[Gambas-user] newbie question virtual classes

[Christopher Brian Jack]

On Wed, 1 Feb 2006, Benoit Minisini wrote:

> > class ListBox;
> > class ListBoxItemIMP;
> >
...
...
> >   ...
> > };
> >
> >
> > -- 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.

You have to admit though I came pretty close with the C++ snippet to what
is happening (in C++ references prevent creation objects).  The only thing
I forgot to do is privitize the copy constructors and the assignment
operators in the 'internal' classes.

> 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:

Yes this is where a limitation in Gambas does indeed necessitate the
special non-creatable classes because there are no "reference" types.
In C++ there is [a reference data type] and you could assign the object
back from the ListView's [] operator to a ListViewItem& but all operations
that would create an object would be prevented by the
constructor, destructor, copy-constructor and assignment operator all
being made private members in the 'internal' class.

Again I think the C++ snippet comes very close in approximation to what
the Gambas scenario is doing (that is, presenting an interface that does
not allow temporary object creations).

Brian