Question

I’m really confused, so I have to ask this. I try to write an application, but I don’t know how to reach the variables of the derived class, which are in a vector in the Base class.
The code is:

class A {
public:
    A() { };

    std::vector<A> aVector;

    void Foo();
}

class B : public A {
public:
    B() { };

    int j;
}

void A::Foo() {
    aVector.push_back( B() );

    // Here I would like to reach B::j, but only the members and variables of A comes in
    aVector[0].j; // wrong
    B b = aVector[0];   // no suitable user-defined conversion from "A" to "B" exists
                        // should I use cast? which one?
}

I’m currently learning inheritance and this kind of things through application programming, and now I’m really stuck.

I looked for other questions, but could not find any that solves my problem. If there is, and I missed, then sorry.

Answer 1

You need to store pointers to A so that your new B object won’t get “sliced” (see explanation here) when pushed into the vector.

Also, when you want to use specifically a child method / variable on a pointer from the base class, you need to cast it into the proper type

std::vector<A*> aVector;
aVector.push_back(new B());
B* b = (B*)aVector[0];
cout << b->j;
// remember to delete the content of aVector

Casting an object like this can be dangerous if you are not 100% sure that it is of the type you’re casting it in.

See this thread for more information on casting (C style, dynamic_cast and static_cast)