CPersistStream Class (Compact 2013)

3/26/2014

IPersistStream

     CPersistStream Class

This class 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 CPersistStream::WriteToStream and CPersistStream::ReadFromStream in your class. These will override the functions here, which do nothing but act as placeholders.
3. Change your NonDelegatingQueryInterface to handle IPersistStream:IPersist.
4. Implement CPersistStream::SizeMax to return an upper bound on the number of bytes of data you save.
5. If you save Unicode data, remember that a WCHAR is 2 bytes.
6. When your data changes, call CPersistStream::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 CPersistStream::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) CPersistStream::GetSoftwareVersion. It reads the version number from the file into CPersistStream::mPS_dwFileVersion before calling CPersistStream::ReadFromStream, so in CPersistStream::ReadFromStream you can check CPersistStream::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

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

Member Functions

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

Overridable Member Functions

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

Implemented IPersistStream Methods

Method	Description
GetSizeMax	Retrieves 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.

Requirements

Header	dshow.h, Streams.h
Library	ole32.lib, Ole32auth.lib, Strmbase.lib, Strmiids.lib, uuid.lib

See Also

Reference

DirectShow Classes

Other Resources

IPersistStream