This tutoiral demonstrates how to build and parse JSON data using Qt's JSON classes, based on a practical example structure.

Sample JSON Data

{
    "name": "BeJson",
    "url": "http://www.bejson.com",
    "page": 88,
    "isNonProfit": true,
    "address": {
        "city": "江苏苏州",
        "country": "中国",
        "street": "科技园路"
    },
    "links": [
        {
            "name": "Google",
            "url": "http://www.google.com"
        },
        {
            "name": "Baidu",
            "url": "http://www.baidu.com"
        },
        {
            "name": "SoSo",
            "url": "http://www.SoSo.com"
        }
    ]
}

Implementation

#include <QtCore/QCoreApplication>
#include <QDir>
#include <QFile>
#include <QDebug>

#include <QJsonArray>
#include <QJsonDocument>
#include <QJsonObject>
#include <QJsonValue>

void serializeJson()
{
    QJsonObject dataObject;

    // Add basic string and numeric fields
    dataObject.insert("name", "BeJson");
    dataObject.insert("url", "http://www.bejson.com");
    dataObject.insert("page", 88);
    dataObject.insert("isNonProfit", true);

    // Add nested address object
    QJsonObject locationObject;
    locationObject.insert("street", "科技园路");
    locationObject.insert("city", "江苏苏州");
    locationObject.insert("country", "中国");

    dataObject.insert("address", locationObject);

    // Add array of link objects
    QJsonObject googleItem;
    googleItem.insert("name", "Google");
    googleItem.insert("url", "http://www.google.com");

    QJsonObject baiduItem;
    baiduItem.insert("name", "Baidu");
    baiduItem.insert("url", "http://www.baidu.com");

    QJsonObject sosoItem;
    sosoItem.insert("name", "SoSo");
    sosoItem.insert("url", "http://www.SoSo.com");

    QJsonArray contactsArray;
    contactsArray.append(googleItem);
    contactsArray.append(baiduItem);
    contactsArray.append(sosoItem);

    dataObject.insert("links", contactsArray);

    // Convert to JSON string
    QJsonDocument doc(dataObject);
    QByteArray jsonData = doc.toJson(QJsonDocument::Compact);

    qDebug().noquote() << QString::fromUtf8(jsonData);

    // Write to file
    QString appDir = QCoreApplication::applicationDirPath();
    QString filePath = QDir::cleanPath(appDir + QDir::separator() + "output.json");
    QFile file(filePath);
    
    if (file.open(QIODevice::WriteOnly)) {
        file.write(jsonData);
        file.close();
        qDebug() << "File written successfully:" << filePath;
    } else {
        qDebug() << "Failed to open file for writing";
    }
}

void deserializeJson()
{
    // Read JSON from file
    QString appDir = QCoreApplication::applicationDirPath();
    QString filePath = QDir::cleanPath(appDir + QDir::separator() + "output.json");
    QFile file(filePath);
    
    if (!file.open(QIODevice::ReadOnly)) {
        qDebug() << "Failed to open file for reading";
        return;
    }
    
    QByteArray jsonData = file.readAll();
    file.close();

    // Parse JSON string
    QJsonDocument doc = QJsonDocument::fromJson(jsonData);
    
    if (doc.isNull() || !doc.isObject()) {
        qDebug() << "Invalid JSON format";
        return;
    }

    QJsonObject dataObject = doc.object();
    
    // Iterate through all keys
    for (auto it = dataObject.begin(); it != dataObject.end(); ++it) {
        QString key = it.key();
        QJsonValue val = it.value();

        if (val.isBool()) {
            qDebug() << key << ":" << val.toBool();
        } 
        else if (val.isString()) {
            qDebug() << key << ":" << val.toString();
        } 
        else if (val.isDouble()) {
            qDebug() << key << ":" << val.toDouble();
        } 
        else if (val.isObject()) {
            qDebug() << key << ":";
            
            QJsonObject locationObject = val.toObject();
            qDebug() << "    street:" << locationObject["street"].toString();
            qDebug() << "    city:" << locationObject["city"].toString();
            qDebug() << "    country:" << locationObject["country"].toString();
        } 
        else if (val.isArray()) {
            qDebug() << key << ":";
            
            QJsonArray linksArray = val.toArray();
            for (int i = 0; i < linksArray.size(); ++i) {
                QJsonObject itemObject = linksArray[i].toObject();
                QString name = itemObject["name"].toString();
                QString url = itemObject["url"].toString();
                
                qDebug() << "    name:" << name << ", url:" << url;
            }
        }
    }
}

int main(int argc, char *argv[])
{
    QCoreApplication app(argc, argv);
    
    serializeJson();
    qDebug() << "---";
    deserializeJson();
    
    return 0;
}

Key Classes Used

• QJsonObject: Represents a JSON object (key-value pairs)
• QJsonArray: Represents a JSON array (ordered list of values)
• QJsonDocument: Container for JSON data, used for conversion between JSON and byte array
• QJsonValue: Represents a single JSON value with type detection methods

Type Detection Methods

QJsonValue provides various methods to check the type of a value:

• isBool() - Check if value is boolean
• isString() - Check if value is string
• isDouble() - Check if value is number
• isObject() - Check if value is nested object
• isArray() - Check if value is array