In order to understand that the code is fundamentally broken,
take a look at _bstr_t class in comutil.h.
See the difference in implementation between

_bstr_t& _bstr_t::operator=(const _bstr_t& s)

and

_bstr_t& _bstr_t::operator=(const wchar_t* s), for example.

Because scope of batrval2 is limited by ExtractAddErrInfo,
class Data_t inside batrval2 will be in the stack and all
assignments will have references to that Data_t.
So, on exit from ExtractAddErrInfo we will have reference
in _bstr_t val to something which does not exist anymore.

> -----Original Message-----
> From: Syed.Alam [mailto:Syed.AlamTARGET.COM]
> Sent: Thursday, March 21, 2002 9:18 AM
> To: DCOMDISCUSS.MICROSOFT.COM
> Subject: Re: bug in _bstr_t ????
>
>
> hmmm....I do not see anythign wrong in the code, or _bstr_t
>
> When comipler creates temp instance of _bstr_t, it is there
> until assigned
> to caller's instance. if u see the BSTR value of _bstr_t in caller's
> context, it will be same BSTR as allocated inside the static function.
>
> this code is running fine on my PC (VC 6.0).
>
> have u run it on ur PC?
>
>
>
>
> -----Original Message-----
> From: Alex Shevchuk [mailto:ashevchukCORILLIAN.COM]
> Sent: Thursday, March 21, 2002 10:59 AM
> To: DCOMDISCUSS.MICROSOFT.COM
> Subject: Re: bug in _bstr_t ????
>
>
> _bstr_t class is using reference counted BSTR inside this class.
> What is happening here is that when you return _bstr_t from
> ExtractAddErrInfo, C++ is creating temporary instance, which
> will live only during assignment operation to _bstr_t val and
> it will be destroyed right after assignment operation.
> It means, that val now has a pointer to already destroyed
> BSTR, hence crash when it is trying to destroy it again
> in the destructor.
> I am not judging your design of ExtractAddErrInfo function, so
> the easiest way to fix it is:
>
> _bstr_T val = CErrorUtil::ExtractAddErrInfo(bstrIn).Copy();
>
> This is not a bug in _bstr_t, just improper use of it.
>
> Regards,
>
> Alex Shevchuk
>
>
>
>
> > -----Original Message-----
> > From: S.Pradhan [mailto:sabyasachi.pradhanKLA-TENCOR.COM]
> > Sent: Wednesday, March 20, 2002 8:26 PM
> > To: DCOMDISCUSS.MICROSOFT.COM
> > Subject: bug in _bstr_t ????
> >
> >
> > I have the following problem.
> >
> > I have a win32 Dll(non COM ordinary Dll).The dlll exposes a
> > method which
> > takes in a _bstr_t as a parameter and returns a _bstr_t
> > parameter.Inside the
> > method i do some manipulation with the passed in bstr and
> > return it.When i
> > call it from another application(say a console app), the
> > function returns
> > succesfully, but at the caller app when the returned bstr
> > goes out of scope
> > it crashes.
> > Why does it happen so.If i replace _bstr_t with CComBSTR or
> > _variant_t data
> > type, everything works fine without any problem.
> > Look like there is a bug in _bstr_t.
> >
> > I know all of you might say, that i am not handling the
> > memory properly when
> > i manipulate the bstr in the method and return.Hence i am
> > giving the code
> > here.
> >
> > Pl et me know if i am doing anything wrong here fundamentally
> > or there is a
> > problem with _bstr_t.
> >
> > The error.h file of the win 32 DLL
> >
> > **************************************************************
> > ************
> > #ifdef ERROR_API_EXPORT
> > #define ERROR_API __declspec(dllexport)
> > #else
> > #define ERROR_API __declspec(dllimport)
> > #endif
> >
> > #include<comdef.h>
> > //#include<atlbase.h>
> > class ERROR_API CErrorUtil
> > {
> >
> > public:
> > static _bstr_t ExtractAddErrInfo(_bstr_t bstrErrMessage);
> > private:
> > // This class is not meant to be instantiated; therefore,
> > the constructor
> > is
> > // made a private member.
> > CErrorUtil(void);
> > };
> >
> > **************************************************************
> > ***********
> >
> > The error.cpp of the win 32 DLL
> >
> > **************************************************************
> > *************
> > #include "error.h"
> >
> > _bstr_t CErrorUtil::ExtractAddErrInfo(_bstr_t bstrErrMessage)
> > {
> >
> > _bstr_t batrval2("Pradhan");
> > bstrErrMessage=batrval2;
> >
> > return bstrErrMessage;
> >
> > /*tried with Following line, still it crashes */
> > //return bstrErrMessage.copy() ;
> > //return ::SysAllocString((OLECHAR*)bstrErrMessage);
> >
> > }
> >
> > **************************************************************
> > *************
> >
> >
> > Console client.cpp
> > **************************************************************
> > *************
> > #include<comdef.h>
> >
> > #include "D:\Scratch Area 2\testproject\Error\error.h"
> > #include<stdio.h>
> > void main()
> > {
> > CoInitialize(NULL);
> > try
> > {
> > _bstr_t bstrIn="Rajesh";
> > _bstr_t val=CErrorUtil::ExtractAddErrInfo(bstrIn);
> >
> > }///it crashes here, when the _bstr_t val gets destroyed.
> > catch(_com_error &e)
> > {
> >
> > }
> > CoUninitialize();
> > }
> > **************************************************************
> > *************
> >
> > ----------------------------------------------------------------
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>
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