Pure virtual functions and binary compatibility

Question

Now, I know it is generally bad to add new virtual functions to non-leaf classes as it breaks binary compatibility for any derived classes which haven't been recompiled. However, I have a slightly different situation:

I have an interface class and implementation class compiled into a shared library, for example:

class Interface {
    public:
        static Interface* giveMeImplPtr();
        ...
        virtual void Foo( uint16_t arg ) = 0;
        ...
}

class Impl {
    public:
        ...
        void Foo( uint16_t arg );
        ....
}

My main application uses this shared library, and could basically be written as:

Interface* foo = Implementation::giveMeImplPtr();
foo->Foo( 0xff );

In other words, the application doesn't have any classes which derive from Interface, it merely uses it.

Now, say I want to overload Foo( uint16_t arg ) with Foo( uint32_t arg ), am I safe to do:

 class Interface {
    public:
        static Interface* giveMeImplPtr();
        ...
        virtual void Foo( uint16_t arg ) = 0;
        virtual void Foo( uint32_t arg ) = 0;
        ...
}

and recompile my shared library without having to recompile the application?

If so, are there any unusual caveats I need to be aware of? If not, do I have any other options other than to take the hit and up-version the library, thus breaking backwards compatibility?

Answer 1

The simple answer is: no. Anytime you change the class definition at all, you potentially lose binary compatibility. Adding a non-virtual function or static members is usually safe in practice, although still formally undefined behavior, but that's about it. Anything else will probably break binary compatibility.