asked on

Newbie COM Question.

Hi All

Please excuse me if this should be asked on the VB fourum. I suspect the problem is in the C code.

I have the following code in a COM object
STDMETHODIMP CHello::get_FullName(BSTR *pVal)
{
      memset(m_FullName, 0 , sizeof(m_FullName));
      strcat(m_FullName, m_FirstName);
      strcat(m_FullName, ", ");
      strcat(m_FullName, m_LastName);
      CComBSTR Tmp(m_FullName);
      *pVal = BSTR(Tmp);
      return S_OK;
}
Which is a simple function to cat two strings togeather and return the result. This is simple but I am new to this COM game.

When I return the result to a VB client and put the text into a text box like such,
Text1.Text = objTestATL.FullName
The text box only displays the first charachter of the string.
If I display it in a message box like such
MsgBox (objTestATL.FullName)
I get the complete string.

If I run it from a C++ client
                        pICG->get_FullName(&FullName);
                        WideCharToMultiByte(CP_ACP,
                               0,
                               FullName,
                               lstrlenW(FullName),
                               m_FirstName,
                               lstrlenW(FullName),
                               NULL,
                               NULL);

I get the full string.

What am I doing wrong that this data is misrepresented in VB.

All help appretiated

tcss

Paul Maker

memset(m_FullName, 0 , sizeof(m_FullName));
strcat(m_FullName, m_FirstName);
strcat(m_FullName, ", ");
strcat(m_FullName, m_LastName);

/* i usually do this to return a string */
CComBSTR t_retval = m_FullName;
*retval = t_retval.Detach();

Member_2_207537

ASKER

That Fixed it.

Can anyone tell me why it fixed it to share double the points with makerp

tcss

nonubik

>*pVal = BSTR(Tmp);

Only casts the CComBstr variable to a BSTR one. More exactly, the BSTR cast operator returns the m_str member of the CComBstr class (which is a BSTR).

BSTR* operator&() throw()
{
      return &m_str;
}

When the method is left, the CCOmBstr variable is destroyed and the m_str is freed

~CComBSTR() throw()
{
      ::SysFreeString(m_str);
}

Thus, the data from the m_str address should not be reliable (I wonder how you didn't get an access violation ;)

>*retval = t_retval.Detach();

The Detach methods returns the m_str member (without freeing) and is the caller duty to free it.

BSTR Detach() throw()
{
      BSTR s = m_str;
      m_str = NULL;
      return s;
}

ASKER CERTIFIED SOLUTION

Paul Maker

membership

This solution is only available to members.

To access this solution, you must be a member of Experts Exchange.

Start Free Trial

nonubik

Seems I was not the "anyone" from above claim...

Member_2_207537

ASKER

You were supposed to be. I stuffed up with the allocation and need to figuare out how to undo it. It appears the best way will be to post a question and give you the points. That will equate to half double the points.

My appologies on the error(s).

https://www.experts-exchange.com/questions/21285587/nonubik-Allocation-error-correction.html

tcss

nonubik

No problem, then. Thanks.