How to make polymorphic field of class in C++? How to use polymorphism correctly in this case?

Question

I have a class

class A 
{
    BasePtr * ptr;
    virtual BasePtr * getPtr() { return ptr; }
    virtual void pure() = 0;
}

And I want to derive two classes from it that create its own versions of ptr:

class B : public A 
{
    B() : A() { ptr = new RoundPtr(); }
};

class C : public A 
{
    C() : A() { ptr = new SquarePtr(); }
}

Then I create a list of pointers to basic class:

A * list[2];
list[0] = new B();
list[1] = new C();

And call this method:

list[i]->getPtr();

But I found that it returns an uninitialized pointer.

So, how to correctly use polymorphism here? How to properly create this field to use different types of it?

This code doesn't compile, but if the errors are fixed in the obvious way then the pointer would be initialised. Please post real code. — john, Jul 17 '19 at 21:32
[Works fine for me](https://ideone.com/GDErEE) once the syntax errors and such are fixed. — Remy Lebeau, Jul 17 '19 at 22:17

score 0 · Accepted Answer · answered Jul 18 '19 at 14:13

Ok, I tested this code and it works perfectly o_0 But I got crashed in more complex program when try to call method of polymorphic object pointer. Maybe I have an additional errors in my project.

The answer is:

#include <iostream>

class Ptr {
    public:
        Ptr(int x, int y) : centerX(x), centerY(y) {}
        virtual ~Ptr() {}
        virtual void foo() = 0;

    protected:
        int centerX, centerY;
};

class RoundPtr: public Ptr {
    public:
        RoundPtr(int x, int y, int radius) : Ptr(x, y), circleRadius(radius) {}
        virtual void foo() { std::cout << "RoundPtr, x: " << centerX << ", y: " << centerY << ", radius: " << circleRadius << std::endl; }

    protected:
        int circleRadius;
};

class SquarePtr: public Ptr {
    public:
        SquarePtr(int x, int y, int w, int h) : Ptr(x, y),
            leftX(centerX - w/2), leftY(centerY - h/2),
            width(w), height(h)
            {}

        virtual void foo() { std::cout << "SquarePtr, x: " << centerX << ", y: " << centerY << ", leftX: " << leftX << ", leftY: " << leftY << ", width: " << width << ", height: " << height << std::endl; }

    protected:
        int leftX, leftY;
        int width, height;
};

class A {
    protected:
        Ptr * ptr;

    public:
        A() : ptr(nullptr) {}
        virtual ~A() {}

        virtual Ptr * getPtr() { return ptr; }
};

class B : public A {
    public:
        B() : A() { ptr = new RoundPtr(0, 0, 10); }
        virtual ~B() { delete ptr; }
};

class C : public A {
    public:
        C() : A() { ptr = new SquarePtr(5, 5, 10, 10); }
        virtual ~C() { delete ptr; };
};

int main()
{
  A * bObj = new B();
  A * cObj = new C();

  bObj->getPtr()->foo();
  cObj->getPtr()->foo();

  delete bObj;
  delete cObj;
}

Can anybody check this code? Do you see any problems with it? Is it good solution? Is it possible to use smart pointers from c++11 here?

How to make polymorphic field of class in C++? How to use polymorphism correctly in this case?

1 Answers1