Gennadiy, thanks for your through and through negative review. Gennadiy Rozental wrote:
"John Torjo" <john.groups@torjo.com> wrote in message news:478B978A.6090503@torjo.com...
Hi all,
* What is your evaluation of the design?
In short: in places too complicated, in places plain wrong.
In details:
1. mutexed_singleton
This should be singleton<synchronized<T> >. Current design mixes two orthogonal concepts: uniqueness of class instance and class methods synchronization and this is obviously wrong.
2. thread_specific_singleton.
The same as above. This should be singleton<thread_specific<T> >. Current design mixes two orthogonal concepts: uniqueness of class instance and thread specific object storage.
One could implement what you propose. It would probably even make things easier on my side. It complicates client code and compromises the replaceability of all the Singleton templates, which is a mistake.
3. public construction and boost::restricted
I don't like this solution and very much prefer private constructor with friend declaration (especially if encapsulated into single macro). This will be a never ending source of confusion (what is boost::restricted, can I have restricted access, how do I create restricted instances etc) and doesn't really protect - once can still create instances of my class with valid C++ code.
You're still free to use a private constructor and a friend declaration.
4. DLL support
Explanation of DLL issues in docs is very unclear. No examples provided. No tests showing that solution actually works. Accordingly I can't really comment on it, but from what I got I really doubt it.
The test just hasn't been automated yet. And, yes, the documentation can use some improvement.
5. instance proxy.
Given complete absence of rationale of this design, I can't really devise why is was chosen, but it seems wrong. We end up with pricy complication with unclear purposes. What's wrong with just returning T& from instance method? The same time we can get rid of all this member_dereference staff and it's limitations.
What limitations are you talking about? The ability to have automatic locking when using a member pointer?
6. lease.
Pretty much the same as above. synchronized<T> should have it's own lock facility. Otherwise instance() should be straightforward access to the instance and no need for any proxies/leases.
Again, fails to see the advantage of a uniform interface.
7. MT support
Setting aside mutexed and thread specific singletons covered above, regular singleton handling of MT is wrong as well. In simple words my primary complain: you can't use define based solution for switching MT/non MT. It should be template parameter. As of now there is no singleton that can work in presence of threads but without disregard to them.
I can't get your point. There probably is none.
8. No Meyer's singleton
What if do not want all this manager staff?
What's "all this manager staff" and what's a "Meyers' singleton"? Regards, Tobias