Re: What is your opinion on Global Objects?

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

Le 21/11/2014 10:01, Dmitry A. Kazakov a écrit :
> On Thu, 20 Nov 2014 22:35:16 +0100, J-P. Rosen wrote:
> 
>>> 1. You are hardcoded to a particular implementation (i.e. no dispatching)
>> Yes, it is more static, therefore safer and more easily provable.
> 
> This is a bit contradictory. Proofs as such are always against some stated
> things. That already presumes several instances. Some instances may
> withstand the proof some don't. Thus the idea of proving an inherent
> singleton looks somewhat dubious. Singleton is only in the usage not a
> property.
Not at all. Where we differ is that you are thinking in terms of classes
first, then instances. You see the properties at class level, and then a
singleton (a class with a single instance) makes little sense.

I am thinking in terms of objects first. Objects have properties. If
several objects share a common behaviour, then I group them into
"equivalence classes", and the common properties of the objects become
the properties of the class. If you think objects first, singletons make
perfect sense.

> And you forgot about reuse. A singleton implementation may use some other
> types implementations or provide its implementation for reuse.
You implicitely think of reuse by inheritance. I favor reuse by composition.

> 
>>> 3. The majority of the program becomes impure
Not really. If your problem requires a global state, then it is
"impure". Whether you use global variables in a package or a global
object gathering the data that you pass to subprograms does not change
anything.

>>> 5. Everything is compile-time constant (i.e. no creating or deleting things dynamically).
>> I would regard this as a huge benefit - dynamicity has always been
>> opposed to simplicity and provability.
> 
> Huh, that reminds me of a relatively fresh discussion about termination of
> a library-level task in Ada. A typical singleton, isn't it? Yet impossible
> to terminate on application exit! 
??? Select ... or terminate works perfectly well for library tasks. I do
that all the time.

-- 
J-P. Rosen
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