How to create a dynamic array of an Abstract class?

Question

Lets say I have an abstract class Cat that has a few concrete subclasses Wildcat, Housecat, etc.

I want my array to be able to store pointers to a type of cat without knowing which kind it really is.

When I try to dynamically allocate an array of Cat, it doesn't seem to be working. Please help?

Cat* catArray = new Cat[200];

Answer 1

You would need to create an array of pointers to Cat:

Cat** catArray = new Cat*[200];

Even if the base class Cat was concrete, you would still run headlong into object slicing if you created an array of Cat.

Note that you should probably use a std::vector instead of an array, and should probably use smart pointers to ensure your code is exception safe.

Answer 2

You cannot directly instantiate an instance of an abstract class, but must instead instantiate an instance of a fully implemented subclass.

So this is legal:

Housecat* theCats = new Housecat[200];

and then you can access each cat through the Cat interface

bool catsMeow = ((Cat*)(&theCats[0]))->CanMeow();

But the compiler has no way of knowing how to instantiate an abstract class; in fact, the very fact that it's abstract means that it cannot be directly instantiated.

Why do this? Because Cat will have an abstract method

bool CanMeow() = 0;

That all inherited cats must implement. Then you can ask if it can meow, with the chance that an instance of Lion will return false.

Answer 3

You cannot round up the cats in fixed size cages, because the compiler has no way of knowing how big the cats will be, nor (metaphor failure) how to initialize them. You are going need to do something like initialize the array to null cat-pointers or something, and herd them later.

Answer 4

By creating an aray of pointers to Cat, as in

 Cat** catArray = new Cat*[200];

Now you can put your WildCat, HouseCat etc instances at various locations in the array for example

 catArray[0] = new WildCat();
 catArray[1] = new HouseCat();
 catArray[0]->catchMice(); 
 catArray[1]->catchMice();

Couple of caveats, when done
a) Don't forget deleting the instances allocated in catArray as in delete catArray[0] etc.
b) Don't forget to delete the catArray itself using

 delete [] catArray;

You should also consider using vector to automate b) for you