Re: Does this look scary to you?

From:
"Leo V" <none@sldkjf.com>
Newsgroups:
microsoft.public.vc.mfc
Date:
Mon, 21 May 2007 14:47:33 -0700
Message-ID:
<eAQbzG$mHHA.5052@TK2MSFTNGP04.phx.gbl>
Okay, I've rewritten the function to hopefully be cleaner.

Please give me your thoughts.

STDMETHODIMP CForm::GetStringValue(LONG lFieldID, /* [out,retval] */ BSTR*
pBstr)
{
    if (!pBstr)
        return E_POINTER;

    _ASSERTE(!(*pBstr) && _T("This should be NULL! It's either not
initialized or is already pointing to allocated memory!"));

    CString csDesc = GetRuntimeText(lFieldID);
    if (csDesc.IsEmpty())
        return E_FAIL; // Is this really a failure?

    try
    {
        *pBstr = csDesc.AllocSysString();
    }
    catch(CMemoryException* pme)
    {
        // NEVER throw an exception over a COM boundary.
        *pBstr = NULL;
        pme->Delete();
    } // catch (CMemoryException*)
    catch(...)
    {
        // NEVER throw an exception over a COM boundary.
        *pBstr = NULL;
    } // catch everything else

    if (*pBstr == NULL)
        return E_OUTOFMEMORY;

    return S_OK;
}

"SvenC" <SvenC@community.nospam> wrote in message
news:66455A59-8729-4033-9D40-4863E210D6E5@microsoft.com...

Leo V wrote:

I'm reviewing some code and I'm concerned about how the code is
dealing with returning a BSTR value. Consider the following function
(shorter version of the real function).

STDMETHODIMP CForm::GetStringValue(LONG lFieldID, BSTR* pBstr)
{
   if (!pBstr)
       return E_POINTER;

   CString csDesc = GetRuntimeText(lFieldID);
   *pBstr = (*pBstr ? csDesc.SetSysString(pBstr) :
csDesc.AllocSysString());

   if (csDesc.IsEmpty())
       return E_FAIL;
   else
       return S_OK;
}

My first concern is the line:
   *pBstr = (*pBstr ? csDesc.SetSysString(pBstr) :
csDesc.AllocSysString());

This seems to be assuming its an input/output parameter. Wouldn't
this code likely crash if someone just declared a BSTR variable
(without assigning it NULL) and called this function? It might
interpret random data as a previously allocated BSTR that must be
freed via SetSysString.


Do you have a typelib where the method is defined? Is it attributes as
[out, retval], [out] or [in, out]? If not you should make that explicit.
If I remember correctly [out] is not automation compatible, so [in, out]
or [out, retval] might be intended.

Further, this code
   if (csDesc.IsEmpty())
       return E_FAIL;
   else
       return S_OK;
will return E_FAIL even though memory was allocated. Most code I
know of would probably not realize they need to free the BSTR
returned if the HRESULT on the function wasn't S_OK. This to me
seems like it could result in a memory leak if someone wasn't careful.


I would change that, also. Something like return *pBstr == NULL ?
E_OUTOFMEMORY : S_OK;

CStrings BSTR methods will throw a CMemoryException when insufficient
memory is available. So the assignment to *pBstr should be put in a
try/catch block, setting *pBstr = NULL in the catch block.

--
SvenC

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