Question by pauldoody25 · Jan 19, 2016 at 07:32 PM · parent child class inheritance parent-child

Putting a Child Class into a List of Parent Type and then casting it back to a Child

I'm trying to set up an inventory system with a List of Inventory_Items to store them. However I want to use weapon or armor classes that inherit from Inventory_Items so they don't have worthless variables like ammo type on a sword or something. I have it set up, however, upon casting the Inventory_Items object in the list back into its original child class, all of the variables belonging to the child class are set to their default values. Is there a way for those variables to be recovered after being converted to a parent class?

         //parent class
         [System.Serializable]
         public class Inventory_Item: MonoBehaviour
         { //data types
       public Inventory_Item()
     {
              //constructor
     }
        }
         //Child Class

           [System.Serializable]
           public class Player_Usable_Item : Inventory_Item 
          {
          Player_Usable_Item() : base
          }
          //database class
          public class Item_Database : MonoBehaviour {

    [SerializeField]
    public List<Inventory_Item> allItems = new List<Inventory_Item>();

    void Awake () 
 {
     allItems.Add (new Inventory_Item());
     allItems.Add ((Inventory_Item)new Player_Usable_Item());
 }

   }
         //trying to access child variables
        Player_Usable_Item test = (Player_Usable_Item)itemDatabase.allItems[3];
        Debug.Log (childVariable + ""); //returns default values

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Bonfire-Boy · Jan 19, 2016 at 09:13 PM 1

Yes you could do that like this...

 Inventory_Item i = itemDatabase.allItems[3];
 Player_Usable_Item p = i as Player_Usable_Item;
 if (p != null)
 {
    // do stuff with p
 }
 else
 {
   // it's not a Player_Usable_Item
 }

However...

You don't need to cast a Player_Usable_Item to the parent class when adding it to the list. A Player_Usable_Item IS an Inventory_Item.

You've made Inventory_Item a $$anonymous$$onoBehaviour, so neither that nor the derived class can have a constructor (and hence can't be created in the way you're doing it in the Awake function).

pauldoody25 Bonfire-Boy · Jan 19, 2016 at 10:35 PM 0

I see. However, once converted all of the new variables belonging only to the child are still all set to their defaults. Is there no way to prevent this as the data is simply lost when it's first put in the parent list?

Bonfire-Boy pauldoody25 · Jan 19, 2016 at 11:21 PM 0

Not sure what you mean, the classes don't appear to have any member variables in the code you've shown. How are you changing the defaults?

You won't be losing data just by putting the references into a List.

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cjdev pauldoody25 · Jan 20, 2016 at 12:17 AM 0

I think what's happening is that if you're casting the derived class to the base class it won't have the variables that are unique to the derived class anymore. So when you cast it back those variables are basically reinstantiated to their default values. Ins$$anonymous$$d of using a base class to organize the inventory items try using an interface ins$$anonymous$$d.

Bonfire-Boy pauldoody25 · Jan 20, 2016 at 12:38 AM 0

@cjdev That is totally wrong. Putting a derived class reference into a list of base class references does not change the derived class object in any way. You can get it from the list, cast it back, and it will retain its values. This is how one uses polymorphism.

Simple example...

     public class A
     {
         public A( ) { }
     }
 
     public class B : A
     {
         public float b = 0f;
         public B(float bt) : base()
         {
             b = bt;
         }
     }
 
         // then put this block into a function
         {
                         List<A> AList = new List<A>( );
         AList.Add( new A( ) );
         AList.Add( new B( 1f ) );
 
         for (int i = 0; i < AList.Count; i++)
         {
             A a = AList[i];
             B b = a as B;
             if (b != null)
             {
                 Debug.Log( "A #" + i + " is a B with b = " + b.b );
             }
             else
             {
                 Debug.Log( "A #" + i + " is not a B ");
             }
         }
         }

This prints

         A #0 is not a B 
         A #1 is a B with b = 1

(ie, the B retains its non-default value of the variable b)

Of course, you can't quite do this with the OP's classes because they are $$anonymous$$onoBehaviours, so the setting of the variable values can't be done using constructors, but the principle holds.

cjdev pauldoody25 · Jan 20, 2016 at 01:12 AM 0

Ah, you're right, my mistake. Not sure what I was thinking there, I think the cast on the new operator got me all mixed up.

Bonfire-Boy pauldoody25 · Jan 20, 2016 at 08:40 AM 0

@cjdev I do see what you mean. Sorry if the tone of my reply was a bit blunt, given the misunderstandings evident in the code in the question I was just concerned about the OP being led in the wrong direction when it comes to how inheritance works, so I jumped on you a bit quick :)

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Putting a Child Class into a List of Parent Type and then casting it back to a Child

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