CPersistStream Class

CPersistStream is the base class for persistent properties of filters (that is, filter properties in saved graphs).

The simplest way to use CPersistStream is to:

1. Arrange for your filter to inherit this class.
2. Implement WriteToStream and ReadFromStream in your class. These will override the functions here, which do nothing but act as placeholders.
3. Change your NonDelegatingQueryInterface to handle IPersistStream.
4. Implement SizeMax to return an upper bound on the number of bytes of data you save.

   If you save Unicode data, remember that a WCHAR is 2 bytes.

5. When your data changes, call SetDirty.

Version Numbers

At some point you might decide to alter or extend the format of your data. You will then wish you had a version number in all the old saved streams so you can tell, when you read them, whether they represent the old or new form. To assist you, this class writes and reads a version number. When it writes, it calls GetSoftwareVersion to inquire as to the version of the software being used at the moment. (In effect, this is a version number of the data layout in the file.) It writes this as the first thing in the data. If you want to change the version, implement (override) GetSoftwareVersion. It reads the version number from the file into mPS_dwFileVersion before calling ReadFromStream, so in ReadFromStream you can check mPS_dwFileVersion to see if you are reading an old version file. Usually you should accept files whose version is no newer than the software version that is reading them.

Protected Data Members

Name	Description
mPS_dwFileVersion	Version number of the file.
mPS_fDirty	Data for this stream must be saved.

Member Functions

Name	Description
CPersistStream	Constructs a CPersistStream object.
SetDirty	Indicates that the object must be saved to the stream.

Overridable Member Functions

Name	Description
GetClassID	Returns the class identifier of this stream.
GetSoftwareVersion	Returns the version number for this file format.
ReadFromStream	Reads the filter's data from the stream.
SizeMax	Returns the number of bytes needed for data (not including version number).
WriteToStream	Writes the filter's data to the stream.

CPersistStream implements IPersistStream. For more implementation information, see the COM Reference in the Microsoft Platform SDK.

Implemented IPersistStream Methods

Name	Description
GetSizeMax	Returns the number of bytes needed for data (including version number).
IsDirty	Checks if the object must be saved.
Load	Loads the data from the stream into memory.
Save	Saves the data from memory to the stream.

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CPersistStream::CPersistStream

CPersistStream Class

Constructs a CPersistStream object.

CPersistStream(
  IUnknown *pUnk,
  HRESULT *phr
  );

Parameters

pUnk

IUnknown interface of the delegating object.

phr

Pointer to the general COM return value. Note that this value is changed only if this function fails.

Return Values

No return value.

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CPersistStream::GetClassID

CPersistStream Class

Retrieves the class identifier for this filter.

HRESULT GetClassID(
  CLSID *pClsID
  );

Parameters

pClsID

Pointer to a CLSID structure. Copy your class ID to here.

Return Values

Returns an HRESULT value.

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CPersistStream::GetSizeMax

CPersistStream Class

Returns the maximum byte size needed for the current stream, including the version number.

HRESULT GetSizeMax(
  ULARGE_INTEGER * pcbSize
  );

Parameters

pcbSize

Size in bytes needed to save this stream, including the version number.

Return Values

Returns an HRESULT value.

Remarks

This member function implements the IPersistStream::GetSizeMax method.

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CPersistStream::GetSoftwareVersion

CPersistStream Class

Returns the software version for this stream.

virtual DWORD GetSoftwareVersion(void);

Return Values

Returns a DWORD containing the version number. Each time the format of the stream is changed, this function should be altered to return a larger number than before.

Remarks

See Version Numbers for an explanation as to why file format version numbers are useful.

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CPersistStream::IsDirty

CPersistStream Class

Indicates whether the object has changed since it was last saved to its current stream.

HRESULT IsDirty( );

Return Values

Returns S_OK if the filter needs saving and S_FALSE if it does not need saving.

Remarks

This member function implements the IPersistStream::IsDirty method.

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CPersistStream::Load

CPersistStream Class

Loads the filter's data from a given stream.

HRESULT Load(
  LPSTREAM pStm
  );

Parameters

pStm

Pointer to the stream from which to be loaded.

Return Values

Returns an HRESULT value.

Remarks

This member function implements the IPersistStream::Load method.

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CPersistStream::ReadFromStream

CPersistStream Class

Reads the filter's data from the given stream.

virtual HRESULT ReadFromStream(
  IStream *pStream
  );

Parameters

pStream

Pointer to an IStream interface from which data is to be read.

Return Values

Returns NOERROR by default; the overriding member function should return a valid HRESULT value.

Remarks

The default version reads nothing; it can be overridden to read data specific to your class.

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CPersistStream::Save

CPersistStream Class

Saves the filter's data to the given stream.

HRESULT Save(
  LPSTREAM pStm,
  BOOL fClearDirty
  );

Parameters

pStm

Pointer to the stream to which data is to be saved.

fClearDirty

Flag that indicates whether to reset the current stream's dirty flag. When called as part of Save, the flag is normally reset; when called as part of Save As, the flag is normally not reset.

Return Values

Returns an HRESULT value.

Remarks

This member function implements the IPersistStream::Save method. It calls WriteInt with the software version, calls CPersistStream::WriteToStream with the stream in pStm, and resets mPS_fDirty.

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CPersistStream::SetDirty

CPersistStream Class

Changes the dirty flag for the current stream.

HRESULT SetDirty(
  BOOL fDirty
  );

Parameters

fDirty

New dirty flag for this stream. TRUE means that the data has not been saved.

Return Values

Returns an HRESULT value.

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CPersistStream::SizeMax

CPersistStream Class

Retrieves the maximum byte size needed for the current stream, not including the version number.

virtual int SizeMax( );

Return Values

Returns the number of bytes needed for data, not including the version number.

Remarks

The default version returns zero; it should be overridden to provide some other appropriate value.

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CPersistStream::WriteToStream

CPersistStream Class

Writes the filter's data to the given stream.

virtual HRESULT WriteToStream(
  IStream *pStream
  );

Parameters

pStream

Pointer to an IStream interface that specifies the filter data's destination stream.

Return Values

Returns NOERROR by default; the overriding member function should return a valid HRESULT value.

Remarks

The default version writes nothing; it can be overridden to write data specific to your class.

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