Re: [boost] [x-files][singleton] incomplete type problem

From: Michael Marcin (mmarcin_at_[hidden])
Date: 2007-12-03 06:10:11

Tobias Schwinger wrote:
> Michael,
> 
> Michael Marcin wrote:
>> I ran into a compiler problem using the singleton library from the vault.
>>
>> singleton's constructor and destructor use instance_proxy before it is 
>> defined. The trivial fix is to make the definition of the constructor 
>> and destructor inline function at the bottom of the file (i.e. after 
>> instance_proxy is defined).
> 
> Actually, that shouldn't be a problem since 'singleton' is a template 
> and things are complete once it's instantiated.
> 

I think the compiler is allowed to check for an incomplete type before 
it is instantiated. I believe this is the same problem that fusion 
as_vector had a while ago.

http://article.gmane.org/gmane.comp.lib.boost.devel/166502

> Got some code that fails to compile?

Yes,

#include <boost/utility/singleton.hpp>

> 
>> singleton_manager.hpp causes warning under msvc 8.0 in the 
>> singleton_manager::cleanup function:
>>
>>   warning C4706: assignment within conditional expression
>>
>> for
>>
>> while (!!(i = ptr_instance->ptr_first))
>>
>> if this could be rewritten without much effort to silence the warning it 
>> would be nice.
> 
> Yes (where interestingly the compiler could figure out this is not a 
> typo here because of the '!!')...
> 
> Maybe
> 
>      while ((i = ptr_instance->ptr_first) != 0l)
> 
> will do the trick...

That works, or

context* i = ptr_instance->ptr_first;
while ( i )
{
    context* next = i->ptr_next;
    i->fnc_dtor(i);
    i = ptr_instance->ptr_first = next;
}

> 
>> Nit - there seems to be a lot of extraneous inline keywords for 
>> functions defined inside the class definition.
> 
> ...intentionally, because some compilers (under certain configurations) 
> distinguish between "implicit" inline and "inline by keyword".

Hmm didn't expect that since the standard says its the same as declaring 
it inline just after the class's closing ';' IIRC. Out of curiosity do 
you remember what compilers do this?

Thanks,

Michael Marcin