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C++/dll exported functions


I want to be able to export functions that would accept strings as parram and return strings from c++ dlls that i would use from a vb application...

_declspec(dllexports) or a def file works fine but i seem to have problems with exporting the string values.

BSTR and _bstr_t seems not to be working.


I presume you can in fact call the C/C++ functions from VB but do not get the results you were hoping for.

Now I should point out that I am _not_ a VB or VB.NET expert - that is why I am in the C++ expert category <grin>.

Which brings me to my next point. Ask yourself which version of VB you are using: VB traditional or VB.NET. If it is the latter then I suspect you will have to use the .NET managed types and not good old BSTRs.

If you are using traditional VB then check the MSDN library knowledgebase article Q187912 (and maybe also Q142840). Remember, the MSDN library can be found online at

I presume from what article Q187912 says you could also use BSTR in place of LPCSTR - but you might like to try it their way first just to see if it fixes your problems. I suspect in this case your problem may be that you are not passing by reference correctly - you forgot to pass by BSTR * or LPCSTR *, however you do not provide enough detail for me to say for sure.


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Ralph McArdell


I am a software developer with more than 15 years C++ experience and over 25 years experience developing a wide variety of applications for Windows NT/2000/XP, UNIX, Linux and other platforms. I can help with basic to advanced C++, C (although I do not write just-C much if at all these days so maybe ask in the C section about purely C matters), software development and many platform specific and system development problems.


My career started in the mid 1980s working as a batch process operator for the now defunct Inner London Education Authority, working on Prime mini computers. I then moved into the role of Programmer / Analyst, also on the Primes, then into technical support and finally into the micro computing section, using a variety of 16 and 8 bit machines. Following the demise of the ILEA I worked for a small company, now gone, called Hodos. I worked on a part task train simulator using C and the Intel DVI (Digital Video Interactive) - the hardware based predecessor to Indeo. Other projects included a CGI based train simulator (different goals to the first), and various other projects in C and Visual Basic (er, version 1 that is). When Hodos went into receivership I went freelance and finally managed to start working in C++. I initially had contracts working on train simulators (surprise) and multimedia - I worked on many of the Dorling Kindersley CD-ROM titles and wrote the screensaver games for the Wallace and Gromit Cracking Animator CD. My more recent contracts have been more traditionally IT based, working predominately in C++ on MS Windows NT, 2000. XP, Linux and UN*X. These projects have had wide ranging additional skill sets including system analysis and design, databases and SQL in various guises, C#, client server and remoting, cross porting applications between platforms and various client development processes. I have an interest in the development of the C++ core language and libraries and try to keep up with at least some of the papers on the ISO C++ Standard Committee site at


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