problem with a stack

Im just trying to push some values into a stack, but the program shows me an error and types:
"Object reference not set to an instance of an object."

the error below points to line 28 in the code below:

Code (CSharp):

1. using UnityEngine;
2. using System.Collections;
3. using System.Collections.Generic;
4.  
5. public class Menu_Stack : MonoBehaviour {
6.  
7.    public GameObject cube = new GameObject();
8.    Stack<int> undoStack = new Stack<int>();
9.  
10.  
11.     void OnGUI() {
12.  
13.         if (GUI.Button(new Rect(10, 70, 120, 30), "EXPAND CUBE SIZE"))
14.         {
15.             expand_and_add_to_stack(); // currently adds scale by 1 unit.
16.         }
17.  
18.         if (GUI.Button(new Rect(10, 125, 120, 30), "UNDO"))
19.         {
20.            reduce_and_delete_from_stack();
21.         }
22.  
23.     }
24.  
25.     void expand_and_add_to_stack()
26.     {
27.         cube.transform.localScale += new Vector3(1, 0, 0);
28.         undoStack.Push(1);
29.     }
30.  
31.     void reduce_and_delete_from_stack()
32.     {
33.       while(undoStack!=null)
34.        {
35.             cube.transform.localScale -= new Vector3(1, 0, 0);
36.             undoStack.Pop();
37.         }
38.     }
39.  
40.  
41. }
42.  

 

Are you initializing those two variables outside of Awake or Start? That shouldn't work, I think, and when I try to run your code it will actually throw an error for that reason. So maybe try something like this:

Code (CSharp):

1.     public GameObject cube;
2.     Stack<int> undoStack;
3.  
4.     void Start () {
5.         cube = new GameObject();
6.         undoStack = new Stack<int>();
7.     }

 

I think the issue is with line 7:

Code (CSharp):

1.    public GameObject cube = new GameObject();

It looks like Unity doesn’t want you to create GameObjects too early on.

As far as I can tell, line 8 is actually okay:

Code (CSharp):

1.    Stack<int> undoStack = new Stack<int>();

I think the undoStack variable wasn’t getting initialized because line 7 was throwing an exception.

 

Line 7 should just be public GameObject cube;
Then in the inspector you should drag something into that slot. Or, if you are instantiating something, then you'll assign the value of that variable when you instantiate your object.