Re: [boost] [config] request for BOOST_NO_FWD_STD_DECLARATION

Subject: Re: [boost] [config] request for BOOST_NO_FWD_STD_DECLARATION
From: John Maddock (boost.regex_at_[hidden])
Date: 2011-02-16 08:40:44

>>>> libc++ does not allow forward declarations of types for std:: , as it 
>>>> uses
>>>> inline namespace to allow versioning.
>>>>
>>>> Forward declarations occur in many libraries (config, fusion,
>>>> interprocess). In each case I would like to just include the actual 
>>>> headers.
>>>>
>>>> I could just #define all cases for LIBCPP, but other libraries might 
>>>> move
>>>> to using inline namespaces in future, so I could introduce a macro?
>>>
>>> I think that sounds a reasonable addition, yes.  Do you have patches?
>>
>> IMO it should be a macro that's defined when forward declaration is
>> possible rather than when it isn't. That way the default would be the
>> safer option. I now think it was a mistake to forward declare for
>> unknown setups in 'container_fwd.hpp'.
>
> I would be happy with that, except I note that (for example) boost fusion 
> currently unconditionally forward declares std::pair, and every compiler 
> in the regression test (except clang with libc++) passes the std_pair 
> test. Therefore the macro would have to be added to every other compiler, 
> else they would become less efficent.

Note that since this is std lib issue, and since there are only a few std 
lib's out there, it's not that much of a change to Boost.Config to add this 
to every std lib config except libc++ (if indeed that's what's required).

As others have noted, in principle this isn't std conforming, just useful.

John.