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Thread: reading JSON file in Qt5

  1. #1
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    Default reading JSON file in Qt5

    Hi,

    I have been trying to figure out how to read a JSON file using Qt5 QJsonDocument with no success so far.
    I can't find any examples online. If someone could give me a simple example I would appreciate it.

    Here is my code::
    Qt Code:
    1. QFile file;
    2. file.setFileName("/tmp/settings.json");
    3. file.open(QIODevice::ReadOnly | QIODevice::Text);
    4. QJsonDocument settdoc;
    5. settdoc.fromBinaryData(file.readAll());
    6. qDebug() << settdoc.isNull();
    To copy to clipboard, switch view to plain text mode 

    settdoc.isNull() returns true, I can read the file into a stream without any issue and I validated the json as well.

    Thank you

    Laszlo
    Last edited by Lykurg; 18th January 2013 at 08:40. Reason: missing [code] tags

  2. #2
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    Default Re: reading JSON file in Qt5

    Does /tmp/settings.json exists? Is it well formed? Also try QJsonDocument::fromJson and see what error (QJsonParseError) you get.

    EDIT: Forget the first questions....

    EDIT 2:
    Qt Code:
    1. QString str = "{"
    2. " \"Herausgeber\": \"Xema\","
    3. " \"Nummer\": \"1234-5678-9012-3456\","
    4. " \"Deckung\": 2e+6,"
    5. " \"Währung\": \"EURO\","
    6. " \"Inhaber\": {"
    7. " \"Name\": \"Mustermann\","
    8. " \"Vorname\": \"Max\","
    9. " \"männlich\": true,"
    10. " \"Hobbys\": [ \"Reiten\", \"Golfen\", \"Lesen\" ],"
    11. " \"Alter\": 42,"
    12. " \"Kinder\": [],"
    13. " \"Partner\": null"
    14. " }"
    15. "}";
    16. QJsonDocument d = QJsonDocument::fromJson(str.toUtf8());
    17. qWarning() << d.isNull();
    To copy to clipboard, switch view to plain text mode 
    See if that works for you and then replace str.toUtf8() through your readAll().
    Last edited by Lykurg; 18th January 2013 at 08:51.

  3. #3
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    Wink [Solved] Re: reading JSON file in Qt5

    Thank you for your fast reply and even more thank you for your example code!!
    It works now! I somehow overlooked that I can use toUtf8() on a QString.
    You example put me in the right track and now I can adjust basic window properties with my json file
    Now just need to work on auto copy this file into the apps resources at runtime.

    Here is my working code::

    Qt Code:
    1. QString settings;
    2. QFile file;
    3. file.setFileName("/tmp/settings.json");
    4. file.open(QIODevice::ReadOnly | QIODevice::Text);
    5. settings = file.readAll();
    6. file.close();
    7.  
    8. QJsonDocument sd = QJsonDocument::fromJson(settings.toUtf8());
    9. qWarning() << sd.isNull(); // <- print false :)
    10. QJsonObject sett2 = sd.object();
    11. qWarning() << sett2.value(QString("title")); // <- print my title
    To copy to clipboard, switch view to plain text mode 


    Thanks again, totally made my morning

    Laszlo

  4. #4
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    Default Re: [Solved] Re: reading JSON file in Qt5

    You are aware that this requires two codec conversions, right?

    Qt Code:
    1. settings = file.readAll()
    To copy to clipboard, switch view to plain text mode 

    converts from UTF-8 to UTF-16 and

    Qt Code:
    1. settings.toUtf8()
    To copy to clipboard, switch view to plain text mode 

    converts from UTF-16 to UTF-8.

    Since start and end encoding are the same (UTF-8) those two conversions are unneeded.

    Why don't you just call fromJson() with the QByteArray returned by file.readAll()?

    Also: the problem with your original code has nothing to do with the content at all, you are calling a static method and are then wondering why it didn't change an instance.

    Static methods can't do that.
    Hint: in Qt methods that are called fromABCD usually return something.

    Cheers,
    _

  5. #5
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    Default Re: [Solved] Re: reading JSON file in Qt5

    QString filename = QFileDialog::getOpenFileName();

    QString val;
    QFile file;
    file.setFileName(filename);
    file.open(QIODevice::ReadOnly | QIODevice::Text);
    val = file.readAll();
    file.close();
    QJsonDocument d = QJsonDocument::fromJson(val.toUtf8());
    QJsonObject Documento = d.object();

    //{}= objet [] Array

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