Re: Doing the Right Thing with IPersistStream
Jason S <jmsachs@gmail.com> wrote:
Suppose I have implemented a COM object CJasonLog with interface
IJasonLog that derives from IPersistStream. (or should it have
IJasonLog and IPersistStream as separate interfaces?)
It is best to implement IJasonLog and IPersistStreamInit (IPersistStream
is obsolete and should not be used) side-by-side on the same object. Do
not derive IJasonLog from IPersistStream[Init]. You may also want to
consider supporting IPersistFile.
If I assign a pointer to IJasonLog into a CComVariant, and something
calls CComVariant::ReadFromStream, will that use my IPersistStream
implementation?
You misunderstand how ReadFromStream works. If you look at the code, the
first thing it does is call VariantClear, so any previous contents of
the variant are irrelevant.
See also OleLoadFromStream
How do I get an IStream * from a filename, anyway? is there a COM
equivalent to fopen()?
With new enough shell version, you can use SHCreateStreamOnFile[Ex].
With older versions, you can read the file into memory and use
CreateStreamOnHGlobal.
Strategies for handling persistence for vectors of the object's
members:
(a) write the # of members to the stream, then write each member to
the stream
(b) write each member to the stream, then write a "terminator" object
to the stream so that when the stream is re-opened later, the object
can know when it's done.
Didn't you say that you already have your file format defined? Shouldn't
you follow that format then?
--
With best wishes,
Igor Tandetnik
With sufficient thrust, pigs fly just fine. However, this is not
necessarily a good idea. It is hard to be sure where they are going to
land, and it could be dangerous sitting under them as they fly
overhead. -- RFC 1925