Why doesn't compiler make a virtual function non virtual when it is possible?

Question

class A{
     void virtual a(){}
};
int main() { 
    std::cout<<sizeof(A);
}

In the above case why doesn't the compiler make the function non virtual and save the space allocated to it. Is there a specific reason for not doing that?

I am using gcc 4.7 compiler, if it is compiler specific.

Answer 1

Because you may create a derived class in a separate translation module.

In theory this could be resolved at link time, but this would involve a lot of work, so in practice that doesn't happen (AFAIK).

Answer 2

The compiler proper probably won't do this because it doesn't know what's in other files.

The linker might be able to do this, but there is no guarantee that a descendant version of A doesn't exist somewhere and will be loaded in a separate module.