BSTR Pointer ...

[SteveBrett]

I have a com wrapper that accepts a pointer to a BSTR as shown below in the code snippet:

conn.SendMsg((LPCSTR)message);
BSTR * recvd = 0;
conn.RecvMsg(recvd);

i need to get the value of recvd but when i run the program it is never naything but 0.

Is this the right way to call a function with a bstr out parameter ?

The dll that xontains the conn class works ok when used with c# via interop so i'm pretty sure it's working ok. i just can't seem to get it to work in c++

many thanks

 

[HyperEngineer]

Try this. It's worth a try.

  conn.SendMsg((LPCSTR)message);
  BSTR* recvd;
  conn.RecvMsg(recvd);

Remove the =0.


HyperEngineer
If it ain't broke, it probably needs improvement.

 

[SteveBrett]

i did try that but get this error:

Warning: attempt to call Invoke with NULL m_lpDispatch!
Warning: attempt to call Invoke with NULL m_lpDispatch!
Run-Time Check Failure #3 - The variable 'recvd' is being used without being defined.

i think the warning as well is a problem ...

any ideas ?

 

[cpjust]

You should ALWAYS initialize pointers to NULL!

What does the definition of conn.RecvMsg() look like?
If it doesn't take a reference to a pointer (or pointer to a pointer) then there's no way the value of recvd could be changed.

 

[SteveBrett]

the definition looks like this:

STDMETHODIMP CSSLConnection::RecvMsg(BSTR* bstrResponse)

and returns:

*bstrResponse = temp.Detach(); // return message

i think with the run-rime check failure i may have more than one comm dll registered - am investigating at the moment - still don't know how to call it though ....

 

[SteveBrett]

okay - only one version of the dll exists now but am still getting the following errors when stepping through

Warning: attempt to call Invoke with NULL m_lpDispatch!
Warning: attempt to call Invoke with NULL m_lpDispatch!
Warning: attempt to call Invoke with NULL m_lpDispatch!

 

[cpjust]

I can't be sure without seeing the documentation for that conn object, but I'm pretty sure you need to allocate the memory yourself and pass a pointer to that memory to RecvMsg().
Something like this:

conn.SendMsg( (LPCSTR)message );
const size_t MAX_STR = 1024;
BSTR recvd[ MAX_STR ] = "\0";
conn.RecvMsg( recvd );
// Now print the string in recvd...

Usually if a function allocates memory for you it will take a pointer to a pointer. i.e. BSTR**

 

[SteveBrett]

fixed it with this:

conn.CreateDispatch("XXXX.XXXX.X");

this was the reason that I got the warnings (Warning: attempt to call Invoke with NULL m_lpDispatch!). CreateDispatch() stops this.

CComBSTR result;
conn.RecvMsg(&result);

RecvMsg accepted a pointer to a BSTR, amended it and then returned the result via the same var. Thanks for all the comments and suggestions.