How can I instantiate a GameObject directly into an array?

You should use the new statement:

 shop_lines[line_number].shop_line_name = Instantiate(menu_text_prefab, new Vector3(0,0,0),Quaternion.identity)

or alternatively use Vector3.zero

 shop_lines[line_number].shop_line_name = Instantiate(menu_text_prefab,Vector3.zero,Quaternion.identity)
 

thanks for the advice - the class constructor is a great tip and i'm trying to get this working with that now but still having problems filling the array...

My current script (below) works as i'd expect it to if i comment out the "shop_lines[line_number].FillShopLine..." function, otherwise i get the error;

NullReferenceException: Object reference not set to an instance of an object ShopLogic.DisplayMenu () (at Assets/scripts/ShopLogic.js:57) ShopLogic.Start () (at Assets/scripts/ShopLogic.js:48)

I have tried including the Instantiate function in the class constructor instead of the For Loop but i still get the same error

In the scene i've noticed the For loop does get to the instantiate functions and instantiates the first two text_menu_prefab gameObjects, but they do net get put into the array. I can also see in the inspector that the array has been set to the correct length (at the moment that's just 3 to keep things simple).

Sorry if i'm being slow but i can't seem to find any way to initialise the array but this (or the original version i posted of this script...) is there some basic principle i'm missing?

(PS i definitely have assigned a prefab to the var menu_text_prefab)

 #pragma strict
  
 var menu_text_prefab : GameObject;
  
 class ShipUpgradeItem {
     var name : String;
     var ship_upgrade_function_call : String;
     var cost : int;
     var cost_increase : float;
     var cost_increase_multiply : boolean;
     var upgrade_unlocked : boolean;
     var max_upgrade_allowance : int;
 }
 var ship_upgrade_array : ShipUpgradeItem[];
  
 class ShopLine {
     var shop_line_name : GameObject;
     var shop_line_cost : GameObject;
     var shop_line_number : int;
     function FillShopLine (name : GameObject,cost : GameObject,number : int){
         this.shop_line_name = name;
         this.shop_line_cost = cost;
         this.shop_line_number = number;
     }
 }
  
 var shop_lines : ShopLine[];
  
 var cost_colour : Color = Color(1,1,0,1);
 var available_colour : Color = Color(1,1,1,1);
 var unavailable_colour : Color = Color(1,0,0,0.8);
 var highlight_colour : Color = Color(0,1,0,1);
  
 private var this_line_colour : Color;
 var line_0_y_position : float = 0;
 var y_spacing : float = -25;
 var left_alignment : float = -225;
 var right_alignment : float = 125;
 var far_right_alignment : float = 225;
  
 var in_game_logic : InGameLogic;
 private var player_pressed_continue : boolean = false;
 private var game_state : GameState;
  
 function Start (){
     in_game_logic = GameObject.Find("InGameLogicHolder").GetComponent(InGameLogic);
     game_state = GameObject.Find("GameStateHolder").GetComponent(GameState);
     DisplayMenu ();
 }
  
 function DisplayMenu(){
     shop_lines = new ShopLine[ship_upgrade_array.Length];
     for (var line_number : int = 0; line_number < shop_lines.Length; line_number++){
         Debug.Log("we have " + shop_lines.Length + " shop lines");
         if (shop_lines[line_number] == null){
             //following line gives a NullReferenceException error 
             shop_lines[line_number].FillShopLine(
             GameObject.Instantiate(menu_text_prefab,Vector3(0,0,0),Quaternion.identity),
             GameObject.Instantiate(menu_text_prefab,Vector3(0,0,0),Quaternion.identity),
             line_number);
             Debug.Log("this is line number " + line_number);
         }
     }
 }