better support for exporting symbols from DLL

[arvid@…]

Currently asio/detail/config.hpp only supports exporting symbols from compilers that supports declspec (i.e. msvc). In order to support many more compilers, including GCC (and gcc compatibles) and sun studio etc. The config header could be simplified to use boost config's BOOST_SYMBOL_EXPORT/IMPORT macro.

Patch against trunk is attached.

[arvid@…]

patch to support exporting symbols in compilers other than msvc

[arvid@…]

actually. with some more testing on darwin, it turns out there's one more change for this to work with GCC. the io_service::service type is not exported, only some of its member functions. My understanding of MSVC's export semantics is that all type metadata (specifically type_info and vtables) are always exported, whereas on GCC, you have to explicitly export a type for its metadata to be exported. I think the io_service::service type should be exported for this reason (otherwise I get a link error failing to find its type_info, presumably originating from a typeid() call inside an asio header).

I'm attaching an updated patch.

[arvid@…]

updated patch to also export io_service::service

[arvid@…]

for reference, this is the error message I get if I don't export io_service::service

Undefined symbols for architecture x86_64:
  "typeinfo for boost::asio::io_service::service", referenced from:
      typeinfo for boost::asio::detail::service_base<boost::asio::socket_acceptor_service<boost::asio::ip::tcp> >in setup_transfer.o
      typeinfo for boost::asio::detail::service_base<boost::asio::deadline_timer_service<libtorrent::ptime, boost::asio::time_traits<libtorrent::ptime> > >in setup_transfer.o
ld: symbol(s) not found for architecture x86_64