Re: Exporting CString from a DLL

From:

"EricW" <empty@empty.com>

Newsgroups:

microsoft.public.vc.language

Date:

Thu, 28 Feb 2008 15:24:37 -0500

Message-ID:

<OCvw6gkeIHA.4144@TK2MSFTNGP05.phx.gbl>

Hi Doug,
I went with the solution of deriving a new class from CString because the
MSDN article below suggested that is the best approach. It also seemed to
suggest the changing of atlstr.h/afxstr.h was required to make this work
completely which I'd greatly prefer not to do but without that you'll get
duplicate definitions. Also I did do something like you suggested in making
a macro for __declspec( ? ) originally I just left it out for time. Anyway,
I think I can live with the solution of modifying atlstr.h for now until
something better comes along.

Thanks,
EricW

http://msdn2.microsoft.com/en-us/library/ms174286(VS.80).aspx

"Doug Harrison [MVP]" <dsh@mvps.org> wrote in message
news:u23es3leac5lr2l8ppdsmnng9nj02v0goa@4ax.com...

On Thu, 28 Feb 2008 11:38:07 -0500, "EricW" <empty@empty.com> wrote:

Hi All,
I'm trying to use a CString variable in a class that is exported from a
DLL and I'm getting the linker errors below. I changed the CString
declaration to CStringEx and exported that class like below which
resulted
in the errors. If I take the regrettable step of commenting out the
declaration of CString, CStringA and CStringW in altstr.h (I'm not using
MFC
at all) and replacing it with the corresponding CStringExA/W it seems
better, but that is solution I'd rather not do. Is there any other way to
use CString in a DLL.

Thanks,
EricW

class __declspec( dllexport ) Test
{
public:
CStringWEx m_strName;
};

CStringWEx is a class I derived from the base CString classes.

ServerEngine.lib(ServerEngine.dll) : error LNK2005: "public: __thiscall
ATL::CStringT<wchar_t,class ATL::StrTraitATL<wchar_t,class
ATL::ChTraitsCRT<wchar_t> > >::~CStringT<wchar_t,class
ATL::StrTraitATL<wchar_t,class ATL::ChTraitsCRT<wchar_t> > >(void)"
(??1?$CStringT@_WV?$StrTraitATL@_WV?$ChTraitsCRT@_W@ATL@@@ATL@@@ATL@@QAE@XZ)
already defined in AdminMsgs.obj

ServerEngine.lib(ServerEngine.dll) : error LNK2005: "public: __thiscall
ATL::CSimpleStringT<wchar_t,0>::operator wchar_t const *(void)const "
(??B?$CSimpleStringT@_W$0A@@ATL@@QBEPB_WXZ) already defined in
AdminMsgs.obj

ServerEngine.lib(ServerEngine.dll) : error LNK2005: "public: class
ATL::CStringT<wchar_t,class ATL::StrTraitATL<wchar_t,class
ATL::ChTraitsCRT<wchar_t> > > & __thiscall ATL::CStringT<wchar_t,class
ATL::StrTraitATL<wchar_t,class ATL::ChTraitsCRT<wchar_t> >

::operator=(class ATL::CStringT<wchar_t,class

ATL::StrTraitATL<wchar_t,class ATL::ChTraitsCRT<wchar_t> > > const &)"
(??4?$CStringT@_WV?$StrTraitATL@_WV?$ChTraitsCRT@_W@ATL@@@ATL@@@ATL@@QAEAAV01@ABV01@@Z)
already defined in ErrorLogOptionsDlg.obj

ServerEngine.lib(ServerEngine.dll) : error LNK2005: "public: __thiscall
ATL::CStringT<wchar_t,class ATL::StrTraitATL<wchar_t,class
ATL::ChTraitsCRT<wchar_t> > >::CStringT<wchar_t,class
ATL::StrTraitATL<wchar_t,class ATL::ChTraitsCRT<wchar_t> > >(void)"
(??0?$CStringT@_WV?$StrTraitATL@_WV?$ChTraitsCRT@_W@ATL@@@ATL@@@ATL@@QAE@XZ)
already defined in AdminMsgs.obj

http://msdn2.microsoft.com/en-us/library/ms174286(VS.80).aspx

To begin, use the following exporting technique. Replace your direct use
of
the __declspec with X_EXPORT, replacing "X" with the name of your DLL,
making sure it has a reasonable chance of being unique:

#ifdef COMPILING_X_DLL
#define X_EXPORT __declspec(dllexport)
#else
#define X_EXPORT __declspec(dllimport)
#endif

Then #define COMPILING_X_DLL only in that DLL's project. This method can
be
used by multiple DLLs, and since the macro names are unique, they don't
conflict.

Also, CString has no virtual functions and thus is not a good candidate
for
public derivation. Consider using plain old CString and extending its
interface through non-member functions.

Finally, if you want to stick with CStringWEx, try to reproduce the
problem
in a minimal DLL consisting of two files, a.cpp and b.cpp, which you can
compile at the command line with:

cl -LD -MD -D_AFXDLL a.cpp b.cpp

If you don't discover the solution along the way, post the files here,
including the definition of CStringWEx.

--
Doug Harrison
Visual C++ MVP