nicolas cellier wrote:
> James Foster a écrit :
> > 
> > On Nov 25, 2008, at 7:45 PM, Igor Stasenko wrote:
> >> My understanding of inheritance is different, in short:
> >> A subclass of particular class is a _specialization_ of base class,
> >> not  _expansion_.
> > 
> > In this context, I sometimes wonder if Square should inherit from 
> > Rectangle (a specialization in which width and height are equal), or 
> > Rectangle should inherit from Square (adding an instance variable). Am I 
> > right that you would have Square inherits from Rectangle (Square being 
> > more specialized)? But then it feels like we are wasting an instance 
> > variable (since Rectangle would have two).
> 
> Beware, you're dangerously sliping to multiple inheritance because your 
> Square might also be a lozenge :)

With the Rectangle as subclass of Square option this wouldn't be a
problem - you would just have two different subclasses of Square.

Could Rectangle be a subclass of Square? Sure:

- Square instance variables: center, size, orientation

- Rectangle adds this instance variable: aspectRatio

We can make Ellipse a subclass of Circle using the same style. We can
even have an #aspectRatio method in Square and Circle which always
returns 1 and then we can move some of the more general code up in the
classes hierarchy if we want to.

For raster graphics it is convenient to define Rectangles as always
parallel to the screen axis so that just two points are enough to fully
identify them. Perhaps calling them RasterRects instead would have made
us think more clearly about them. We later moved to vector graphics
(Balloon) but were stuck with the historic baggage.

-- Jecel
P.S.: I am aware that even in this scheme you might want Parallelogram
to inherit from both Rectangle and Lozenge (actually, Rhombus is more
generic) and then you have the multiple inheritance problem again. Which
is what Traits are for...