How to create a dynamic array of objects polymorphically?

Question

How can I create a dynamic array of objects polymorphically, if I have an abstract class with derived classes, and without using STL data structure classes? (vectors, list, etc)

A static array of objects

TwoDimensionShape *shapes[2];       
shapes[0] = &Triangle("right", 8.0, 12.0);  
shapes[1] = &Rectangle(10);

I know I can't do this because you cannot create objects of an abstract class:

cin >> x;
TwoDimensionShape *s = new TwoDimensionShape [x];

EDIT:

Thanks to Nick, this works:

  int x = 5;
  TwoDimensionShape **shapes = new (TwoDimensionShape*[x]);

imreal · Accepted Answer · 2012-11-20 02:04:41Z

4

You can create an array of pointers to that class:

TwoDimensionShape **s = new TwoDimensionShape*[x];

And then construct each object with it's specific type:

s[0] = new Triangle("right", 8.0, 12.0);  
s[1] = new Rectangle(10);

Similar to what you had. Remember to delete when you don't need anymore.

edited Nov 20, 2012 at 2:04

answered Nov 20, 2012 at 1:39

imreal

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6 Comments

Dog Over a year ago

I have a problem, int x = 5; TwoDimensionShape **shapes = new (TwoDimensionShape *)[x]; The compiler says "A value of type TwoDimensionShape * cannot be used to initialize an entity of type TwoDimensionShape **

imreal Over a year ago

You are missing an asterisk on shapes and one afer TwoDimensionShape: TwoDimensionShape **shapes = new (TwoDimensionShape*)[x];

Dog Over a year ago

Huh, that's weird, when I clicked to edit and it was in there, but nonetheless it still gives an error

imreal Over a year ago

Can you update the question on the top to see how the code is after this change?

Dog Over a year ago

i figured it out, the [x] is supposed to be inside the parrentheses after the *, I will post an update

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