[Gambas-user] newbie question virtual classes

[Benoit Minisini]

On Wednesday 01 February 2006 07:07, Christopher Brian Jack wrote:
> On Tue, 31 Jan 2006, johnf wrote:
> > > Laurent
> >
> > OK so I can - but do not need to use it.  All right then what do doc's
> > mean showing the virtual class.  As you can see I really don't understand
> > the Doc's reference to the virtual class.  To me a virtual class is a
> > class that describes an object for use in other objects.  Helper class or
> > a class I inherit.  Anyway I don't understand what is being said in the
> > doc's John
>
> Hmm are you coming to Gambas from a C++ background?  You can consider
> "virtual class" to mean a class with pure virtual methods and private
> constructor/destructors (which yields a virtual class that you cannot
> create or copy but can pass by reference) but that has friends within
> lower level components that implement the class (it can use the class as a
> base).  Thus, though you cannot create the class directly or use it as a
> base class in your Gambas programs, the friends that are implementing the
> class can be accessed as ListBox.ListBoxItem[n] where the "virtual" class
> here is only defining an interface to access the items of the listbox.
>
> This bit of C++ (pardon me in advance if some of my syntax is off)
> attempts to show what's going on.  It is possible in C++ to return a
> reference to an interface whose implentor and interface objects are not
> directly instantiable or useable as a base class using inheritence:
>
> class ListBox;
> class ListBoxItemIMP;
>
> class _ListBoxItem {
>   friend class _ListBoxItenIMP;
>   private:
>     _ListBoxItem() {}
>     virtual ~_ListBoxItem {}
>   public:
>     std::String Text()=0
>   ...
> };
>
> class _ListBoxItemIMP : private _ListBoxItem {
>   friend class ListBox;
>   private:
>     Listbox& host;
>   private:
>     _ListBoxItemIMP() {...}
>     virtual ~_ListBoxItemIMP {...}
>     SetParent(ListBox &parent) : host(parent) {...}
>   public:
>     std::String Text() {...}
>     ...
> };
>
> class ListBox : public Control {
>   public:
>     ListBox(Control &parent) {...}
>     virtual ~ListBox() {...}
>   private:
>     std::vector<_ListBoxItemIMP> data;
>   pubilc:
>     ListBoxItem& operator [] (int idx) {return data[idx];}
>     int Count() {return data.length();}
>   ...
> };
>
>
> -- Christopher Brian Jack
>

'Virtual' class in Gambas has nothing to do with 'virtual' class in C++. I 
admit I badly chosen the word, but C++ did too, in a different way.

A virtual class is actually a way for an object of a real class to look like 
another class, said virtual. Maybe 'pseudo-class' would be a better word.

You cannot instanciate a virtual class. You can only use it temporarily inside 
an expression.

For example: MyTreeView[key] returns a '.TreeViewItem' from its key, but you 
cannot put it inside a variable. You can only use it inside an expression, 
like MyTreeView[key].Text, or inside a WITH...END WITH control structure:

WITH MyTreeView[key]
  PRINT .Text
  PRINT .Key
  PRINT .Parent.Text
  ...
END WITH

I did it to avoid object allocations. Gambas creates no object when you access 
an item inside a treeview, so things are faster.

As the documentation is automatically generated, the documentation of 
properties that use virtual class is not very understandable at the moment. 
Sorry for the inconvenience!

I hope it is clearer now :-)

Regards,

-- 
Benoit Minisini