How to prevent a derived class being an abstract class in c++?

Question

I have a question about Polymorphism. The following example would result in a error of "cannot declare variable ‘obj’ to be of abstract type ‘B’"

I know that the problems lies in the pure virtual method of function A::foo(), which make A an abstract class. Furthermore, class B inherent from class A. If i didn't implement the method foo() in the body of B, does that necessarily make B an abstract as well? and hence causing the error? But what's the logic of this? There might be a lot other derived classes from my base class A, the function foo might be applicable in some derived class, but may be totally useless for Class B. Of course I can declare an empty function B::foo() which absolutely does nothing and make the code run. But, is there any better solution or explanation of this behavior? Thanks!

struct A
{
    A() : x(0)  {}
    virtual int foo() const = 0;   // if I remove this line, code will run with no problem.
    int x;
};

struct B :  A {};

int main()
{
    B obj;
    cout << obj.x << endl;

    return 0;
}

Answer 1

At first, you are right, not implementing foo in A makes A abstract, not implementing foo in B makes that one abstract as well.

The whole sense of polymorphism is that you provide a common interface for a set of classes. Any derived type can behave as a base type in general, the specific behaviour would deviate only in the details (e. g. like steering in a car, bike or ship, the mechanism is different, but you can steer all of them).

If it doesn't make sense to call foo on B, then first ask 'Why?'! Somehow, B cannot be a true A then, just like a screw driver cannot be a hammer (but both can be tools).

If you find yourself in such trouble, then most likely your design is flawed.