Re: Ada-Singleton-Why does it work like this?

[Jean-Pierre Rosen <rosen@adalog.fr>]

patrick.gunia@googlemail.com a �crit :
> [...] When you say
> that Singleton isn�t necessary in Ada this might come from the fact
> that it can�t be implemented as in other languages, though using such
> a construct might increase readability of the code.

I think there is a contradiction between "class" and "singleton". The
term "class" is used, because it includes a set of objects that are
equivalent for some point of view, i.e. a mathematical equivalence class.

Now, if the class gathers only one object, the equivalence relationship
does not make much sense...

In some cases, you need a stand-alone object. Fine, but you don't need
to define a class for that. Note that Ada allows you  to define
singleton arrays, tasks, and protected objects without an explicit type
to enforce that notion.

Some languages do not allow you to define an object unless it belongs to
a class; that's why you have to jump through complicated hoops to define
those "singleton classes", i.e. structures that are made to hold many
objects, but with appropriate design patterns to make sure there is only
one. It is simply not necessary in Ada. Note that it /can/ be
implemented like in other languages, and I don't think it would be more
complicated - just that the (unnecessary in the Ada case) complication
shines more brightly.

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