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

From:

Stuart Redmann <DerTopper@web.de>

Newsgroups:

comp.os.ms-windows.programmer.win32,microsoft.public.vc.language

Date:

Wed, 04 Apr 2007 16:13:06 +0200

Message-ID:

<ev0f22$aa0$1@news.dtag.de>

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.

Ok.

`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.

Thanks.

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

<code>

#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*/[])
{
    ::CoInitialize(NULL);

    try
    {
        // Create the document and fill it with
        // some example data.
        IXMLDOMDocument2Ptr spDocument(
            __uuidof(DOMDocument40));

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

        spDocument->appendChild(spCurrentNode);

        // 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",
            MB_OK | MB_ICONINFORMATION);
    }
    catch(const _com_error& e)
    {
        ::MessageBox(NULL, e.ErrorMessage(), _T("Error"),
            MB_OK | MB_ICONERROR);
    }

    ::CoUninitialize();

    return 0;
}

</code>

Thank you very much (the same for Igor).

Stuart