Re: [boost] Threadsafe singleton - questions

From: Tobias Schwinger (tschwinger_at_[hidden])
Date: 2007-08-24 19:05:43

Robert Ramey wrote:
> c) the synopsis specifies
> 
> static unspecified instance;
> 
> But the Expression Symatics specifies D::instance->m which seems
> inconsistent to me.  

Why? 'm' is a member of the derived class. Obviously it's a user-defined 
thing, so I can't specify what it is.

> I Rather expect the synopsis to say
> 
> D:instance.m
> 
> or
> 
> D::get_instance().m

D::instance is a pointer-like proxy. We can't overload operator., we 
can't expose the object directly (to ensure initialization) and we can't 
implement automatic locking if we expose the object by returning it.

Nice bonus: There seamless interoperability with anything that accepts a 
smart pointer.

> d) what about access to object which don't require thread locking - for 
> example
> const members.  Do they need locking/leasing also? I would guess not but?

As explained in my previous reply 'mutexed_singleton' locks every access 
(imagine you're iterating a list to get the const result - if that list 
is not locked it might get modified and the program might e.g. access 
freed memory) while 'singleton' puts synchronization in the 
responsibility of the user. If your objects are immutable (all member 
functions are const and no mutable members are present) you can just use 
'singleton' and no locking is done.

> e) What does D::instance return ? a reference or pointer to the single 
> instance of D
> or something else. I would expect the former, but the synopsis contains
> 
> static "unspecified" instance

That's right.

> rather than
> 
> static T instance as I would expect.

No, it's a proxy of unspecified type.

Implementation detail: That proxy's operator-> returns T* for 
'singleton' and yet another proxy for 'mutexed_singleton'.

> f) can "const" static objects be created?

I'm not sure I've tried, yet.

What would it be good for (as it doesn't make a difference if the 
singleton instance is immutable and it would yield inaccessible code if 
it wasn't)?

Maybe it should be possible for completeness...

> 
> g) what happens if same interface used by different DLLS.  Is one instance
> still created?

Yes.

Thanks for your comments.

Regards,
Tobias