A simple class for parsing JSON data into a QVariant hierarchy and vice versa.

Related tags

JSON json qt

The qt-json project is a simple collection of functions for parsing and serializing JSON data to and from QVariant hierarchies.

NOTE: Qt5 introduced a native JSON object class. If you are targeting Qt5, you should use that instead.


Parsing JSON

The parser is really easy to use. Let's say we have the following QString of JSON data:

  "encoding" : "UTF-8",
  "plug-ins" : [
  "indent" : {
    "length" : 3,
    "use_space" : true

We would first call the parse-function:

#include "json.h"

bool ok;
// json is a QString containing the JSON data
QtJson::JsonObject result = QtJson::parse(json, ok).toMap();

if(!ok) {
  qFatal("An error occurred during parsing");

Assuming the parser completed without errors, we can then go through the hierarchy:

qDebug() << "encoding:" << result["encoding"].toString();
qDebug() << "plugins:";

foreach(QVariant plugin, result["plug-ins"].toList()) {
    qDebug() << "  -" << plugin.toString();

QtJson::JsonObject nested = result["indent"].toMap();
qDebug() << "length:" << nested["length"].toInt();
qDebug() << "use_space:" << nested["use_space"].toBool();

The previous code would print out the following:

encoding: "UTF-8"
  - "python"
  - "c++"
  - "ruby"
length: 3
use_space: true

Serializing JSON

To write JSON data from Qt object is as simple as creating and assigning data to a QVariantMap/JsonObject:

QtJson::JsonObject contributor;
contributor["name"] = "Luis Gustavo";
contributor["age"] = 22;

QByteArray data = QtJson::serialize(contributor);

The byte array 'data' contains valid JSON data:

  "name": "Luis Gustavo",
  "age": 22

Serializing JSON pretty-print

By default, the serialization will create a minified version, like following:

{"name":"Luis Gustavo","age":22}

If you are debugging or logging, you may prefer to enable pretty-print mode globally, before serialize:


QByteArray data = QtJson::serialize(contributor);
// ...
QByteArray data = QtJson::serialize(other_contributor);

Obviously, you can disable it with:


After creating the QVariantMap, you can create a QVariantList/JsonArray and append the QVariantMaps.

QtJson::JsonObject friend1, friend2, friend3;
friend1["id"] = 1;
friend1["name"] = "Mackenzie Hamphrey";

friend2["id"] = 2;
friend2["name"] = "Melanie Molligan";

friend3["id"] = 3;
friend3["name"] = "Sydney Calhoun";

QtJson::JsonArray friends;

QtJson::JsonObject obj;
obj["friends"] = friends;

This way you create a nested structure:

    "friends": [
            "id": 1,
            "name": "MackenzieHamphrey"
            "id": 2,
            "name": "MelanieMolligan"
            "id": 3,
            "name": "SydneyCalhoun"

If you continue this process recursively, you nest more levels into the JSON structure.

Using Builders

For simplicity you can use builders, if you prefer.

For example, create a JsonObject:

QtJson::JsonObject json = QtJson::objectBuilder()
    ->set("field_1", 10)
    ->set("field_2", "A string")
    ->set("field_3", true)
    ->set("field_4", QtJson::objectBuilder()
        ->set("sub_field_1", 10.4)
        ->set("sub_field_n", "Another string")

Or create a JsonArray:

QtJson::JsonArray json = QtJson::arrayBuilder()
    ->add("anything else")

Take a look at this example that rewrite the previous one:

QtJson::JsonObject obj = QtJson::objectBuilder()
    ->set("friends", QtJson::arrayBuilder()
            ->set("id", 1)
            ->set("name", "Mackenzie Hamphrey")
            ->set("id", 2)
            ->set("name", "Melanie Molligan")
            ->set("id", 3)
            ->set("name", "Sydney Calhoun")


Send in a pull request and bug the maintainer until it gets merged and published. Make sure to add yourself to AUTHORS.

  • The library will be no longer necessary

    The library will be no longer necessary


    The Qt5 will be released in the end of June with many new features such as support for JSON: http://doc.qt.nokia.com/qt5-snapshot/json.html

    In a short time the qt-json library will be no longer necessary and I want to thank anybody who is involved in it.

    opened by gustavosbarreto 8
  • Add QtJson::Object class implementation

    Add QtJson::Object class implementation


    It's useful to me to use json object with json[key] = value semantics. This patch set adds QVariant-based QtJson::Object class that implements such an ability. Now it's possible to use the following:

    QtJson::Object json;
    json["a"] = 0;
    json["b"]["c"] = 1;
    json["b"]["d"] = 2;
    json["b"]["e"]["f"] = QString("hello");
    opened by milabs 3
  • Updated Documentation

