On 2015年05月01日 04:09, Joshua Cranmer 🐧 wrote: > On 4/30/2015 1:25 PM, ISHIKAWA, Chiaki wrote: >> * 787 nsCOMPtr <nsIInputStream> inboxInputStream = >> do_QueryInterface(m_outFileStream); >> 788 rv = MsgReopenFileStream(m_tmpDownloadFile, inboxInputStream); >> >> Before, as in the current release, m_outFileStream is not buffered. >> And the code on line 787 produces non-null inboxInputStream. >> >> However, once m_outFileStream is turned into a buffered output stream >> using, say, >> >> m_outFileStream = NS_BufferOutputStream(m_outFileStream, 64 * 1024 ); >> >> the code on line 787 produces nullptr. >> >> Is this to be expected? > > In short, yes. What happens is that the original m_outFileStream happens to > be of type nsFileStreams (or something like that), which inherits from both > nsIInputStream and nsIOutputStream. When you wrap that in a buffered output > stream, the resulting type of m_outFileStream is of nsBufferedOutputStream, > which does not inherit nsIInputStream; therefore the cast to nsIInputStream > fails. >> >> Up until now, I thought of do_QueryInterface() as mere sugar-coating for >> certain type-mutation or something. But I now know I am wrong. > > do_QueryInterface is the equivalent of a type-checked downcast, e.g. > (ClassName)foo in Java. (Regular C++ downcasts are not dynamically > type-checked). >> >> I read a page about do_QueryInterface() but it does not >> explain the principle very much. >> >> Is the reason of failure something like as follows. >> I am using a very general class hierarchy. >> >> >> A base class >> | >> +---+---+ >> B C B and C are derived from base class A >> | >> --+--+ >> | >> D D is derived further from Class D. >> >> Let's say Class B and C are derived from Class A. >> Class D is further derived from Class C. >> Let us assume there are corresponding XPCOM class/object A', B', C', D'. >> >> By using do_QueryInterface() on objects, >> we can follow the path of direct "derives" relation >> B' <= do_QueryInterface (A') (or is it the other way round?) >> >> and maybe between B' and C' (? Not sure about this.) >> >> but we can NOT follow the direction of >> B' <= do_QueryInterface (D') >> That is >> X = do_QeuryInterface(Y) is possible only when X is the direct or >> indirect descendant of Y? > > No, you are incorrect. The issue is the dynamic type of the object (if you > have A *x = new B, the static type of x is A whereas the dynamic type is B). > In the pre-modified code, the dynamic type of m_outFileStream supported the > interface in question, but your modification changed the dynamic type to one > that did not support the interface. >
Thank you. I am learning bit by bit. I did not realize that the do_QueryInterface has something to do with "dynamic type". Hmm, a lot to learn just to be able to use less buggy mail client :-) Thank you again. CI _______________________________________________ dev-platform mailing list dev-platform@lists.mozilla.org https://lists.mozilla.org/listinfo/dev-platform
