Re: exporting data from a dll - unresolved externals

From:

"Ben Voigt [C++ MVP]" <rbv@nospam.nospam>

Newsgroups:

microsoft.public.vc.language

Date:

Tue, 8 Jan 2008 09:22:36 -0600

Message-ID:

<O16JBogUIHA.4684@TK2MSFTNGP06.phx.gbl>

"2b|!2b==?" <user@domain.invalid> wrote in message
news:_9udncYmgLGlah_anZ2dnUVZ8uWdnZ2d@bt.com...

I have a DLL project that I need to export a data variable from. I have
defined the variable in a .cpp file in the project like this

//C++ file (DLL Project)

class MYPROJ_API data_type
{
//Impl here ...
};

MYPROJ_API data_type exported_data;

//header file (project that consumes DLL)
extern MYPROJ_API data_type exported_data ;

It looks to me like the type is incomplete here, because the class
declaration isn't seen from the header file.

where MYPROJ_API is amacro that handles declspec import/export

dllexport of C++ classes is a bad idea. Optimally DLLs should only export
functions. You can pass around pointers to objects as long as they are
accessed through virtual functions (the client should only see an interface
definition with pure virtual functions and no data). Also memory should
always be freed in the same module that allocated it.

I checked the DLL project to ensure that the symbols are exported
correctly in the generated library - and they all are there - including
the exported data - yet when I try to build the other dependent project, I
am getting linkage errors (unresolved externals).

BTW, I have checked my library paths etc, and all is in order - anyone
come accross this before? - what am I missing?