easiest way to parse JSON in Qt 4.7

Question

I need to parse JSON object through Qt. What is the easiest/fastest way to do it?

See [Best JSON parser for Qt? ](http://stackoverflow.com/questions/1825892?tab=newest#tab-top). — Matthew Flaschen, Nov 12 '10 at 23:44
Looks like Qt5 has some JSON classes. http://qt-project.org/doc/qt-5.0/qtcore/json.html — Jared Price, Nov 05 '13 at 17:30
http://stackoverflow.com/questions/4987560/how-to-serialize-to-json-in-qt/16174375 — dtech, Jun 08 '14 at 18:22

score 18 · Answer 1 · answered May 23 '14 at 21:53

JSON parsing is now supported in Qt 5. Here's how to load and parse a document:

#include <QByteArray>
#include <QFile>
#include <QJsonObject>
#include <QJsonDocument>

// ...

// Read JSON file
QFile file("/path/to/file.json");
file.open(QIODevice::ReadOnly);
QByteArray rawData = file.readAll();

// Parse document
QJsonDocument doc(QJsonDocument::fromJson(rawData));

// Get JSON object
QJsonObject json = doc.object();

// Access properties
qDebug() << json["something"].toString();

The question is about version 4.7 – dtech May 23 '14 at 22:02 — dtech, May 23 '14 at 22:02

score 17 · Accepted Answer · edited Dec 21 '12 at 20:47

17

Try QJson.

QJson is actively developed (and used by KDE, if I'm not mistaken). The best is to checkout the source code directly and built it yourself. There is no dependencies to QJson (except for Qt and CMake). It's pretty simple to use too, have a look at some usage examples :

http://qjson.sourceforge.net/usage/

edited Dec 21 '12 at 20:47

Joshua D. Boyd

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answered Nov 13 '10 at 00:20

Symbiosoft

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score 9 · Answer 3 · answered Apr 04 '13 at 08:10

Here is Qt style json encoder/decoder

#include "json.h"

#include <QScriptEngine>
#include <QScriptValueIterator>

Json::Json()
{
}


QString Json::encode(const QMap<QString,QVariant> &map)
{
    QScriptEngine engine;
    engine.evaluate("function toString() { return JSON.stringify(this) }");

    QScriptValue toString = engine.globalObject().property("toString");
    QScriptValue obj = encodeInner(map, &engine);
    return toString.call(obj).toString();

}

QMap<QString, QVariant> Json::decode(const QString &jsonStr)
{
    QScriptValue object;
    QScriptEngine engine;
    object = engine.evaluate("(" + jsonStr + ")");
    return decodeInner(object);
}

QScriptValue Json::encodeInner(const QMap<QString,QVariant> &map, QScriptEngine* engine)
{
    QScriptValue obj = engine->newObject();
    QMapIterator<QString, QVariant> i(map);
    while (i.hasNext()) {
        i.next();
        if (i.value().type() == QVariant::String)
            obj.setProperty(i.key(), i.value().toString());
        else if (i.value().type() == QVariant::Int)
            obj.setProperty(i.key(), i.value().toInt());
        else if (i.value().type() == QVariant::Double)
            obj.setProperty(i.key(), i.value().toDouble());
        else if (i.value().type() == QVariant::List)
            obj.setProperty(i.key(), qScriptValueFromSequence(engine, i.value().toList()));
        else if (i.value().type() == QVariant::Map)
            obj.setProperty(i.key(), encodeInner(i.value().toMap(), engine));
    }
    return obj;
}


QMap<QString, QVariant> Json::decodeInner(QScriptValue object)
{
    QMap<QString, QVariant> map;
    QScriptValueIterator it(object);
    while (it.hasNext()) {
        it.next();
        if (it.value().isArray())
            map.insert(it.name(),QVariant(decodeInnerToList(it.value())));
        else if (it.value().isNumber())
            map.insert(it.name(),QVariant(it.value().toNumber()));
        else if (it.value().isString())
            map.insert(it.name(),QVariant(it.value().toString()));
        else if (it.value().isNull())
            map.insert(it.name(),QVariant());
        else if(it.value().isObject())
            map.insert(it.name(),QVariant(decodeInner(it.value())));
    }
    return map;
}

