Re: Problem with writing XML DOM tree to memory buffer.

Stuart Redmann <>
Wed, 04 Apr 2007 16:13:06 +0200
Stuart Redmann wrote:
 > Hello newsgroup,
 > I tried to write a small test application that creates an XML DOM tree
 > and writes the contents
 > into a string buffer (saving to file works perfectly). Unfortunately, I
 > seem to go wrong
 > at some point (the text displayed contains only question marks :(
 > Can anybody give me some suggestions?

[snipped erroneous code snippet]

In addition to Igor's reply, you don't need to call
`Detach()' om `CComBSTR' object; it will leak BSTR.


`CComBSTR' class already has `operator BSTR'. Also, calling
`GlobalLock' is not enough. You should also call
`GlobalSize' in order to determine actual size of written
data. Stream doesn't append terminating NUL automatically.


Here's working example (made with VC2005, though it should
be easily adaptable to VC6):


#include <comutil.h>
#include <comdef.h>
#include <atlstr.h>

_COM_SMARTPTR_TYPEDEF(IUnknown, __uuidof(IUnknown));
_COM_SMARTPTR_TYPEDEF(IDispatch, __uuidof(IDispatch));

#import <msxml4.dll>

_COM_SMARTPTR_TYPEDEF(IStream, __uuidof(IStream));

using namespace MSXML2;

int _tmain(int /*argc*/, _TCHAR* /*argv*/[])

        // Create the document and fill it with
        // some example data.
        IXMLDOMDocument2Ptr spDocument(

        IXMLDOMElementPtr spCurrentNode =
        spCurrentNode->setAttribute(L"value", 3.14159);


        // Let the document persist into a memory stream.
        IStreamPtr spStream;
        HRESULT hr = ::CreateStreamOnHGlobal(
            NULL, TRUE, &spStream);
        if(FAILED(hr)) _com_issue_error(hr);

        _variant_t vtStream(spStream.GetInterfacePtr());
        hr = spDocument->save(vtStream);

I should have seen this one (tracing into the save call should have done the trick).
The big question is what IXMLDOMDocument is doing with the Boolean value I have
passed (I would have expected that the save method had fired an E_INVALIDARG).

        if(FAILED(hr)) _com_issue_error(hr);

        HGLOBAL hMem = NULL;
        hr = ::GetHGlobalFromStream(spStream, &hMem);
        if(FAILED(hr)) _com_issue_error(hr);

        LPVOID lp = ::GlobalLock(hMem);
        CStringA strXmlText((LPCSTR)lp, ::GlobalSize(hMem));

        ::MessageBoxA(NULL, strXmlText, "XML Text",
    catch(const _com_error& e)
        ::MessageBox(NULL, e.ErrorMessage(), _T("Error"),
            MB_OK | MB_ICONERROR);


    return 0;


Thank you very much (the same for Igor).


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