Question

I have this code, but I don’t see where I went wrong here. It seem to compile OK but I cannot access Computer or Appliance functions. Can someone please help me understand how can I make an array that holds different objects on this code example I have here?

#include <iostream>
using namespace std;

class Technics
{
private:
    int price, warranty;
    static int objCount;
    double pvn;
    char *name, *manufacturer;
public:
    Technics()
    {
        this->objCount++;
    };

    Technics(int price)
    {
        this->objCount++;
        this->price = price;
    }

    ~Technics(){
        this->objCount = this->objCount - 2;
    };

    static int getObjCount()
    {
        return objCount;
    }

    void setPrice(int price)
    {
        this->price = price;
    }

    int getPrice()
    {
        return this->price;
    }

    void resetCount()
    {
        this->objCount = 0;
    }
};
int Technics::objCount = 0;

class Computer : public Technics
{
private:
    int cpu, ram, psu, hdd;
public:
    Computer() {}
    Computer(int price)
    {
        this->setPrice(price);
    }

    void setCpu(int cpu)
    {
        this->cpu = cpu;
    }

    int getCpu()
    {
        return this->cpu;
    }
};

class Appliance : public Technics
{
private:
    int height;
    int width;
    char* color;
    char* type;

public:
    Appliance(){}
    Appliance(int height, int width)
    {
        this->height = height;
        this->width = width;
    }

    void setWidth(int width)
    {
        this->width = width;
    }

    int getWidth()
    {
        return this->width;
    }
};

void main()
{
    //Creating array
    Technics *_t[100];

    // Adding some objects
    _t[0] = new Computer();
    _t[1] = new Computer();
    _t[2] = new Appliance();

    // I can access only properties of Technics, not Computer or Appliance
    _t[0]->

    int x;
    cin >> x;
}

Answer 1

The line:

_t[0] = new Computer();

Creates a computer object and stores it as a Technics base pointer in the array (i.e. for all intents and purposes while in that array, it is a Technics object).

You need to cast back to the derived class to access members that are more derived than those in Technics:

static_cast<Computer*>(_t[0])->Your_Member();

Use dyncamic cast if you don’t know which derived type it is – it will return the casted pointer on success and NULL on fail so it’s kind of a type-check – it has big runtime overhead though, so try to avoid it 🙂

EDIT in response to your closing comment:

//Calculate the length of your dynamic array.

//Allocate the dynamic array as a pointer to a pointer to Technics - this is like
//Making an array of pointers each holding some Technics heirarchy object.
Technics** baselist = new Technics*[some_length];

//Populate them the same way as before:
baselist[0] = new Computer();
baselist[1] = new Appliance();

PS: you could also use std::vector which is dynamically changeable as opposed to just created at run time – it’s the best option if your allowed to use it. It saves you making your own resizable array code. Google it 😉