overriding virtual function return type differs and is not covariant

Question

Ah, SO came back just in time.

I am getting a strange error:

 \'B::blah\': overriding virtual function return type differs and is not covariant from          


        

Answer 1

objects declared on the runtime stack must be known to the compiler at compile time:

void f(const A & a) {
   Inner inr = a.blah( );

}

inr must be of both static and dynamic type "Inner" (not "Inner2") to be allocated off the stack -- so if blah returned an Inner2 it would be used to construct an Inner and it's "2ness" would be lost (or sliced as K Dragon mentions).