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[Solved] Basic QIODevice subclass in Qt4

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    paucoma
    wrote on last edited by
    #28

    Hi Gerolf: thanks for fixing the comments.
    In the first section I am suposing we dont declare the macro Q_DISABLE_COPY(CryptDevice)

    • In that case, will the compiler automatically generate a copy constructor?
      ** If so, are the following legal?
      *** CryptDevice cdev_4(cdev_1);
      *** CryptDevice cdev_5 = cdev_1;
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      giesbert
      wrote on last edited by
      #29

      it will try to create it, but it will automatically call the base copy c'tor, which is private --> compiler error

      Nokia Certified Qt Specialist.
      Programming Is Like Sex: One mistake and you have to support it for the rest of your life. (Michael Sinz)

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      • P Offline
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        paucoma
        wrote on last edited by
        #30

        Thanks for that clarification, when I try these things I don't know about, I get errors, and it is sometimes hard to understand what the error means, you guys here are really helping me understand whats happening, I'm greatfull! :)

        P.S. I need to read more carefully, sorry.. you did do the correct assumption in your first posting.
        [quote] will not work, as QIODevice has Q_DISABLE_COPY [/quote]

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          goetz
          wrote on last edited by
          #31

          Just a short note: It may be handy to provide some custom signal encryptedBytesWritten() just like signals in "QSslSocket:: encryptedBytesWritten()":http://doc.qt.nokia.com/4.7/qsslsocket.html#encryptedBytesWritten. This counts the bytes that were actually written to the underlying device (it might differ from the bytes that went in!).

          http://www.catb.org/~esr/faqs/smart-questions.html

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            andre
            wrote on last edited by
            #32

            Not with this implementation it doesn't ;-)

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              giesbert
              wrote on last edited by
              #33

              And with this implementation, when write returns, the bytes are written :-)
              But I should look into this in the my SimpleCryptDevice impl :-) Thanks.

              Nokia Certified Qt Specialist.
              Programming Is Like Sex: One mistake and you have to support it for the rest of your life. (Michael Sinz)

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              • P Offline
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                paucoma
                wrote on last edited by
                #34

                Attempting to find out where the bytes are being emited in the case of QFile for example.
                I have done a search in QFile.cpp, QIODevice.cpp and even QObject.cpp and none of them actually has the words "emit bytesWritten"

                I have found: void bytesWritten(qint64 bytes); in qiodevice.h
                Under a signal declaration header macro Q_SIGNALS which I have read to be for "Using Qt with 3rd Party Signals and Slots mechanisms":http://doc.qt.nokia.com/latest/signalsandslots.html#3rd-party-signals-and-slots

                And then I searched for the Q_EMIT macro with no findings (except for a comment in qobject) and I have no further ideas.

                Any suggestions on where to find out where this signal is being emited?

                p.s. This is just a simple curiosity I had, if someone knows it off the top of their head it would be nice to know, otherwise I wouldn't want anybody to waste their time trying to answer the question.

                p.p.s. Gerolf: Just noticed your "quoted" signature, was laughing for a good long minute and the smile will probably still be on my face when I wake up tomorrow. :D

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                  giesbert
                  wrote on last edited by
                  #35

                  QIODevice is a base class. It defines the interface.

                  If you look at the docs of QFile, "it's stated":http://doc.qt.nokia.com/latest/qfile.html#signals :

                  bq. Unlike other QIODevice implementations, such as QTcpSocket, QFile does not emit the aboutToClose(), bytesWritten(), or readyRead() signals. This implementation detail means that QFile is not suitable for reading and writing certain types of files, such as device files on Unix platforms.

                  qbuffer.cpp emits it on line 82.

                  qprocess.cpp emits it on line 980.

                  qwindowspipewriter.cpp emits it on line 163.

                  For all sockets, I did not check now.

                  Nokia Certified Qt Specialist.
                  Programming Is Like Sex: One mistake and you have to support it for the rest of your life. (Michael Sinz)

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                  • P Offline
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                    paucoma
                    wrote on last edited by
                    #36

                    Hehe, thanks Gerolf. Man, what bad luck having chosen QFile for a "random" QIODevice to see where it would emit the signal.

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                      andre
                      wrote on last edited by
                      #37

                      [quote author="Volker" date="1300991199"]Just a short note: It may be handy to provide some custom signal encryptedBytesWritten() just like signals in "QSslSocket:: encryptedBytesWritten()":http://doc.qt.nokia.com/4.7/qsslsocket.html#encryptedBytesWritten. This counts the bytes that were actually written to the underlying device (it might differ from the bytes that went in!).[/quote]

                      Wouldn't that be overkill, because you already have access to the underlying QIODevice directly? If you're interested in what was written, couldn't simply connect to that IODevices' bytesWritten() signal?

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                      • G Offline
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                        giesbert
                        wrote on last edited by
                        #38

                        [quote author="Andre" date="1301002233"]
                        Wouldn't that be overkill, because you already have access to the underlying QIODevice directly? If you're interested in what was written, couldn't simply connect to that IODevices' bytesWritten() signal?[/quote]

                        Only if it's not a QFile!

                        bq. Unlike other QIODevice implementations, such as QTcpSocket, QFile does not emit the aboutToClose(), bytesWritten(), or readyRead() signals. This implementation detail means that QFile is not suitable for reading and writing certain types of files, such as device files on Unix platforms.