    Updated Documentation

    replaced readme.txt with markdown version.

    added info regarding nested JSON structure creation

    added note regarding Qt5's native QJsonObject

    Included Qt4.8 class reference links for the classes used in the examples, as well as a link to the example solve by ereilin in stackoverflow

    opened by joum 3
  • Unicode string problem

    Unicode string problem


    I moved from Qt 5.0.2 to Q t4.8.4. On Qt5 I used json parser provided by Qt. On Qt 4.8.4 I needed find own. I like your simple json package. It work fine except unicode strings. I send json via websocket client. I also writting sent json into file. When sending string with special polish characters (like "ść") I see in log file "msg" : "ść" server returns me "msg" : "ść". On QT5 everything worked fine. Need I some extra code to handle unicode? I construct json like this:

    o["msg"] = ui->editMsg->text().toUtf8();

    And "send" function is:

    void send(const QString &msg)
      logg.send("Sent", msg);


    opened by dibok 2
  • Why class?

    Why class?

    Why is QtJson API encapsulated in a class? As far as I know, there are no non-static field in the class, the class has no state, therefore, I'd say it's useless.

    Also, it exposes the private function in .h.

    Why not just leave the public function in QtJson namespace without a class and those private functions as static in the .cpp

    To explain what I mean I'll try attach code/commit...

    opened by vojtechkral 2
  • Update json.h

    Update json.h

    opened by BriFuture 1
  • Reduces spaces

    Reduces spaces

    As the serialized output is not suitable for end-users (due to the lack of newlines and proper indentation), the spaces can be removed too.

    Feel free to close this PR in case it is not in line with the goals of the project.

    opened by george-hopkins 1
  • Added QStringList inclusion.

    Added QStringList inclusion.

    This should fix problems with compilation on OS/2 + Qt4.

    [ 34%] Building CXX object CMakeFiles/rssguard.dir/src/gui/dialogs/formdatabasecleanup.cpp.obj H:/rssguard/src/qt-json/json.cpp: In function 'QVariant QtJson::clone(const QVariant&)': H:/rssguard/src/qt-json/json.cpp:120:37: error: invalid use of incomplete type 'class QStringList' cloneList(v, data.toStringList()); ^ In file included from C:/usr/include/QtCore/qdatetime.h:45:0, from C:/usr/include/QtCore/QDateTime:1, from H:/rssguard/src/qt-json/json.cpp:23: C:/usr/include/QtCore/qstring.h:94:7: error: forward declaration of 'class QStringList' class QStringList; ^ CMakeFiles/rssguard.dir/build.make:714: recipe for target 'CMakeFiles/rssguard.dir/src/qt-json/json.cpp.obj' failed make[2]: *** [CMakeFiles/rssguard.dir/src/qt-json/json.cpp.obj] Error 1 make[2]: *** Waiting for unfinished jobs.... CMakeFiles/Makefile2:128: recipe for target 'CMakeFiles/rssguard.dir/all' failed make[1]: *** [CMakeFiles/rssguard.dir/all] Error 2 Makefile:146: recipe for target 'all' failed make: *** [all] Error 2

    opened by martinrotter 1
  • Wrong conversion to double!

    Wrong conversion to double!

    I tested and found that the use of:

    str = QByteArray::number(value, 'g', 20); //line 123 json.cpp

    is a really problem (eg: 41.32 converted in "41.320000000000000284" !!!!!!!)

    it seems depend on the precision you used (20). Partially(not completely tested) solved in the following way:

    str = QByteArray::number(value, 'g'); //line 123 json.cpp

    Thanks for your nice job.

    opened by cydside 1
  • Use template to serialize QVariantHash and QVariantMap

    Use template to serialize QVariantHash and QVariantMap

    Use template to serialize QVariantHash and QVariantMap, So the code looks cleaner.

    BTW: I suggest expanding tab to spaces, the code otherwise looks strange in editors with different tab setting.

    opened by sunxiaoguang 1
  • Corrected serialization of long values.

    Corrected serialization of long values.

    Shouldn't be calling sanatize string when serializing long values. It adds quotes around the number thus implying it's a string. There shouldn't be anything to santizes anyway.

    opened by klnusbaum 1
  • Looking for a maintainer

    Looking for a maintainer

    I am not the right person to maintain qt-json anymore, actually I haven't been for a long time. I have professionally moved away from the Qt ecosystem and lack the time and passion that I once had for this.

    So any person willing to dedicate a little time for this project please step forward.

    ~I will be moving this repository under the new qt-json organization.~

    opened by gaudecker 0
  • Improve the reliability of the parser

    Improve the reliability of the parser

    See Parsing JSON is a Minefield.

