Re: Ada is getting more popular!

["J-P. Rosen" <rosen@adalog.fr>]

Le 31/10/2010 06:02, Yannick Duchêne (Hibou57) a écrit :
[...]
> So, shall we define two root types, one with Size and Coordinate, and
> another with Move ?
> 
In my view, it is very important to make the distinction between
(class-wide) subprograms and methods.

A method is bound to a specific type. You don't draw a square the same
way that you draw a circle, each object has its own /method/ of
implementing the abstract notion. OTOH, the algorithm for moving a
square is the same as for moving a circle: it is not a method, but a
subprogram that operates on all graphical objects => perfectly
implemented in Ada with a procedure operating on a class-wide type.

Note that a class-wide subprogram is unique, cannot be redefined, and
you are guaranteed that no descendant of graphical objects can change it
to behave differently.

FWIW, here are my criteria for using inheritance:
1) you have several types, that are too different to be modelled as
variations of a single type (use discriminated types otherwise);
2) however, these types have some common logical properties, with
different implementations
3) you have to maintain an heterogenous container of these various types
4) you want to call the common operations on all objects in the container

Yes, graphical objects meet these criteria. You maintain a list of
objects on the screen, and you want to Paint them all by iterating over
the list.

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           J-P. Rosen (rosen@adalog.fr)
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