Qt parsing JSON using QJsonDocument, QJsonObject, QJsonArray

Question

I'm using Qt5. I am trying to obtain values from a json object. Here is what the json object looks like that I am trying to get data from:

{
    "success": true,
    "properties": [
        {
            "ID": 1001,
            "PropertyName": "McDonalds",
            "key": "00112233445566778899aabbccddeeff"
        },
        {
            "ID": 1002,
            "PropertyName": "Burger King",
            "key": "10112233445566778899aabbccddeeff"
        },
        {
            "ID": 1003,
            "PropertyName": "Taco Bell",
            "key": "20112233445566778899aabbccddeeff"
        }
    ]
}

How can I create three arrays that contain properties[x].ID, properties[x].PropertyName, and properties[x].key in Qt?

Edit:

Using QScriptEngine I tried this:

QString data = (QString)reply->readAll();

QScriptEngine engine;

QScriptValue result = engine.evaluate(data);

qDebug() << result.toString();

Debug is saying "SyntaxError: Parse error"

Actually right now I'm trying to use QScriptEngine. I'm looking at this example at the moment: makina-corpus.org/blog/access-json-webservice-qt-c — Jared Price
– Jared Price, Commented Nov 6, 2013 at 21:07

NG_ · Accepted Answer · 2015-01-05 20:51:20Z

79

I figured it out:

QStringList propertyNames;
QStringList propertyKeys;
QString strReply = (QString)reply->readAll();
QJsonDocument jsonResponse = QJsonDocument::fromJson(strReply.toUtf8());
QJsonObject jsonObject = jsonResponse.object();
QJsonArray jsonArray = jsonObject["properties"].toArray();

foreach (const QJsonValue & value, jsonArray) {
    QJsonObject obj = value.toObject();
    propertyNames.append(obj["PropertyName"].toString());
    propertyKeys.append(obj["key"].toString());
}

edited Jan 5, 2015 at 20:51

NG_

7,2618 gold badges50 silver badges70 bronze badges

answered Nov 6, 2013 at 21:43

Jared Price

5,4337 gold badges47 silver badges76 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

Myanju Over a year ago

What is reply->readAll(); Reply from what? HTTP server reply? You accessed database via HTTP get/post method? I'm just curious.

Myanju Over a year ago

I have networkReply Like this

[{"Serial Number":"0000000123","Trip Number":"0000000234","Destination":"blr","Passenger Name":"name"}, {"Serial Number":"0000000125","Trip Number":"0000002314","Destination":"rtg","Passenger Name":"name3"}, {"Serial Number":"0000000125","Trip Number":"0000002345","Destination":"udp","Passenger Name":"name2"}, {"Serial Number":"0000000124","Trip Number":"0000009878","Destination":"drm","Passenger Name":"name1"}]

How can I parse this using your answer?

Jared Price Over a year ago

Well the difference there is that you have an array where I started with an object in my example. I would assume that you could call QString strReply = (QString)reply->readAll(); then QJsonDocument jsonResponse = QJsonDocument::fromJson(strReply.toUtf8()); then QJsonArray jsonArray = jsonResponse.array();. Disclaimer: I didn't actually try this, but I would think it would work.

locka Over a year ago

This is extremely inefficient. When you assign readAll() to a QString you're causing a byte array to be converted from utf8 bytes to a utf16 QString. And then when you call fromJson you're causing the utf16 string to be converted back into a new utf8 byte array. Change strReply to a QByteArray and you avoid all this pointless conversion. More to the point, if you're running this code on a memory constrained device you avoid about 4x the memory overhead.

BSMP · Accepted Answer · 2018-02-17 06:44:31Z

Here is an example from How To Manipulate JSON With C++ and Qt.

// reads a json file from disk to QVariantMap
// originally from http://erickveil.github.io/2016/04/06/How-To-Manipulate-JSON-With-C++-and-Qt.html
bool readJsonFile(std::string file_path, QVariantMap& result)
{
    // step 1
    QFile file_obj(QString::fromStdString(file_path));
    if (!file_obj.open(QIODevice::ReadOnly)) {
    std::cout << "Failed to open " << file_path << std::endl;
    exit(1);
    }

    // step 2
    QTextStream file_text(&file_obj);
    QString json_string;
    json_string = file_text.readAll();
    file_obj.close();
    QByteArray json_bytes = json_string.toLocal8Bit();

    // step 3
    auto json_doc = QJsonDocument::fromJson(json_bytes);

    if (json_doc.isNull()) {
        std::cout << "Failed to create JSON doc." << std::endl;
        return false;
    }
    if (!json_doc.isObject()) {
        std::cout << "JSON is not an object." << std::endl;
        return false;
    }

    QJsonObject json_obj = json_doc.object();

    if (json_obj.isEmpty()) {
        std::cout << "JSON object is empty." << std::endl;
        return false;
    }

    // step 4
    result = json_obj.toVariantMap();
    return true;
}

// writes a QVariantMap to disk
bool writeJsonFile(QVariantMap point_map, std::string file_path)
{
    QJsonObject json_obj = QJsonObject::fromVariantMap(point_map);
    QJsonDocument json_doc(json_obj);
    QString json_string = json_doc.toJson();

    QFile save_file(QString::fromStdString(file_path));
    if (!save_file.open(QIODevice::WriteOnly)) {
        std::cout << "failed to open save file" << std::endl;
        return false;
    }
    save_file.write(json_string.toLocal8Bit());
    save_file.close();
    return true;
}

Saeid Amini · Accepted Answer · 2022-01-12 10:59:13Z

0

I'm using QT Network for my API request to Firebase but for some reason, I received a response full of data but when parsing to an array I received null.

void firebaseHandler::parseResponse(const QString &response){
    @TODO for some reason sometimes the arrayData is empty 

    QJsonDocument jsonDocument = QJsonDocument::fromJson(response.toUtf8());
    QJsonValue kind =  jsonDocument.object().value("kind");
    QJsonArray arrayData = jsonDocument.array();

     qDebug() << "json response is " << jsonDocument.toJson();
}

edited Jan 12, 2022 at 10:59

Saeid Amini

1,3055 gold badges20 silver badges27 bronze badges

answered Jan 6, 2022 at 13:25

dave tamo

1

Collectives™ on Stack Overflow

Qt parsing JSON using QJsonDocument, QJsonObject, QJsonArray

3 Answers 3

4 Comments

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

4 Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related