Typically it is a not magic number, it is a magic string, which solves most problems. I would recommend to keep it this way. If you want to use number there, write it to string first. then write the string.
Alex On Sat, Oct 26, 2013 at 6:49 PM, Constantin Makshin <cmaks...@gmail.com>wrote: > Since QDataStream versions form a contiguous sequence of integer numbers > and new ones are added in [some] minor Qt updates, I'd say you have a LOT > of time before these numbers stop fitting into 7-8 bits. > > As for the magic number issue, you may store it *after* the stream version > number -- if the version is wrong (i.e. larger than your application > understands), there's no need to worry about the magic number because the > stream should be considered unreadable anyway. > On Oct 27, 2013 3:44 AM, "Charley Bay" <charleyb...@gmail.com> wrote: > >> The QDataStream docs give examples on serializing for compatible >> formats across versions defined by QDataStream: >> >> <http://doc-snapshot.qt-project.org/qdoc/qdatastream.html#setVersion> >> >> Other examples on the web are similar, like: >> >> <http://doc.qt.digia.com/qq/qq05-achtung.html> >> >> The recommendation is something like: >> >> //--------------- >> //... >> QDataStream out(&file); >> >> // Write a header with a "magic number" and a version >> out << (quint32)0xA0B0C0D0; >> out << (qint32)123; >> //--------------- >> >> Then, reading in is: >> >> //--------------- >> //...- >> QDataStream in(&file); >> >> // Read and check the header >> quint32 magic; >> in >> magic; >> if (magic != 0xA0B0C0D0) >> return XXX_BAD_FILE_FORMAT; >> >> // Read the version >> qint32 version; >> in >> version; >> //--------------- >> >> QUESTION #1: Is it safe to write the 32-bit magic-number before >> reading-and-setting the "QDataStream::version()" ? It seems like >> these examples are not cross-platform, as the 32-bit value may have >> varying formats on different platforms. >> >> Other reading suggests that it is "safe" to assume >> "QDataStream::version()" wholly fits into an 8-bit value, so this >> should not be a problem with that value. >> >> QUESTION #2: Is it forever safe to assume an 8-bit value is >> sufficient to store "QDataStream::version()"? >> >> Because of my concern regarding "#1", it seems like four >> "magic-quint8" values should be serialized instead of one >> "magic-quint32" value. >> >> Or, am I just paranoid? >> >> --charley >> >> P.S.: I'll close with the quote from the Digia page above because I >> found it funny: >> >> "We have two choices, either to attack I/O now and get it over with, >> or to postpone I/O until near the end. >> >> Neither prospect is very attractive." >> -- Donald E. Knuth >> _______________________________________________ >> Interest mailing list >> Interest@qt-project.org >> http://lists.qt-project.org/mailman/listinfo/interest >> > > _______________________________________________ > Interest mailing list > Interest@qt-project.org > http://lists.qt-project.org/mailman/listinfo/interest > >
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