Does type casting work

Here is an arbitary example

Type TBase
	Field FOne:Int
'---------------------------------------------
Function Create:TBase ()
	Local 	Temp:TBase 	= New TBase
		Temp.FOne		= 1
	Return Temp
End Function
EndType
'----------------------------------------------
Type TExtend	Extends TBase
Field FTwo:Int
'---------------------------------------------
Function Create:TExtend ()
		Local	Temp:TExtend	= New TExtend
				Temp.FTwo		= 2
		Return Temp
	End Function
End Type

Global mine:TBase		= TBase.Create()
Print mine.FOne
Print TExtend(mine:TBase).FOne

When run it prints the value of Mine:Tbase.Fone
then it casts TBASE to TExtend. BUT it dosent even do a field copy?
Because when I then ask for FOne, its a different number

Tell me, when you're using C++ and you do this:

BaseClass* c = new BaseClass();
DerivedClass* d = (DerivedClass*)c;

Do you expect d to be a valid DerivedClass?

Dont know I cannot program in c++

but in bmax, if at the end of above you

Global newmine:TExtend= TExtend(mine:TBase)

then yes I expect newmine to be an extend type, and it is.

The problem isnt that it doesnt create a valid object, the problem is that it doesnt copy any of the fields

Type TBase
    Field FOne:Int
    '---------------------------------------------
    Function Create:TBase ()
        Local Temp:TBase = New TBase
        Temp.FOne= 1
        Return Temp
    End Function
EndType
    '----------------------------------------------
Type TExtend Extends TBase
    Field FTwo:Int
    '---------------------------------------------
    Function Create:TExtend ()
        Local Temp:TExtend= New TExtend
        Temp.FTwo= 2
        Return Temp
    End Function
End Type

Local mine:TBase = TBase.Create()
Print mine.FOne

Local mine2:TExtend = TExtend( mine )
Print mine2.FOne

Input

Does this work? It shouldn't.

Thats the same as my code.
It works in so far as local mine2:Textend = Textend (mine) creates a valid extend object.

But it doent work in that mine2 contains NON of the values of mine.

But all you are doing is saying look it doesnt work, which is what I said. What I would like to know, is exactly what has the textend(mine) done, other than create a textends type. Surly it should have field copied the base fields into the extend type TAnyType(ABaseObject)

Basicaly you are saying
TAnyType(ABaseObject) is the same as
New TAnyType

In that they both create a TAnyType.
Surly the cast method should copy somthing from ABaseObject?

TExtend(mine) doesn't create a TExtend object. What version of BlitzMax are you using?

Think of it this way, or even draw up a venn diagram if you have to, it's very simple though:
A pie is a dessert, but a dessert is not a pie.

Casting works only when you create an object of the correct type and either upcast or downcast to its appropriate base or derivation. When you create a TBase and cast it to a TExtend, it should not ever work and it definitely should not be creating a new object. Your program should, in fact, throw a runtime error if you attempt to cast a new instance of TBase (note: NOT a new instance of TExtend) to a TExtend and then use said TExtend. It's like saying that dessert is the same thing as pie and vice versa when in fact pie is a derivative of dessert and dessert is not a derivative of pie.

Casting is not copying.

What can I say

Global mine:TBase	= TBase.Create()
Print mine.FOne
Global newmine:TExtend	= TExtend(mine)
Print mine.FOne
Print newmine.FOne+","+newmine.FTwo
mine.FOne:+999
newmine.FOne:+ 1
Print mine.FOne
Print newmine.FOne+","+newmine.FTwo

Put this at the end of the first example.

I am using todays version of Bmax, and if you run the above it dosent crash.

TExtend(Mine) does seem to create a Textend Object, yet this object Dosent have any of the values of Mine.

