Com object calling process and bstr processing

Posted on 1998-12-04
Last Modified: 2013-11-20
I have a comm object and a client program that I am working on. The com object has a method that has a BSTR* parameter and returns a BSTR* pointer.

Questions :

1. Suppose the name of the method is Reverse, what would the calling sequence be in the client program. The client program is going to pass a CString object and is expecting to store the value in a CString object when it is returned.

2. In the com object how would one go about looping through and getting and setting individual characters in the string ?


P.S. More points to be awarded when I get the answer.
Question by:andrewmchorney
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Expert Comment

ID: 1325787
I'm assuming your idl looks something like this:

HRESULT Reverse( [in,out] BSTR bstr );

Then your client call will look like this:

CString str( "abcdefg" );
BSTR bstr = str.AllocSysString();
pI->Reverse( bstr );
. // do something with bstr
::SysFreeString( bstr );

The server implementation should look like this:

HRESULT CServerClass::Reverse( BSTR bstr )
    WCHAR * pwszTmp = new WCHAR[ wcslen( bstr ) + 1 ];
    if ( pwszTmp )
        wcscpy( pwszTmp, bstr );
        WCHAR * lpwsz = &pwszTmp[ wcslen( pwszTmp ) - 1 ];
        int ndx = 0;
        while ( lpwsz >= pwszTmp )
            bstr[ ndx++ ] = *lpwsz--;
    if ( pwszTmp )
        delete pwszTmp;

The basic assumptions here are that a BSTR is the same as a LPWSTR (you'll notice the WinAPI says this is so, and the headers define it the same).  That means you can do string related operations, albeit with unicode functions.  Secondly, you only need to use a BSTR * if the server is allocating memory and returning it.  In the example you gave, the client is allocating the memory, so you don't need to pass a BSTR *.  Obviously, I could have used the _wcsrev() function to reverse the string, but the idea was to show how the string manipulation works.

Accepted Solution

tsollas earned 10 total points
ID: 1325788
doh... consider my comment above as the answer.

Author Comment

ID: 1325789
I have gotten most of the work done before getting your message. My project has 2 interfaces with a function in each one. The first one has a short parameter and returns a short and compiles and works well. The second one has a BSTR * parameters and is to return a BSTR object. The call to the com function (ReverseString) complains about the parameter and I have not figured out the error. Can you take a look at what is wrong. There are more points to be awarded when this is solved.

Com function

BSTR StringFunctions::ReverseString(BSTR FAR* strInputString)
   // TODO: Add your dispatch handler code here
   CString strResult(*strInputString);

     // code to do reversal goes here and is not relevent to the question
    return strResult.AllocSysString();

Client Code :

void CTestReverseString::OnReverseStringCommand()
      // TODO: Add your control notification handler code here
            // TODO: Add your control notification handler code here
      IStringFunctions m_StringFunctions;

      BSTR strToBeReversed;
      BSTR strReversedString;

      // Get the variable values from the dialog

      // Load the com object
      if (!m_StringFunctions.CreateDispatch("mchorney2.StringFunctions"))
            // Com object not loaded
            AfxMessageBox("mchorney2.StringFunctions not found");

      strToBeReversed = m_strSourceString.AllocSysString();
      // Reverse the string but does not compile
                m_strReversedString = m_StringFunctions.ReverseString(

      // Release connection to com object

      // Update the dialog


Author Comment

ID: 1325790

I just figured out the error and the program works well.


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