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Thread: Convert QByteArray to object and vice versa

  1. #1
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    Default Convert QByteArray to object and vice versa

    I have a data structure:

    Qt Code:
    1. typedef struct
    2. {
    3. QString IpAddress;
    4. int Port;
    5. } VideoSource;
    6.  
    7. Q_DECLARE_METATYPE( VideoSource )
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    All I want to do is be able to convert this to a byte array in order to send on a socket, and get the object back (deserialize) on the other end. So I am doing this:

    Qt Code:
    1. int videoSourceMetaTypeId = qRegisterMetaType< VideoSource >();
    2. ...
    3. ...
    4. VideoSource vs;
    5. vs.IpAddress = "127.0.0.1";
    6. vs.Port = 5678;
    7. QVariant videoSourceAsVariant;
    8. videoSourceAsVariant.setValue( vs );
    9. QByteArray videoSourceBytes = videoSourceAsVariant.toByteArray();
    10. int numBytes = videoSourceBytes.count(); // this is 0
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    Not sure why the byte array's size ends up being 0.
    So how do I get the object back? Was trying this but obviously it's wrong...

    Qt Code:
    1. QVariant testVar( videoSourceBytes );
    2. if( testVar.canConvert< VideoSource >() ) // returns false
    3. {
    4. VideoSource testVs = testVar.value< VideoSource >();
    5. }
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  2. The following user says thank you to DiamonDogX for this useful post:


  3. #2
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    Default Re: Convert QByteArray to object and vice versa

    It's because QVariant doesn't know how to convert your object to byte array and as far as I remember there is no way to teach it do it.

    What you should do is reimplement operators (<< and >>) for QDataStream to be able to stream your object into a datastream that can operate on a byte array.
    Your biological and technological distinctiveness will be added to our own. Resistance is futile.

    Please ask Qt related questions on the forum and not using private messages or visitor messages.


  4. #3
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    Default Re: Convert QByteArray to object and vice versa

    Ok. I am going to work on that. Do you know why this doesn't compile?

    VideoSource.h:

    Qt Code:
    1. #ifndef VideoSource_h_
    2. #define VideoSource_h_
    3.  
    4. #include <QString>
    5. #include <QDataStream>
    6.  
    7. #pragma pack 1
    8. typedef struct
    9. {
    10. QString IpAddress;
    11. quint16 Port;
    12. } VideoSource;
    13.  
    14. QDataStream & operator<< ( QDataStream & stream, const VideoSource & vs );
    15. QDataStream & operator>> ( QDataStream & stream, VideoSource & vs );
    16.  
    17. Q_DECLARE_METATYPE( VideoSource )
    18.  
    19. #endif
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    VideoSource.cpp:

    Qt Code:
    1. #include "VideoSource.h"
    2.  
    3. QDataStream & operator<< ( QDataStream & stream, const VideoSource & vs )
    4. {
    5. return stream;
    6. }
    7.  
    8. QDataStream & operator>> ( QDataStream & stream, VideoSource & vs )
    9. {
    10. return stream;
    11. }
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    I get errors like:
    Error 2 error C3646: 'QDataStream' : unknown override specifier
    Error 3 error C2143: syntax error : missing ';' before '&'
    Error 4 error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
    Error 5 error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
    Error 6 error C2556: 'int &operator <<(QDataStream &,const VideoSource &)' : overloaded function differs only by return type from 'QDataStream &operator <<(QDataStream &,const VideoSource &)'
    Error 7 error C2371: 'operator <<' : redefinition; different basic types
    Last edited by DiamonDogX; 20th July 2009 at 19:50.

  5. #4
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    Default Re: Convert QByteArray to object and vice versa

    Looks like you need:

    #include <QDataStream>

    [EDIT]

    I see you have that in your header file. [/EDIT]

    [EDIT2] Whats with the ; on line 9 of your header file [EDIT2]
    Last edited by drescherjm; 20th July 2009 at 19:32.
    John

  6. #5
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    Default Re: Convert QByteArray to object and vice versa

    Took out the following line and it worked:

    Q_DECLARE_METATYPE( VideoSource )

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