Thread: Inheritance and Array question

My question is that can you put objects from several different objects into an array of pointers from a base class?(i think the answer is yes b/c its an array of pointers) and how would you access data from the derived class?

for ex.(Illustrative code)

Code:

class ComplexShape 
{
public: BasicShape *pBasicShape[100];
...
...
}

class BasicShape : public ComplexShape 
{
public: int someInt;
...
...
}

class triangle : public BasicShape
{
public : int base
...
...
}

how would i access base if i were to put a pointer to a triangle in '[1]' ?
ive tried something like this(below), but doesnt work:

Code:

ComplexShape Z;
Z.pBasicShape[1] = new triangle; //assume a default constructor is provided
Z.pBasicShape[1]->base = 10;

You array of pointers is of type Shape however a triangle is derived from BasicShape and ComplexShape. Is a triangle supposed to be derived from Shape as well and if so why isn't that represented? In order for this to work, a triangle must be deriveable from a Shape object. The overall representation of your classes and the hierarchy they represent seems a little off.

An array of pointers does the trick.

To access data (or member functions) in derived classes that are not part of the base class, it is necessary to (a) have an instance of the derived class (b) know it is of the derived class (c) if you have a pointer to base, convert it to a pointer to that derived class and (d) access the member you need to.

In your example, this means that (a) Z.pBasicShape[1] would need to be a triangle (b) your code your need to know that (c) Z.pBasicShape[1] would need to be converted to a pointer to triangle, and (d) the base member of that triangle would need to be accessed. You have not done that, so the compiler has no way of knowing that Z.pBasicShape[1] is a triangle, and therefore does not know it contains a data member named base.

In code the things you need to do would be;

Code:

   triangle *temp;
   temp = dynamic_cast<triangle *> (Z.pBasicShape[1]);   // attempt to convert the pointer to a triangle, assuming Shape is a polymorphic base class

   if (temp != NULL)   // the conversion has succeeded
       temp->base = 10;
   else
       the_shape_is_not_a_triangle();

Having to do this is poor design practice. It is better to provide core functionality in the base class, which is tailored by derived classes (eg overriding of a virtual member function) rather than adding functionality to a derived class that does not exist in the base class.

Incidentally, for your example to make sense at all, triangle needs to be derived from Shape, not from BasicShape.

whoops

Code:

Shape *pBasicShape[100];

should be:

Code:

BasicShape *pBasicShape[100];

Originally Posted by grumpy

An array of pointers does the trick.

To access data (or member functions) in derived classes that are not part of the base class, it is necessary to (a) have an instance of the derived class (b) know it is of the derived class (c) if you have a pointer to base, convert it to a pointer to that derived class and (d) access the member you need to.

In your example, this means that (a) Z.pBasicShape[1] would need to be a triangle (b) your code your need to know that (c) Z.pBasicShape[1] would need to be converted to a pointer to triangle, and (d) the base member of that triangle would need to be accessed. You have not done that, so the compiler has no way of knowing that Z.pBasicShape[1] is a triangle, and therefore does not know it contains a data member named base.

In code the things you need to do would be;

Code:

   triangle *temp;
   temp = dynamic_cast<triangle *> (Z.pBasicShape[1]);   // attempt to convert the pointer to a triangle, assuming Shape is a polymorphic base class

   if (temp != NULL)   // the conversion has succeeded
       temp->base = 10;
   else
       the_shape_is_not_a_triangle();

Having to do this is poor design practice. It is better to provide core functionality in the base class, which is tailored by derived classes (eg overriding of a virtual member function) rather than adding functionality to a derived class that does not exist in the base class.

Incidentally, for your example to make sense at all, triangle needs to be derived from Shape, not from BasicShape.

didnt think of a dynamic type cast i think thats what i will need to do. and I am using a polymorphic base class. thnxs for the help, ill try this out in a bit

title cost and onHand is in any derived class - move them to the Base class
you don't need dynamic cast when storing pointers in the base pointers array
onHand is private - make a functions for modifying it 9they will make a validation before modifying value)

0.0 isn't an int.

Code:

class Cd : public InventoryItem
{
private:
	String title;
	float cost;
	int onHand;
	int numTrack;
	float playTime;
public:
	Cd() : title(""), cost(0.0), onHand(0.0), numTrack(0), playTime(0.0){}
	Cd(String t, float c, int oH, int nT, float pT) :
		title(t), cost(c), onHand(oH), numTrack(nT), playTime(pT){}
	float sellPrice();
};

The same code occurs for every class that inherits from InventoryItem.