base class in your main executable and subclasses (say, plugins) defined in DLLs, what happens when you want to get a plugin?

I am looking for an article/answer that'd clarify what happens when you

  • load a DLL
  • call a DLL's function that returns a plugin* (it has virtual functions)
  • use that plugin in your main executable's code
  • delete, unload

I am thinking about the vtable and other C++ issues. For instance, if you unload the DLL stil having some plugins running... The "code" will be gone?

Solution

  • Since you are talking about plugins, you must be doing something like LoadLibrary. Assuming Windows:

    • Load a library. You would normally call LoadLibrary followed by GetProcAddress. The DLL is loaded into the process address space and you have the pointer to the function exposed.
    • Call a DLL's function that returns a plugin* (it has virtual functions). You would cast a the return value from GetProcAddress to the function pointer type, and call it. Everything should work as normal.
    • Use that plugin in your main executable's code. There is nothing special.
    • Delete the plugin. I assume you use a function in the DLL to do so, and it should be fine. Directly calling delete can be dangerous, as the DLL may have a separate memory manager (depending on what runtime you use).
    • Unload the DLL. You call FreeLibrary, and the code is gone. The pointers you previously get from GetProcAddress become invalid.

    I am not aware specific vtable issues. If you unload the DLL while the code is still running, I would assume the program will crash at this point, as the code address space becomes invalid.