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From: Petr Kocmid (pkocmid_at_[hidden])
Date: 2000-06-01 16:21:20

> From: Branko Èibej [mailto:branko.cibej_at_[hidden]]
> First, I'm not convinced that it's a good idea to be able to instantiate
> a class both as a singleton and an "ordinary" object. If interface
> compatifility is what you're after, there are other (better) ways to
> do that.

Well, I expected just that comment! Glad someone read it at all ;-).
The basic idea on behind of allowing any class to my singleton_ptr
mechanism is the non-invasiveness. Take it positively: I have no
special requirements on classs which instance is to be taken as a
There are many conditions when such non-invasiveness comes handy:
legacy code, third party dll classes or so.
What better way you have on mind exactly?

> Petr Kocmid wrote:
> > - idea to have a singleton object of any arbitrary type which will come
> > on my mind later, declared by simple template expansion on that type
> > and intermix singleton functionality with class hiearchy architecture.

> If I understand your source correctly, you can't define a hierarchy of
> singletons, something like this:
> class A { virutal void foo() {/*...*/} };
> class B : public A { virtual void foo {/*...*/ };
> // We want to create singleton B ...
> singleton_ptr<B>();
> // ... and access it as an A
> singleton_ptr<A>.foo();

singleton_ptr<A>->foo() exactly. Yes, it calls A::foo(), not B::foo().
In what you write, singleton_ptr<A> and singleton_ptr<B> will made
even two different instances!
In the example I presented a way, how to create a singleton-like hierarchy:
by having singleton_ptr embedded in base class, you can derive and
virtualize from the base class at will. Singleton part of object and
hierarchy part of object are quite separate things.

> This will always call A::foo. It'd be nice to have a way to "register"
> a derived class singleton and use the base class interface to access it.

Actually I am using a different mechanism for that sort of things.
I call it DynamicClass. That's the generic object, which is allowed
to add members and methods dynamically with it's own virtualisation
scheme. But that's nothing to do with singletons! ;-)
But yes, it is possible to extend singleton_factory mechanism to get
some sort of virtual method registration, but I see it as overkill.

> What system are you using g++ on, and which version? As far as I know,
> recent versions shouldn't have that problem.

2.95.2 on debian and cygwin. I am sure I had some bloat somewhere,
but it's long time. Now you made me unsure if it was not some 2.8x...

Petr Kocmid (Koèmíd, if your mailer shows that damned diacritics properly)

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