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  4. How does the "toString" method of qt data structures work?
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How does the "toString" method of qt data structures work?

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  • Q Offline
    Q Offline
    QAndrew
    wrote on last edited by QAndrew
    #1

    I found myself trying to save the textual representation of a QVariantMap (QMap<QString, QVariantMap>> actually) like as if I were to write
    qDebug() << myMap
    which yields the correct text data put to the console, but instead have the result in a file. I tried with QDataStream class but it can only handle binary form, I managed to do it finally like this:
    QDebug outDebug(&file);
    outDebug << myMap;
    But it really bothers me that I have no idea what is going on behind the scenes. I assume either QDebug & QDebug::operator<<(const QByteArray & b) or QDebug & QDebug::operator<<(const QString & s) functions are called after an implicit conversion, but how does that implicit conversion work?

    Also I have a custom (non-QObject)! class that is among the stored QVariants and it's textual repesentation is an empty string. Is there a way to customize that?

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    • FranckynosF Offline
      FranckynosF Offline
      Franckynos
      wrote on last edited by
      #2

      YourClass
      {
      friend QDataStream & operator << (QDataStream & out, const YourClass& t)
      {
      out << t._nom;
      return out;
      }
      };

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      • Q Offline
        Q Offline
        QAndrew
        wrote on last edited by
        #3

        That'll work, ty, but how come a QDebug instance can decompress the binary to textual? And how does one do it without QDebug instance?

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        • Q Offline
          Q Offline
          QAndrew
          wrote on last edited by QAndrew
          #4

          And actually I can't make your code snippet work. I implemented the friend operator method as described by you and doc but it's not called like it should be.

          QVariant(BitPattern, )
          empty string................^

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          • SGaistS Offline
            SGaistS Offline
            SGaist
            Lifetime Qt Champion
            wrote on last edited by
            #5

            Hi,

            Have a look at the QDebug documentation, there's an example on how to write a debug stream operator for your custom type.

            Interested in AI ? www.idiap.ch
            Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

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            • Q Offline
              Q Offline
              QAndrew
              wrote on last edited by QAndrew
              #6

              Aha, I see, somehow I skipped trough that paragraph of the doc, thanks!

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