Re: [cpp-netlib-devel] MSVC Problem with 'extern' placeholders

[Christian H. <chh...@gm...>]

Dean,

I just put a typedef on front of:

typedef to_upper_placeholder to_upper_;
typedef selectors::source_selector source_;
typedef selectors::destination_selector destination_;

I changed the test code to:

msg << transform(to_upper_(), source_());

And there were no problems. Does that makes sense to you?

Christian


On 6/11/07, Christian Henning <chh...@gm...> wrote:
> Dean, if you remove the extern specifier it compiles fine on my
> machine. I'm using MSVC 7.1. Why did you decided to implement it this
> way? Is it possible to just typedef this information?
>
> Thanks,
> Christian
>
>
>
> On 6/11/07, Dean Michael Berris <mik...@gm...> wrote:
> > Hi Everyone,
> >
> > I've only recently been developing with MSVC, and while I've been
> > trying to implement the transformations to the message class I hit a
> > nasty snag. Let me identify what the problem is.
> >
> > First, in boost/network/message/transformers/to_upper.hpp I have the following:
> >
> >     struct to_upper_placeholder {
> >         template <class Selector>
> >             struct type : public impl::to_upper_transformer<Selector> { };
> >     } ;
> >
> >     extern to_upper_placeholder to_upper_;
> >
> > I also have in boost/network/message/transformers/selectors.hpp:
> >
> >     namespace selectors {
> >         struct source_selector { };
> >         struct destination_selector { };
> >     }; // namespace selectors
> >
> >     extern selectors::source_selector source_;
> >     extern selectors::destination_selector destination_;
> >
> > What I'm concerned about, is that the MSVC linker keeps asking for
> > where to look for these externs when I have the following function
> > template in boost/network/transformers.hpp :
> >
> >
> >     template <class Algorithm, class Selector>
> >         inline impl::transform_impl<Algorithm, Selector>
> >         transform(Algorithm, Selector) {
> >             return impl::transform_impl<Algorithm, Selector>();
> >         };
> >
> > This is used in the unit test (which fails to compile in MSVC 8)
> > libs/network/test/message_transform_test.cpp :
> >
> >     using namespace boost::network;
> >
> >     message msg;
> >     msg << source("me");
> >     BOOST_CHECK_EQUAL ( source(msg), "me" );
> >     msg << transform(to_upper_, source_);
> >     BOOST_CHECK_EQUAL ( source(msg), "ME" );
> >
> > Clearly, the 'transform' template method only requires the type
> > information of to_upper_ (an extern to_upper_placeholder) and source_
> > (an extern selectors::source_selector) but MSVC seems to require that
> > they be linkable from somewhere.
> >
> > Any thoughts on the matter? I haven't tried this in GCC yet, but I'm
> > confident that GCC would know that I don't really need to link to
> > these instances -- that I just need the type to be deduced in the
> > 'transform' template and not care about the actual instances.
> >
> > Help and insight would be most appreciated.
> >
> > Thanks and hope to hear from any one of you soon!
> >
> > (Please update to Revision 17 to get the latest unit tests and
> > implementations from trunk/)
> >
> > --
> > Dean Michael C. Berris
> > http://cplusplus-soup.blogspot.com/
> > mikhailberis AT gmail DOT com
> > +63 928 7291459
> >
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