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

From: Nat Goodspeed (nat_at_[hidden])
Date: 2008-01-16 09:28:58

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. 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?
   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?
   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?