derived class as argument for base class and other rules for inheritance

This topic contains 14 replies, has 7 voices, and was last updated by Mark Sibly 1 year, 2 months ago.

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January 19, 2018 at 8:37 am #13020

Participant

in the following code I can give a derived class as parameter for an argument defined as the base class. The same is true for assignation. Is it expect behaviour?
If I cast a derived class from the base one I can access the derived method but not the Base one!
What may I and what shouldn’t I do. What are the rules with these inheritance things?

Namespace myapp

#Import "<std>"

Using std..

Function Main()
	
	Local oa:=New A()
	Local ob:=New B()
	
	haha(oa)
	haha(ob)
	
'	hoho(oa)
	hoho(ob)
	
	Local oa2:A
	Local ob2:B
	
	oa2=ob
'	ob2=oa
	
	oa.a=7
	Print oa.a
	Local ob3:=Cast<B>(oa)
	

	ob3.doMore()
	
	Print ob3.a 'not OK
	

	
End

Class A
	Field a:Int
	Method do() Virtual
		Print "a"
	End
	
End

Class B Extends A
	Field b:Int
	Method do() Override
		Print "b"
	End
	
	Method doMore()
		Print "yo!"
	End
End

Function haha (o:A)
	o.do()
End

Function hoho (o:B)
	o.do()
	o.doMore()
End

Namespace myapp

#Import "<std>"

Using std..

Function Main()

Local oa:=New A()

Local ob:=New B()

haha(oa)

haha(ob)

' hoho(oa)

hoho(ob)

Local oa2:A

Local ob2:B

oa2=ob

' ob2=oa

oa.a=7

Print oa.a

Local ob3:=Cast(oa)

ob3.doMore()

Print ob3.a 'not OK

End

Class A

Field a:Int

Method do() Virtual

Print "a"

End

Class B Extends A

Field b:Int

Method do() Override

Print "b"

End

Method doMore()

Print "yo!"

End

Function haha (o:A)

o.do()

End

Function hoho (o:B)

o.do()

o.doMore()

End

January 19, 2018 at 2:36 pm #13035

nerobot

Participant

Probably, it’s a lang bug. The code looks correct.

January 19, 2018 at 9:30 pm #13041

Simon Armstrong

Participant

Shouldn’t the downcasting on line 26 be an error?

January 19, 2018 at 10:05 pm #13043

Mark Sibly

Keymaster

Look all right to me. Pulling out the noise, code is:

Class A
End

Class B Extends A
End
	 
Function Main()
	
	Local a:=New A
	
	Local b:=Cast<B>( a ) 	'cast fails (ie: returns null) as 'a' is not of type B
	
	Assert( b )
End

Class A

End

Class B Extends A

End

Function Main()

Local a:=New A

Local b:=Cast( a ) 'cast fails (ie: returns null) as 'a' is not of type B

Assert( b )

End

Shouldn’t the downcasting on line 26 be an error?

Don’t think so. B is a subclass of A, so Cast( a ) is semantically correct (ie: compilable) if ‘a’ is of class A (or any other superclass of B). In general, the compiler can’t know at compile time what the true class of ‘a’ is though.

January 19, 2018 at 10:49 pm #13045

Jesse

Participant

I too expected that to be an error since class a doesn’t know about class b but class b does know about class a…But I like that it returns a null.

January 20, 2018 at 8:04 am #13055

abakobo

Participant

ok thanks

I thought invoking a method on a Null instance was giving an error..

January 20, 2018 at 8:39 am #13056

Mark Sibly

Keymaster

I thought invoking a method on a Null instance was giving an error..

It was and it should. In debug mode I get the following error…

Attempt to invoke method on null instance

…on line 29, ob3.doMore()

This is 100% correct behaviour – you can’t invoke a method on a Null instance. You could a while back but haven’t been able to for some time now.

January 20, 2018 at 10:12 am #13059

abakobo

Participant

Strange… Here with latest dev branch using “i686-6.2.0-posix-dwarf-rt_v5-rev1” and windows 10 I get the following. (see picture, whole code and console prints are visible, code attached as file too.)

