Re: Creating object simulate C++ object in lua

[Kenneth Gangstoe <kennethg@...>]

Excellent response, Kelmar, but does anyone know if there are a preferred
way to do this in Lua 5.0?

- Kenneth

Quoting Kelmar K. Firesun (kfiresun@ix.netcom.com):
> 
> ----- Original Message ----- 
> From: "nerurin" <nerurin@hotmail.com>
> To: "Multiple recipients of list" <lua-l@tecgraf.puc-rio.br>
> Sent: Thursday, August 15, 2002 3:00 PM
> Subject: Creating object simulate C++ object in lua
> 
> 
> > Hi all,
> > 
> > I wonder what is the "easiest" way to create a class defination in 
> > Lua, that function like C++ class. The intention is, I want to 
> > prototype some idea, but I do not prefer to write in C++ right way.
> > 
> > Thanks in advance!
> > 
> > 
> 
> In 4.0 the easist way to do this is to set up an index tag 
> method for your tables that will look for a super object inside 
> of a table looking for properties/functions that were not found
> in the first table.  The following is the code I use:
> 
> function delegate(tab, item)
>     local base = rawget(tab, "inherits")
>     if type(base) == "table" then
>         return base[item]
>     else
>         return nil
>     end
> end
> 
> settagmethod(tag{}, "index", delegate)
> 
> Now, if the table "tab" has an item called "inherits" the tag 
> method will check for that item, if found and it is a table, it
> will look for "item" in that table.  This continues up the chain
> until the item is found or "inherits" isn't found.
> 
> When you write your objects you create them as such:
> 
> BaseObject = { myval = "A Value" }
> 
> InheritedObject = { inherits = BaseObject, newval = "Another Value" }
> 
> print(InheritedObject.myval)
> 
> This will cuase the value "A Value" to be displayed.  This
> functionally works great with functions as well.  This following
> example shows how you might define a set of simplistic GUI objects:
> 
> GUIObject =
> {
>     x = 10,
>     y = 10,
>     Width = 100,
>     Height = 100
> }
> 
> function GUIObject:OnDraw()
>     DrawRect(self.x, self.y, self.Width, self.Height)
> end
> 
> function GUIObject:OnClick()
>     -- Do something.
>     print("GUIObject clicked!")
> end
> 
> -- Then...
> 
> StaticText = 
> {
>     inherits = GUIObject,
>     Caption = "Static Text"
> }
> 
> function StaticText:OnDraw()
>     DrawText(self.x, self.y, self.Caption)
> end
> 
> Now, when you call StaticText's OnDraw() method, you'll see the
> caption being displayed; however, the "x" and "y" values come from
> the inherited GUIObject class.  Presumably the user wishing to
> create an instance of a StaticText object would override the "x"
> and "y" values to set the correct possition of the object.
> 
> If you were to call StaticText's OnClick() method, the result
> would be that of the OnClick() defined in the GUIObject base class.
> 
> The colon in the function definitions passes a hidden "self"
> parameter to the function.  This is akin to the C++ "this" pointer.
> 
> There are a few limitations to this setup:
> 
> There are no private/protected methods and variables, everything 
> can look inside the guts of your object and possibly play with the 
> values.
> 
> This is possibly not the most efficent way to do inheritance.  I 
> don't know of a better method inside of Lua though, I'd be
> intrested in hearing about other ways to do this.
> 
> I hope this helps you out and that I explained it clearly enough.
> 
> Kelmar K. Firesun (IRL: Bryce Simonds)
> 
>