Creating array of different objects - c++

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 ;)

Answer 2

That's because _t is a pointer to Technics not Computer or Appliance.
Give Technics an "object type" parameter e.g. an enum that is TechnicsType.Computer for Computer and TechnicsType.Applicance for Appliance, check that and cast to the appropriate type to get the class methods.

Answer 3

The solution is very very simple :)
The super-class must have the virtual functions of the subclasses declared in the class definition.
For example: if the super-class computer have a sub-class called laptop that have a function int getBatteryLife();, so the computer class must have the definition virtual int getBatteryLife() to be called in the vector of pointers of the type computer.

Answer 4

Because _t is a Technics pointer array and, there is not possible to access to derived classes attributes. Use a Visitor Pattern like this or downcast your pointer:
// visitor pattern
class Visitor
{
void accept(Appliance &ref) { // access Appliance attributes };
void accept(Computer & ref) { // access Computer attributes };
};
class Technics
{
....
virtual void visit(Visitor &) = 0;
};
class Appliance
{
....
virtual void visit(Visitor &v) { v.accept(*this); }
};
class Computer
{
....
virtual void visit(Visitor &v) { v.accept(*this); }
};

Answer 5

Yes, you can only access the properties of Technics, since your variable has type Technics. You have to cast it to your Computer or Appliance class in order to execute your additional methods.
You really have to think about your design here. Is it really appropiate? Why do you have all of the objects inside the same container? Especially if you have different methods to call..this doesn't make sense..
If you really want to call different methods, you probably have to use a switch statement to decide what type you have, then call the appropiate methods (I guess you want to iterate through the whole container, or else it doesn't make sense to have a big container with different objects).