QList<QVariant> Json::decodeInnerToList(QScriptValue arrayValue)
{
    QList<QVariant> list;
    QScriptValueIterator it(arrayValue);
    while (it.hasNext()) {
        it.next();
        if (it.name() == "length")
            continue;

        if (it.value().isArray())
            list.append(QVariant(decodeInnerToList(it.value())));
        else if (it.value().isNumber())
            list.append(QVariant(it.value().toNumber()));
        else if (it.value().isString())
            list.append(QVariant(it.value().toString()));
        else if (it.value().isNull())
            list.append(QVariant());
        else if(it.value().isObject())
            list.append(QVariant(decodeInner(it.value())));
    }
    return list;
}

This looks very nice... but does not compile... undefined reference to QScriptEngine::newObject ...... and I added #include in the json.h file. Is it even possible to use this with QT 4.8.1. ? — user568021, Dec 16 '13 at 19:51
Awesome solution! It would be a nice header-only library, just include-and-use rather all the all build library thing with infinite dependences. Something to turn that QMap into an object would make it perfect. I managed to do that I'll leave the code here to be used with your code. — Jack, Sep 12 '17 at 03:24

score 9 · Answer 4 · edited Dec 20 '12 at 12:19

9

If you don't want to depend on external libraries you could use the QScriptEngine

http://qtwiki.remdex.info/Parsing_JSON_with_QT_using_standard_QT_library

edited Dec 20 '12 at 12:19

Morten Kristensen

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answered Nov 13 '10 at 12:47

guruz

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Linkrot. From some random "info" domain. Thanks for nothing. – o9000 May 09 '16 at 14:33

score 6 · Answer 5 · answered Jan 22 '14 at 05:38

6

I know this answer is late, but I recently created a project to help create code which is meant to compile on both Qt4 and Qt5 and deals with JSON.

https://code.google.com/p/qjson4/

This library is indented to be a drop in replacement for QJsonDocument in Qt4 and will use Qt's classes when used in Qt5. So it should be a fairly smooth transition. It's not 100% complete, but the major features are in there :-).

answered Jan 22 '14 at 05:38

Evan Teran

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Great answer which I have upvoted. Just a small addition to your code in `QJsonValue.h`: `Q_DECLARE_METATYPE(QJsonValue)` to make this class usable in some special cases. – jonathanzh Mar 25 '16 at 16:12
Sure, I can add that :-) – Evan Teran Mar 25 '16 at 17:19

score 4 · Answer 6 · answered Jul 10 '15 at 12:53

I would recommend qjson-backport, as it uses the same API as in Qt5.

You can conditionally load the library when you use Qt4, and use the default implementation when using Qt5.

My qjson.pri file looks like:

!greaterThan(QT_MAJOR_VERSION, 4): {

    INCLUDEPATH += \
        $$PWD

    SOURCES += \
        $$PWD/qjson.cpp \
        $$PWD/qjsonarray.cpp \
        $$PWD/qjsondocument.cpp \
        $$PWD/qjsonobject.cpp \
        $$PWD/qjsonparser.cpp \
        $$PWD/qjsonvalue.cpp \
        $$PWD/qjsonwriter.cpp

    HEADERS += \
        $$PWD/qjson_p.h \
        $$PWD/qjsonarray.h \
        $$PWD/qjsondocument.h \
        $$PWD/qjsonobject.h \
        $$PWD/qjsonparser_p.h \
        $$PWD/qjsonvalue.h \
        $$PWD/qjsonwriter_p.h

}

easiest way to parse JSON in Qt 4.7

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