Re: [Boost-users] Having trouble includingmystaticlibrary(whichuses regex) in my MFC app

From: John Maddock (john_at_[hidden])
Date: 2007-05-10 04:48:19

Martin Zeng wrote:
> Hi John,
>
> Thanks for you help, and now my MFC is compiling without the link
> error. There are still some things I would still like to know.
>
> I noticed that just by including the libs/regex/src/winstances.cpp
> file is enough to get rid of the the error
>> LNK2005: "public: static unsigned int __cdecl
>> std::char_traits<unsigned short>::length(unsigned short const *)"
>> (?length@?$char_traits_at_G@std@@SAIPBG_at_Z) already defined in
>> msvcprtd.lib(MSVCP71D.dll)
>
> so I was just wondering is it ok just to add only the winstances.cpp
> to my source directory as opposed to adding the entire libs/regex/src
> directory?

Probably :-)

> I ask this because my static library was created with using stdafx.h
> (precompiled header), so I had to copy over the winstances.cpp file
> into my project directory and put in #include "stdafx.h" at the top as
> opposed to just referring to the libs/regex/src/winstances.cpp file
> directly. I prefer not to do this to every single file in the
> libs/regex/src directory if I don't need to include them anyway.
>
> Also, what is the cause of this link error. Does it have anything to
> do with VC++ compiler and how it ignores inline directives in debug
> mode as this person seems to suggest:
> http://aspn.activestate.com/ASPN/Mail/Message/2509074
>
> In my experience, it seems that by having:
> boost::wregex expression(L"[ab]");
> will cause the link error to happen. Is there going to be a fix for
> this problem?

I hope it's fixed for Boost-1.34, but it's hard to test every possible build 
combination, so it's hard to be 100% sure :-(

The problem is caused by certain template instances being declared as extern 
in the std lib headers (they're instantiated in the C++ runtime). 
Unfortunately, the std lib headers only declare *either* the unsigned short 
or wchar_t versions of these as external, even though the std lib actually 
contains *both*.

In order to be link-compatible with as many build options as possible 
Boost.Regex instantiates *both* unsigned short and wchar_t variants of it's 
templates, but then when you build with certain specific build options this 
causes a conflict with the std lib :-(   Basically I've made life easier for 
90% of users, at the expensense of the 10%.  The problem could be solved 
reduced but not completely solved by not putting any template instances in 
the lib, but that would be at the expense of some code-bloat and much longer 
compile times - currently compiling with Boost.Regex placed in a 
pre-compiled header is more or less instantanious, and I'd like to keep that 
if at all possible.

HTH, John.