How to add derived class objects to an array of base class type?

Question

unfortunately, I can't use std::vector and have to use plain C++ arrays. I got the following code:

class Base
{

}

class DerivedCar : Base
{
public:
    DerivedCar(int a) a(a) {};
private:
    int a;
}

class DerivedHouse : Base
{
  public:
    DerivedHouse(float b) b(b) {};
private:
    float b;  
}

class Vector
{
    Vector() :
    index(0)

    void add(const DerivedCar& car)
    {
       vec[index] = new DerivedCar(car.a);
       index++;
    }

    void add(const DerivedHouse& house)
    {
       vec[index] = new DerivedHouse(house.b);
       index++;
    }

private:
    Vector vec[100];
    int index;
}

int main()
{
    Vector vector;
    DerivedCar car(100);
    DerivedHouse house(2.f);

    vector.add(car);
    vector.add(house);
}

I would like to have an array of type Base and add objects of a derived type. Is there a better approach to this other than the way I did? What would be the best way to keep copying of objects at a minimum.

Why does Vector (not to be confused with std::vector) have an array of itself inside of itself? Do you mean Base* vec[N]? — tadman
– tadman, Commented Sep 12, 2018 at 18:32
If you must avoid std::vector, at least implement the same API. push_back not add. — tadman
– tadman, Commented Sep 12, 2018 at 18:33
class DerivedCar : Base - you almost certainly want class DerivedCar : public Base and similar elsewhere. — user2100815
– user2100815, Commented Sep 12, 2018 at 18:33
Arrays by definitions may only contain objects of the same type. In order to overcome this you will need an extra layer of indirection, for example by storing pointers to (polymorphic) base class (as suggested above) or some sort of variant combining all the types to be stored. — user7860670
– user7860670, Commented Sep 12, 2018 at 18:42

Öö Tiib · Accepted Answer · 2018-09-12 20:17:57Z

1

How to add derived class objects to an array of base class type?

You can not put derived class objects into raw array or std::vector of base class because the derived class objects are usually larger and so simply do not fit there.

Is there a better approach to this other than the way I did?

Better approaches are certainly out there. One good example of such containers with polymorphic elements is boost::base_collection. Read its documentation and its source code. If you do not understand some detail in it then ask about that detail in Stack Overflow.

What would be the best way to keep copying of objects at a minimum.

Containers that only contain pointers to objects and intrusive containers keep copying of objects at minimum. However such containers do not manage the objects and so responsibility of objects life time has to be taken by something outside.

edited Sep 12, 2018 at 20:17

answered Sep 12, 2018 at 20:09

Öö Tiib

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Comments

jackw11111 · Accepted Answer · 2018-09-13 03:12:12Z

0

Here is a possible way that implements a linked list:

class Base
{

};

class DerivedCar : public Base
{
public:
    DerivedCar(int a) { _a = a; };
private:
    int _a;
};

class DerivedHouse : public Base 
{
public:
    DerivedHouse(float b) { _b = b; };
private:
    float _b;
};

class Object
{
public:
    const Base *data;
    const Object *next; 
};

class Vector 
{
public:
    void add(const Base& v)
    {
        Object item;
        item.data = &v;
        head.next = &item;
        index++;
    }
private:
    Object head;
    int index = 0;
};

int main()
{
    Vector vector;
    DerivedCar car(100);
    DerivedHouse house(2.f);
    vector.add(car);
    vector.add(house);
}

answered Sep 13, 2018 at 3:12

jackw11111

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