Post Instantiation Inheritance?

BlitzMax Forums/BlitzMax Programming/Post Instantiation Inheritance?

BLaBZ

(Posted 2011) [#1]

I'd like to instantiate an extension of a base class after the base class had already been instantiated, for example,

Type Base
	Field Name:String
End Type

Type Extension Extends Base
	Field ExtensionName:String
End Type

b:Base = New Base
e:Extension = Extension(b)

e.ExtensionName = "TestExtension"

Print e.ExtensionName

I want to do this because I want to be able to "Extend" the actions of the base class without "modifying" the initial base class.

If you have an alternative approach I'm very interested to hear.

Thanks

Last edited 2011

col	(Posted 2011) [#2]

Is this what you want :-

Type Base
	Field Name:String
End Type

Type Extension Extends Base
	Field ExtensionName:String
End Type

b:Base = New Base
e:Extension = New Extension

e.ExtensionName = "TestExtension"

Print e.ExtensionName

Last edited 2011

BLaBZ

(Posted 2011) [#3]

No,
Lets say for example I have code that works exactly the way I want it with the base class, so I package it into a module.

Now I don't want to modify that module at all because it's perfect, but I do want to add behavior.

col	(Posted 2011) [#4]

But that codes does that no?

Type Base
	Field Name:String
End Type

Type Extension Extends Base
	Field ExtensionName:String
End Type

b:Base = New Base
e:Extension = New Extension

e.Name = "Test"
e.ExtensionName = "TestExtension"

Print e.Name
Print e.ExtensionName

BLaBZ

(Posted 2011) [#5]

In that code you can access the base classes functionality by instantiating the extension class, but what if you already have an instance of the Base class and now you want to convert it to the extension class,

I want to "upgrade" my INSTANCE of my base class to one of its extensions.

Does that make sense?

Jesse

(Posted 2011) [#6]

the reason your example won't work is because a base class does not know how to convert itself into the extended class. That is not how extended and casting work. when you create a base type object, only the space for the base object is allocated. When you create an extended type object, the space for both the base and the extended object is allocated. when you create an extend class you can assign it to a base type variable but that will only give you access to the base type part of the extended class ( sort of, its a little bit more complicated than that) and that is why you can cast an extended class back and forth and not the pure instance of a base class. You can't simply just cast it. It won't work!

the only thing I can suggest you do is what col said. you can create a base or a extended and assign them accordingly! and is what programmers usually do when interested in extending functionality.

BLaBZ

(Posted 2011) [#7]

Ok, understood,

Just looking for a more elegant way to extend without modifying code.

Jesse

(Posted 2011) [#8]

maybe it will work for you if you create a type that has both types as fields and instantiate the field variables depending on your needs.

col	(Posted 2011) [#9]

How about this :-

Type Base
	Field Name:String
End Type

Type Extension Extends Base
	Field ExtensionName:String
	
	Method Create:Base()
		Return Self
	EndMethod
End Type

Local b:Base = New Extension.Create()
b.Name = "Base"

Local eb:Extension = Extension(b)
eb.ExtensionName = "Extension"

Print eb.Name
Print eb.ExtensionName

EDIT :-
You can create a means in the inherited type to return itself as the base type, then you can cast back up with :-

Print b.Name
Print Extension(b).ExtensionName

Its still not like a complete 'lift' into the inherited type because it means creating from the inherited to receive the base and then having to use a 'up cast', but it will function the same.

Last edited 2011