RE: [sv-ec] virtual methods

David,

I think most of the committee understands this well, having experience in OOP. I am a newcomer to OOP, so I was not sure of this.

I think the sentences I originally quoted from 8.19 are very unclear, as you agreed, and should be reworded.

As for my last two questions, I understood correctly and came to the correct conclusions, but I think the LRM should be explicit about them. I had to make deductions based on what the LRM does not say, not based on what it does say.

I will file a Mantis on these items.

I have to teach this material, so it was important to me to understand it myself. Enjoy your vacation.

Thanks very much,
Shalom 

> -----Original Message-----
> From: David Jones [mailto:djones@xtreme-eda.com] 
> Sent: Friday, January 02, 2009 3:16 PM
> To: Bresticker, Shalom
> Cc: sv-ec@server.eda.org
> Subject: Re: [sv-ec] virtual methods
> 
> > Leaf l = new;
> > Derived d = l;
> >
> > 1. From d, can I access a method M or property P that is 
> defined in Leaf, but not in Derived?
> 
> No.
> 
> > I understand that if M or P is defined in both Derived and 
> then overriden in Leaf, then d.P (or M) gives me Derived::P 
> (or M), not Leaf::P (or M), but is d.P (or M) legal if there 
> is no Derived::P (or M)?
> 
> Methods and properties are different, as there is no such 
> thing as a virtual property, so lumping P amd M in the same 
> statement will be confusing. The "virtual" feature works with 
> methods only.
> 
> If you define D::P and then Leaf::P, these are two separate 
> properties. Leaf::P can never be accessed from d.
> 
> In your example, d.M()  will call Derived::M if M is not 
> virtual in Derived, or Leaf::M if it is.
> 
> Any access from d must reference a definition available to 
> Derived, without recourse to Leaf. If you have a member 
> Derived::P then a reference from d will always pick up 
> Derived::P. It can't see Leaf at all. You must have a member 
> Derived::M for a call d.M() to be legal; Leaf::M will be 
> called only if Derived::M exists and is virtual.
> 
> >
> > 2. Declaring a method as virtual in Derived (e.g., pow()) 
> means that d.pow() will give me Leaf::pow() instead of 
> Derived::pow(). Is there any equivalent of a virtual method 
> for properties? That is, if property P is defined in both 
> Derived and Leaf, is there any way for d.P to give me 
> Derived::P instead of Leaf::P? There does not seem to be.
> 
> There is no equivalent.  You can "fake it" by defining a 
> virtual method getSomeProperty() in Derived, and then 
> implementing it appropriately in both Derived and all its 
> subclasses. This is common in OOP.
> 
> Would it help you or the committee to understand this better 
> if I prepared a document explaining how this stuff works 
> under the hood?
> Sometimes understanding the implementation helps to 
> understand the concept. If you want such a document then you 
> MUST reply IMMEDIATELY:
> I am 9 hours behind Israel timewise, and I'm about to leave 
> on a 2-week vacation to Hawaii, so if you want this I will 
> have to get to it today.
> 
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Intel Israel (74) Limited

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