Attachments:

method_on_null_instances.jpg

derived_class_cast.zip

January 20, 2018 at 8:37 pm #13116

Mark Sibly

Keymaster

Can you post output of console? You can copy and paste from console by clicking in console, then using edit menu to select-all/copy (Yes, this should be smoother, my fault really).

[edit]Ok, can reproduce, seems to be a mingw thing. Under msvc x64 I get:

ob is Null
-now trying to invoke the doMore() method on ‘ob’ Null intance-

Attempt to invoke method on null instance

ie: it should die before the ‘yo!’ Will check it out…

January 20, 2018 at 9:18 pm #13122

Diffrenzy

Keymaster

Testing that upload of .monkey2 files is now allowed .

Attachments:

derived_class_cast.monkey2

January 21, 2018 at 11:54 am #13141

cocon

Participant

This one seems to have a link to an object instance somewhere in memory, but the displayed value is Null.

Class A
End
 
Class B Extends A
End
	 
Function Main()
	Local a := New A
	Local b := Cast<B>(a)
	Print(Typeof(b) = Typeof<Object>)
	Print(b = Null)
End

Class A

End

Class B Extends A

End

Function Main()

Local a := New A

Local b := Cast(a)

Print(Typeof(b) = Typeof<Object>)

Print(b = Null)

End

However trying overloading the To operator makes the program behave correctly.

Class A
	Method To:B()
		Local b := New B
		b.b = Self.a
		Return a
	End

Class B
Method To:A()
	Local a := New A
	a.a = Self.b
	Return a
End

Class A

Method To:B()

Local b := New B

b.b = Self.a

Return a

End

Class B

Method To:A()

Local a := New A

a.a = Self.b

Return a

End

However I know that the point of the post is to stress test the compiler (I am not trying to patch the code example). But other than that you would consider that as a programming practice, super-casting is always ambiguous, because most of the times you can’t be sure how the derived type translates to their parent.

As a point of reference in C# language this operation is not permitted. So perhaps Monkey would introduce the same restriction, to promote more formal and strict habits.

class A
{
}

class B : A
{
}

class MainClass
{
	public static void Main(string[] args)
	{
		var a = new A();
		var b = (B)a; // <-- This won't happen...
		// System.InvalidCastException has been thrown
		// Unable to cast object of type 'TestSuperCasting.A' to type 'TestSuperCasting.B'.
	}
}

class A

{

}

class B : A

{

}

class MainClass

{

public static void Main(string[] args)

{

var a = new A();

var b = (B)a; // <-- This won't happen...

// System.InvalidCastException has been thrown

// Unable to cast object of type 'TestSuperCasting.A' to type 'TestSuperCasting.B'.

}

January 21, 2018 at 9:29 pm #13157

Mark Sibly

Keymaster

This one seems to have a link to an object instance somewhere in memory, but the displayed value is Null.

Please post runnable examples including Main() – how am I meant to know what goes before that etc?

January 22, 2018 at 5:10 am #13160

Mark Sibly

Keymaster

var b = (B)a; // <– This won’t happen…
> // System.InvalidCastException has been thrown

I’m not too keen on making Cast throw an exception or runtime error, as cast currently doubles as an ‘intanceOf’ style operation, eg: you can go…

If Cast( a ) Print "A as a B!"

Changing this behaviour would break a ton of my own code and would mean I would have to add ‘InstanceOf’ or similar to the language. I’m quite happy with current behaviour.

January 22, 2018 at 12:37 pm #13166

cocon

Participant

I updated the previous post to include the code sample.

January 22, 2018 at 10:21 pm #13170

Mark Sibly

Keymaster

Cheers, I thought that was probably it but for the sake of clarity and to minimize confusion it’s always nice to be as clear as possible.

Same answer though, Cast wont be throwing an exception any time soon. Even if it did, I’d still like something ‘castish’ which is faster than “If i instanceOf T then t:=Cast<T>( i )” which effectively needs to do 2 downcasts.

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derived class as argument for base class and other rules for inheritance

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