    There is a ton of work to make qt-json pass the test cases in the article. If someone has the time to check them, I'm sure that would be much appreciated by the users.

    opened by gaudecker 0
  • Occasionally needed: json parse w/ an ordered QVariantMap, e.g. Prefs dialog setup

    Occasionally needed: json parse w/ an ordered QVariantMap, e.g. Prefs dialog setup

    This can be accomplished by subclassing QVariantMap:

    #include <QtCore>
    class OrderedVariantMap : public QMap<QString, QVariant> {
    // Test:
    //    OrderedVariantMap test_map;
    //    test_map.insert("xxx", 1);
    //    test_map.insert("aaa", 2);
    //    test_map.insert("kkk", 3);
    //    test_map["321"] = 4;
    //    test_map["000"] = 5;
    //    test_map["123"] = 6;
    //    qDebug() << "QMap.keys()" << test_map.keys();
    //    qDebug() << "QMap.orderedKeys()" << test_map.orderedKeys();
    //    QVariant test_variant;
    //    test_variant.setValue(test_map);
    //    qDebug() << "test_variant.typeName()" << test_variant.typeName();
    //    OrderedVariantMap test_map_recovered = qvariant_cast<OrderedVariantMap>(test_variant);
    //    qDebug() << "test_map_recovered.orderedKeys()" << test_map_recovered.orderedKeys();
    // Test results:
    //    QMap.keys() ("000", "123", "321", "aaa", "kkk", "xxx")
    //    QMap.orderedKeys() ("xxx", "aaa", "kkk", "321", "000", "123")
    //    test_variant.typeName() OrderedVariantMap
    //    test_map_recovered.orderedKeys() ("xxx", "aaa", "kkk", "321", "000", "123")
        OrderedVariantMap ( );
        ~OrderedVariantMap ( );
        clear ( );
        void // QMap::iterator
        insert ( const QString &key,
                 const QVariant &value );
        operator[] ( const QString &key );
        const QVariant
        operator[] ( const QString &key ) const;
        const QString
        orderedKey ( int index ) const;
        const QVariant
        orderedValue ( int index ) const;
        orderedKeys ( ) const ;
        QStringList Ordered_Keys;


    #include "OrderedVariantMap.h"
    OrderedVariantMap::OrderedVariantMap ( ) : QMap ( ) {
    OrderedVariantMap::~OrderedVariantMap ( ) {
    OrderedVariantMap::orderedKeys ( ) const {
        return Ordered_Keys;
    OrderedVariantMap::clear ( ) {
    void // QMap::iterator
    OrderedVariantMap::insert ( const QString &key,
                                const QVariant &value ) {
        QMap::insert(key, value);
    OrderedVariantMap::operator[] ( const QString &key ) {
        return QMap::operator [](key);
    const QVariant
    OrderedVariantMap::operator[] ( const QString &key ) const {
        return this->value(key);
    const QString
    OrderedVariantMap::orderedKey ( int index ) const {
        return Ordered_Keys[index];
    const QVariant
    OrderedVariantMap::orderedValue ( int index ) const {
        return this->value(Ordered_Keys[index]);

    I've made the changes to json.h/.cpp which I'll send if you're interested.


    kcrossen at mail server gmail.com

    opened by kcrossen 0
  • Too many QString constructions/destructions around

    Too many QString constructions/destructions around

    Hi there, profiler suggests (and it is right this time) that below mentioned methods create myriads of QString temporary instances that can be easily avoided as written below - using static QString instance for the very static contents.

    diff -r 181aa426115e qtjson/json.cpp --- a/qtjson/json.cpp Wed Nov 21 10:04:39 2012 +0100 +++ b/qtjson/json.cpp Wed Nov 21 14:13:44 2012 +0100 @@ -503,11 +503,12 @@ */ int Json::lastIndexOfNumber(const QString &json, int index) {

    • static const QString numericCharacters("0123456789+-.eE"); int lastIndex;
     for(lastIndex = index; lastIndex < json.size(); lastIndex++)
    •            if(QString("0123456789+-.eE").indexOf(json[lastIndex]) == -1)
    •            if(numericCharacters.indexOf(json[lastIndex]) == -1)
      @@ -521,9 +522,11 @@ */ void Json::eatWhitespace(const QString &json, int &index) {
    • static const QString whitespaceChars(" \t\n\r");
    • for(; index < json.size(); index++) {
    •            if(QString(" \t\n\r").indexOf(json[index]) == -1)
    •            if(whitespaceChars.indexOf(json[index]) == -1)
    opened by JanKovis 0
  • Empty string parses as null string

    Empty string parses as null string

        "data": [

    When parsed, "" becomes a null QString, as opposed to an empty QString. The difference is significant. This seems to be an issue in Json::parseString, where the initial result string is created as QString s, rather than QString s("") or QString s = "".

    opened by Mzyxptlk 3
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