Re: [Boost-users] [review] Fast track review of Boost.Utility/Singleton

From: Tobias Schwinger (tschwinger_at_[hidden])
Date: 2008-01-18 04:17:05

Nat Goodspeed wrote:
> Tobias Schwinger wrote:
> 
>> thanks for your very positive review.
> 
>    Thanks for a well-designed library!
> 
>>>    Very little is said about BOOST_SINGLETON_PLACEMENT and 
>>> BOOST_SINGLETON_PLACEMENT_DECLARATION. The reference material mentions 
>>> them without any explanation.
>> That "Singletons and dynamic libraries" section explains their purpose.
>>
>> What exactly is missing?
>>
>> I'm not too fond of specifying what those macros expand to and I don't
>> really want to mention them in the introductory section either, as for
>> me they are kind-of workarounds for the imperfect ABIs we have to deal
>> with.
> 
>    Agreed: I don't want you to document the expansions. But the entirety 
> of the reference coverage consists of the words "Placement declaration" 
> and "Placement definition," with examples showing how to write them. 
> What effects can I expect?
>    The "Singletons and dynamic libraries" section says: "There must be 
> at most one single instance of a Singleton, even if it is exposed by a 
> dynamic library... Keeping the former can be problematic if the 
> management code gets inlined into the client or if symbols are not being 
> merged automatically. As a solution, this library provides two macros, 
> BOOST_SINGLETON_PLACEMENT and BOOST_SINGLETON_PLACEMENT_DECLARATION to 
> compile the effective code into a specific source file."

>    These macros are a workaround for a problem. The problem appears to 
> be more than one instance of the same singleton class in a given 
> process. 

That's one symptom.

> How would I know I'm susceptible? What's an example scenario? 
> On a lame platform, two or more dynamic libraries in the process each 
> #include a header for a particular singleton class definition, and 
> contain 'instance' invocations? How would I know my platform is lame?

The issue deserves careful investigation even with a "non-lame" ABI in
place :-).

>    What's the solution: putting BOOST_SINGLETON_PLACEMENT_DECLARATION 
> inside the header class definition, and BOOST_SINGLETON_PLACEMENT inside 
> exactly one of the consumer dynamic libraries? What would happen if both 
> libraries contained BOOST_SINGLETON_PLACEMENT? If I'm writing a dynamic 
> library for use by others, and I use someone else's singleton class 
> containing BOOST_SINGLETON_PLACEMENT_DECLARATION, how would I decide 
> whether to include BOOST_SINGLETON_PLACEMENT in my library? Or should 
> BOOST_SINGLETON_PLACEMENT be used in the main program instead?

It should probably always be used if the Singleton is in some way part 
of a dynamic library.

>    For me, the documentation doesn't yet answer these two key questions: 
> (1) How would I know I need BOOST_SINGLETON_PLACEMENT? (2) Where should 
> I put it?

OK, I'll address them when I revise the docs.

Regards,
Tobias