Overriding protected field members in C++ not working?

October 25, 2022

In the following, I expected class Child‘s protected field member _AorB to be of type B, and not A, but reality shows otherwise.

What am I mis-understanding, and how can I adjust the code for the desired behavior?

class A{
    public:
       void doit(){
           std::cout<<" this is A!"<<std::endl;
       }
    
};

class B{
    public:
       void doit(){
           std::cout<<" this is B!"<<std::endl;
       }
    
};

class Parent{
    public:
      void doit(){
          _AorB.doit();
      }
      protected:
        A _AorB;
};

class Child: public virtual Parent{
    protected:
      B _AorB;
};

int main()
{
    cout<<"Hello World";
    auto c = Child();
    c.doit(); // I expected this to print "This is B" because c is Child(), and Child class's _AorB is of type B.
    return 0;
}

>Solution :

You can make such changes:

template <typename AorB>
class Parent{
    public:
      void doit(){
          _AorB.doit();
      }
      protected:
        AorB _AorB;
};

class Child: public virtual Parent<B> {
}

Also take a look at What are the rules about using an underscore in a C++ identifier?

Reserved in any scope, including for use as implementation macros:

identifiers beginning with an underscore followed immediately by an uppercase letter