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Howdy :-)
The problem here is discussed in the |nsCOMPtr| user's manual at
<http://www.mozilla.org/projects/xpcom/nsCOMPtr.html#ref_assert_NoQuery
Needed>
In this case, there are two possible |nsIStreamObserver| interface
pointers present in an |nsFtpControlConnection|. Querying must always
return the same one (that's an axiom of COM). Any |nsCOMPtr| (except
an |nsCOMPtr<nsISupports>|) that holds a |nsIStreamObserver| reference
must hold the COM-correct one, i.e., the one that would be returned by
|QueryInterface|. That's an invariant of |nsCOMPtr|.
Your casts select which |nsIStreamObserver| interface you are trying
to stuff into the |nsCOMPtr|. The one that happens to select the same
one |QueryInterface| would return succeeds. The other one fails (at
run-time).
Here's what you need to know: avoid casts, particularly old-style
casts. If you think you needed to cast, you probably _really_ needed
to query. You can't rely on the relationship of the C++ types to the
COM types. Your assignments should read:
nsCOMPtr<nsISupports> sup1 = NS_STATIC_CAST(nsIStreamListener*,
fcc);
nsCOMPtr<nsISupports> sup2 = NS_STATIC_CAST(nsIStreamProvider*,
fcc);
nsCOMPtr<nsIStreamObserver> obs1 = do_QueryInterface(fcc);
Hope this helps,
______________________________________________________________________
Scott Collins <http://ScottCollins.net/>
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