Why must unused virtual functions be defined?

Question

I find it quite odd that unused virtual functions must still be defined unlike unused ordinary functions. I understand somewhat about the implicit vtables and vpointers which are created when a class object is created - this somewhat answers the question (that the function must be defined so that the pointers to the virtual function can be defined) but this pushes my query back further still.

Why would a vtable entry need to be created for a function if there's absolutely no chance that virtual function will be called at all?

class A{
    virtual bool test() const;
};

int main(){
    A a; //error: undefined reference to 'vtable for A'
}

Even though I declared A::test() it was never used in the program but it still throws up an error. Can the compiler not run through the program and realise test() was never called - and thus not require a vtable entry for it? Or is that an unreasonable thing to expect of the compiler?

Answer 1

The OP says that he already knows about vtables and vpointers, so he understands that there is a difference between unused virtual functions and unused non-virtual functions: an unused non-virtual function is not referenced anywhere, while a virtual function is referenced at least once, in the vtable of its class. So, essentially the question is asking why is the compiler not smart enough to refrain from placing a reference to a virtual function in the vtable if that function is not used anywhere. That would allow the function to also go undefined.

The compiler generally sees only one .cpp file at a time, so it does not know whether you have some source file somewhere which invokes that function.

Some tools support this kind of analysis, they call it "global" analysis or something similar. You might even find it built-in in some compilers, and accessible via some compiler option. But it is never enabled by default, because it would tremendously slow down compilation.

As a matter of fact, the reason why you can leave a non-virtual function undefined is also related to lack of global analysis, but in a different way: if the compiler knew that you have omitted the definition of a function, it would probably at least warn you. But since it does not do global analysis, it can't. This is evidenced by the fact that if you do try to use an undefined function, the error will not be caught by the compiler: it will be caught by the linker.

So, just define an empty virtual function which contains an ASSERT(FALSE) and proceed with your life.

Answer 2

Because it would inevitably be a very difficult problem to solve on the compiler writer's part, when the usefulness of being able to leave virtual functions undefined is at best dubious. Compiler authors surely have better problems to solve.

Besides, you ARE using that function even though you don't call it. You are taking its address.

Answer 3

The whole point of virtual functions is that they can be called through a base class pointer. If you never use the base class virtual function, then, why did you define it ? If it is used, they you either have to leave the parent implementation (if it's not pure virtual), or define your own implementation, so that code using your objects through the base class can actually make use of it. In that case, the function is used, it's just not used directly.