How to add object to Json file - c++

Question

I am trying to log data from 3 sensors to a json file. All I want to be able to accomplish is to write the Speed, Latitude and Longitude to a Json file, with an object containing each of the above. That is a json file that contains one route object, n sub objects each of which contain speed, latitude, longitude.
These 3 values I get from 3 global QList lists. Below is the json file which is stored locally. (The double values are not actual values, just for testing purposes)
{
"Sensordata": [
{
"Speed": 1,
"GPSLat":-12.5687,
"GPSLong":26.125546
},
{
"Speed": 1,
"GPSLat":-12.5687,
"GPSLong":26.125546
}
]
}
This is what the json must look like and when I add it must be formatted in the same way
void MainWindow::save_to_json() {
QFile file_obj(".../SensorData.json");
if(!file_obj.open(QIODevice::ReadOnly)){
qDebug()<<"Failed to open "<<"SensorData.json";
exit(1);
}
QTextStream file_text(&file_obj);
QString json_string;
json_string = file_text.readAll();
file_obj.close();
QByteArray data_json = json_string.toLocal8Bit();
QJsonDocument doc = QJsonDocument::fromJson(data_json);
QJsonObject rootObj = doc.object();
QJsonValue SensorData = rootObj.value("SensorData");
if(!SensorData.isArray())
{
// array expected - handle error
}
QJsonArray SensorDataArray = SensorData.toArray();
QJsonObject newObject;
newObject["Speed"] = speed_array.takeFirst();
newObject["GPSLat"] = gps_lat.takeFirst();
newObject["GPSLong"] = gps_long.takeFirst();
SensorDataArray.push_back(newObject);
}
ASSERT: "!isEmpty()" in file /home/username/Qt/5.12.1/gcc_64/include /QtCore/qlist.h, line 347
11:32:55: The program has unexpectedly finished.
11:32:55: The process was ended forcefully.
This is the error the above code creates.

Answer 1

To modify the data, given your example, you need to check if the contained data in the QJsonDocument is an array or a simple object. In your case, I suppose you want to append data to an array. Try something like this:
// Read the data
const QString filename = "example.json";
QJsonDocument doc = read(filename);
// Check that it's an array and append new data
QJsonValue sensorData = doc.value("SensorData");
if (!sensorData.isArray()) {
// if the doc is empty you may want to create it
}
// Get the array and insert the data
auto array = sensorData.array();
array.append(QJsonObject{
{"Speed", speed_array.takeFirst()},
{"GPSLat", gps_lat.takeFirst()},
{"GPSLong",gps_long.takeFirst(),
});
// Restore your sensor data
doc.setObject(QJsonObject{{"SensorData", array}});
// Write the new data
write(filename, doc);
A helper functions to read/write JSON documents may avoid the mistake of open/closing a file:
QJsonDocument read(const QString& filename) {
QFile file(filename);
file.open(QIODevice::ReadOnly | QIODevice::Text);
const QString val = file.readAll();
file.close();
return QJsonDocument::fromJson(val.toUtf8());
}
void write(const QString& filename, const QJsonDocument& document) {
QFile file(filename);
file.open(QFile::WriteOnly | QFile::Text | QFile::Truncate);
file.write(document.toJson());
file.close();
}
Updates
To not overwrite the original doc, you must update the field of the root object or use QJsonValueRef.
// Get a reference to your array
QJsonObject root = doc.object();
QJsonValueRef ref = root.find("SensorData").value();
// get the array and insert the data
QJsonArray array = ref.toArray();
array.append(QJsonObject{
{"Speed", speed_array.takeFirst()},
{"GPSLat", gps_lat.takeFirst()},
{"GPSLong",gps_long.takeFirst(),
});
// Update the ref with the new data
ref = array
// update the modified data in the json document
doc.setObject(root);