                        Nokia Certified Qt Specialist.
                        Programming Is Like Sex: One mistake and you have to support it for the rest of your life. (Michael Sinz)

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                          goetz
                          wrote on last edited by
                          #39

                          [quote author="Andre" date="1301002233"]
                          [quote author="Volker" date="1300991199"]Just a short note: It may be handy to provide some custom signal encryptedBytesWritten() just like signals in "QSslSocket:: encryptedBytesWritten()":http://doc.qt.nokia.com/4.7/qsslsocket.html#encryptedBytesWritten. This counts the bytes that were actually written to the underlying device (it might differ from the bytes that went in!).[/quote]

                          Wouldn't that be overkill, because you already have access to the underlying QIODevice directly? If you're interested in what was written, couldn't simply connect to that IODevices' bytesWritten() signal?[/quote]

                          In this case yes, you're right. I had that QSslSocket in mind, but you do not have access to the "regular" TCP socket in that case.

                          http://www.catb.org/~esr/faqs/smart-questions.html

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                            giesbert
                            wrote on last edited by
                            #40

                            There is a more enhanced eexample, also in the wiki now: "Simple Crypt IO Device":http://developer.qt.nokia.com/wiki/Simple_Crypt_IO_Device . It uses the "SimpleCrypt class":http://developer.qt.nokia.com/wiki/Simple_encryption to encrypt the data and stores it via a custom IO device in any other device.

                            Nokia Certified Qt Specialist.
                            Programming Is Like Sex: One mistake and you have to support it for the rest of your life. (Michael Sinz)

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                              andre
                              wrote on last edited by
                              #41

                              Is there a reason you set the key twice in the first example?

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                                giesbert
                                wrote on last edited by
                                #42

                                Ups, no,

                                It was first in the c'tor, then the additional function.
                                I should add a c'Tor without the key...

                                Thanks for the hint.

                                Nokia Certified Qt Specialist.
                                Programming Is Like Sex: One mistake and you have to support it for the rest of your life. (Michael Sinz)

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                                  andre
                                  wrote on last edited by
                                  #43

                                  I hope you don't mind that I edited the wiki entry itself too :)

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                                    giesbert
                                    wrote on last edited by
                                    #44

                                    All is fine :-)
                                    I now changed the code and also on gitorious.

                                    Nokia Certified Qt Specialist.
                                    Programming Is Like Sex: One mistake and you have to support it for the rest of your life. (Michael Sinz)

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                                    • P Offline
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                                      paucoma
                                      wrote on last edited by
                                      #45

                                      Wow, Fantastic, Marvelous, Wonderful, Excellent!!
                                      You guys Rock!
                                      I've tried the classes out and great, I'm sure many will benefit from your brilliant contributions!

                                      I've added a couple modifications to your SimpleCryptDevice class.
                                      SimpleCryptDevice.h
                                      @
                                      // ----- construction -----
                                      /**

                                      • Constructor.
                                      • Constructs a SimpleCryptDevice instance.
                                      • The underlyingDevice needs to be set with setDevice.
                                      • The crypt key can be set by setKey.
                                      • @param parent parent object, see Qt documentation
                                        /
                                        SimpleCryptDevice(QObject
                                        parent=0);
                                        // ----- methods ----
                                        /**
                                      • Sets the underlyingDevice
                                        /
                                        void setDevice(QIODevice
                                        underlyingDevice) {m_underlyingDevice = underlyingDevice;}
                                        /**
                                      • Returns the last error that occurred.
                                        /
                                        int lastError() const {return m_crypto.lastError();}
                                        signals:
                                        /
                                        *
                                      • This signal is emitted when SimpleCryptDevice writes a block of raw data to the underlying device.
                                        /
                                        void blockWritten();
                                        @
                                        SimpleCryptDevice.cpp
                                        @
                                        SimpleCryptDevice::SimpleCryptDevice(QObject
                                        parent)
                                        {
                                        }
                                        int SimpleCryptDevice::writeBlock(const QByteArray& bytesToWrite)
                                        {
                                        //...
                                        //..
                                        //.
                                        emit blockWritten();
                                        return realBytesWritten;
                                        }
                                        return 0;
                                        }
                                        @

                                      Justification of modification:

                                      • blockWritten() signal
                                        ** to provide feedback to a QProgressBar.
                                        *** I externally calculate how many blocks will be written dependent on the block size chosen
                                      • Bare constructor
                                        ** to connect the signals before I define the device.
                                      • lastError()
                                        ** to have access to the last error

                                      Although the compiled code works fine, I would apreciate if you point out any error / malpractice I have done.

                                      Thanks loads again!

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                                        andre
                                        wrote on last edited by
                                        #46

                                        There is a potiential danger, that you have not set the underlying device before you start using the device if you take it out of the constructor. Because of the guarantee what was there before your modification, there is no check for that. Other than that, I can think of no major issues.

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                                        • G Offline
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                                          giesbert
                                          wrote on last edited by
                                          #47

                                          That was exactly why the underlying device was part of the constructor. It does not make sense to reuse the SimpleCrypDevice with another base device. And also not without an underlying device.

                                          The signal looks good. I will add it to gitorious and the wiki.

                                          Nokia Certified Qt Specialist.
                                          Programming Is Like Sex: One mistake and you have to support it for the rest of your life. (Michael Sinz)

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