How to implement my own IStream derived object

Posted on 2004-04-19
Last Modified: 2012-05-04
I want to implement a class derived from the IStream interface?
I want this to be used as the stream of an IStorage object .


Question by:romanm
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Expert Comment

ID: 10864888
do you mean istream? (not capital IStream)

if so, then check out this site.
it's a good (and the only one i can find online) guide to how the iostream works, and how to derive a class from it

Author Comment

ID: 10865125
No, IStream an interface used by structured storage.

Accepted Solution

_ys_ earned 125 total points
ID: 10871576
Of course it's possible. Albeit quite a bit of work.

You need to:
- [obviously] create your component and implement IStorage.
- roll out your own IMoniker implementation (typically on your class factory)
- implement IMoniker::BindToStorage to specifically return your custom IStorage implementation

Of course, anyone using your component would then have know how to use monikers properly in order to create your component.

Implementations of IMoniker and IStorate et al. are non-trivial. What's wrong with the default implementation? You just playing around?

I presented a write up on monikers before, although you need to know the basics well to read it:

Author Comment

ID: 10871926
What I am trying to do is to make a source/write filter for msxml,
I want to create an encrypted form of a file that I could use as a source file for SAX parsing.

Author Comment

ID: 10872853
// I have found a simple solution to my problem:

class CMyXMLSourceFilter : public CUnknown, public ISequentialStream
      long __stdcall Read(void* pv, unsigned long cb, unsigned long* pcbRead);
      long __stdcall Write(const void* pv, unsigned long cb, unsigned long* pcbRead);
      static HRESULT CreateInstance(IUnknown * pUnknownOuter, CUnknown ** ppNewComponent);
      DECLARE_IUNKNOWN      // IUnknown

      void open(bool b);
      CMyXMLSourceFilter(IUnknown* pUnknpwnOuter);
      virtual ~CMyXMLSourceFilter() {};
      FILE*      f;
      // INondelegatingUnknown
      virtual HRESULT __stdcall  NondelegatingQueryInterface( const IID& iid, void ** ppv);
      virtual void FinalRelease();

CMyXMLSourceFilter::CMyXMLSourceFilter(IUnknown* pUnknpwnOuter) : CUnknown(pUnknpwnOuter)

void CMyXMLSourceFilter::open(bool b)
      if( b )
            f = fopen("C:\\test.stg", "wb" );
            f = fopen("C:\\test.stg", "rb" );

HRESULT CMyXMLSourceFilter::CreateInstance(IUnknown * pUnknownOuter, CUnknown ** ppNewComponent)
      *ppNewComponent = new CMyXMLSourceFilter(pUnknownOuter);

      return S_OK;

// INondelegatingUnknown
HRESULT CMyXMLSourceFilter::NondelegatingQueryInterface( const IID& iid, void ** ppv)
      if(iid == __uuidof(ISequentialStream))
            return FinishQI(static_cast<ISequentialStream*>(this),ppv);
            return CUnknown::NondelegatingQueryInterface(iid,ppv);

void CMyXMLSourceFilter::FinalRelease()

long CMyXMLSourceFilter::Read(void* pv, unsigned long cb, unsigned long* pcbRead)
      size_t l = fread(pv, 1, cb, f);
      if( pcbRead != NULL )
            *pcbRead = l;
      return 0;
long CMyXMLSourceFilter::Write(const void* pv, unsigned long cb, unsigned long* pcbRead)
      size_t l = fwrite(pv, 1, cb, f);
      if( pcbRead != NULL )
            *pcbRead = l;
      return 0;

// And the usage:

      ISAXXMLReaderPtr pSax;
      CMyXMLSourceFilter* cc1;
      CMyXMLSourceFilter::CreateInstance(NULL, (CUnknown**)&cc1);
      _variant_t var1(pcc);
      pcc = NULL;

I'll award the points for effort ;)

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