Can you please post a copy of cast that works? (Objects not Types)

I am willing to admit that I have totaly missunderstood the meaning of casting, (But NOT soon;-)

1
1
~>Unhandled Exception:Attempt to access field or method of Null object
~>
t
~>StackTrace{
~>@C:/Documents and Settings/Owner/Desktop/tf.bmx<25,1>
~>Function tf
~>Global mine:TBase=$00ea1d30
~>Global newmine:TExtend=Null
~>}
~>

It does crash.

Compiling:uu.bmx

flat assembler  version 1.64
3 passes, 0.1 seconds, 4050 bytes.
Linking:uu.exe

1
1
0,0
1000
1,0

Not on mine


Full code

Type TBase
	Field FOne:Int =3
'---------------------------------------------
Function Create:TBase ()
	Local 	Temp:TBase 	= New TBase
		Temp.FOne		= 1
	Return Temp
End Function
EndType
'----------------------------------------------
Type TExtend	Extends TBase
Field FTwo:Int=4
'---------------------------------------------
Function Create:TExtend ()
		Local	Temp:TExtend	= New TExtend
				Temp.FTwo		= 2
		Return Temp
	End Function
End Type

Global mine:TBase	= TBase.Create()
Print mine.FOne
Global newmine:TExtend	= TExtend(mine)
Print mine.FOne
Print newmine.FOne+","+newmine.FTwo
mine.FOne:+999
newmine.FOne:+ 1
Print mine.FOne
Print newmine.FOne+","+newmine.FTwo

Are you compiling in debug or release mode?

Release

Compile in debug.

Doesnt work in debug.
(But Maybe thats a promblem in Debug ;)

Wrong. If it doesn't work in debug, then your code is bad. It could be bad in any number of ways, but currently it's bad in that you're accessing memory that is not allocated for the type in question.

A new object is not created. I don't know where you get this idea, honestly. What you're doing is accessing memory that you just shouldn't be accessing. Release mode removes checks for these things, and the problem is your code isn't checking for these faults because it's being compiled in release.

So, in fact, your code is bad and you're wrong.

(Editing your post cannot save you from the browser cache)

After you click OK on the Unhandled Exception in debug mode, look over to the Debug tab of the IDE. There you will see Function untitled... or whatever your test code is named.

If you expand that you will see newmine is infact null which is what you get if you try and cast something in the wrong direction. BlitzMax allows you to downcast but upcasting results in a null object, not some copy of the orginal type with extra fields as seems to be your expectation.

@My old post.
I Know I realized how stupid that was as soon as I posted it. It was edited straight away. Sorry

So does Type casting work?

@Casting works only when you create an object of the correct type and either upcast or downcast to its appropriate base or derivation

Type TExtend extends TBase

Surly the base of Textend is TBase. And surly a derivation of TBase if TExtend

etc.

Oh come on Post me some code where it works, cos Ive spent all day on this (and the overridding thing) and Im stuck. I admit it. I CANNOT figure it out by myself. Please sombody tell how to use the cast stuff.

SuperStrict

Type Base
    Field BField%
End Type

Type Derived Extends Base
    Field DField%
End Type

' This is a legitimate use of casting
Local d:Derived = New Derived   ' A Derived is a Base but a Base is not a Derived, REMEMBER.
Local b:Base = Base(d)          ' Same as b = d, but explicitly downcasted for the sake of example
d = Derived(b)                  ' Pretty much the same as d=d

' This is not
b = New Base
d = Derived(b) ' A Derived is a Base but a Base is not a Derived
b = Base(d)

I'd recommend this excellent OOP tutorial http://www.blitzmax.com/Community/posts.php?topic=59233

Tar.

Can the Local b:base by created/invoked/named any other way than the one you have used?

@Assari Already got it

No.

Ahhhh. Then yes I have TOTALY missed the point of casting :(

I thought casting was
"Ive got a base, and I want to make a derived, so I cast the base, and Yippy I have a Derived"

I would like to appologise If Ive been ratty on this or the Override Create thread.

Casting base to derived only works if the stuff saved within the base was already a derived casted to base. (ie you can only downcast the inheritance tree if the element was first upcasted the inheritance tree)