Re: [boost] [any] new version

Subject: Re: [boost] [any] new version
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2011-09-01 13:00:38

On 09/01/2011 08:16 AM, Ilya Bobir wrote:
> On Wed, Aug 31, 2011 at 10:26 PM, Andrey Semashev
> <andrey.semashev_at_[hidden]>  wrote:
>> [...]
>>
>> In addition, on most platforms there will be multiple instances of
>> type_id<T>::value for a single given T if the application uses type_id<T>  in
>> multiple modules.
>> [...]
>
> Can you elaborate on this?
>
> Are not you supposed to provide an explicit initialization for the
> type_id<T>::value for every T you are using and that would be the only
> instance?
>
> I can not find anything in the standard that would contradict your
> statement, at the moment, except that if you treat a template
> instantiation as a class in this case you may say that 9.4.2 para. 5
> applies to template instantiations and then there should be no more
> than once instance of a static member for a particular set of template
> arguments.

Yes, except that the Standard doesn't specify how that translates to a 
multi-module application (i.e. which consists of exe and dll/so/dylib). 
Because the Standard doesn't know of modules.

Suppose you use boost::any with types int and string in your main 
executable and in one of the dlls linked in. Exe and dll will have 
separate instances of both type_id<int>::value and 
type_id<string>::value, each of them being initialized independently. 
What's worse, exe and dll will have separate next_id() functions with 
its local static counter. Depending on the circumstances, 
type_id<int>::value in exe may become initialized equal to 
type_id<string>::value in dll, for instance. Needless to say what will 
happen if you try to interoperate with boost::any's created in different 
modules.

In practice, I'm aware of only one platform (Linux), which will by 
default attempt to maintain a single instance of these ids, but even 
there care must be taken.