How to create/read/write JSON files in Qt5

Example: Read json from file

/* test.json */{   "appDesc": {      "description": "SomeDescription",      "message": "SomeMessage"   },   "appName": {      "description": "Home",      "message": "Welcome",      "imp":["awesome","best","good"]   }}void readJson()   {      QString val;      QFile file;      file.setFileName("test.json");      file.open(QIODevice::ReadOnly | QIODevice::Text);      val = file.readAll();      file.close();      qWarning() << val;      QJsonDocument d = QJsonDocument::fromJson(val.toUtf8());      QJsonObject sett2 = d.object();      QJsonValue value = sett2.value(QString("appName"));      qWarning() << value;      QJsonObject item = value.toObject();      qWarning() << tr("QJsonObject of description: ") << item;      /* in case of string value get value and convert into string*/      qWarning() << tr("QJsonObject[appName] of description: ") << item["description"];      QJsonValue subobj = item["description"];      qWarning() << subobj.toString();      /* in case of array get array and convert into string*/      qWarning() << tr("QJsonObject[appName] of value: ") << item["imp"];      QJsonArray test = item["imp"].toArray();      qWarning() << test[1].toString();   }

OUTPUT

QJsonValue(object, QJsonObject({"description": "Home","imp": ["awesome","best","good"],"message": "YouTube"}) ) "QJsonObject of description: " QJsonObject({"description": "Home","imp": ["awesome","best","good"],"message": "YouTube"}) "QJsonObject[appName] of description: " QJsonValue(string, "Home") "Home" "QJsonObject[appName] of value: " QJsonValue(array, QJsonArray(["awesome","best","good"]) ) "best" 

Example: Read json from string

Assign json to string as below and use the readJson() function shown before:

val =   '  {       "appDesc": {          "description": "SomeDescription",          "message": "SomeMessage"       },       "appName": {          "description": "Home",          "message": "Welcome",          "imp":["awesome","best","good"]       }    }';

OUTPUT

QJsonValue(object, QJsonObject({"description": "Home","imp": ["awesome","best","good"],"message": "YouTube"}) ) "QJsonObject of description: " QJsonObject({"description": "Home","imp": ["awesome","best","good"],"message": "YouTube"}) "QJsonObject[appName] of description: " QJsonValue(string, "Home") "Home" "QJsonObject[appName] of value: " QJsonValue(array, QJsonArray(["awesome","best","good"]) ) "best" 

Sadly, many JSON C++ libraries have APIs that are non trivial to use, while JSON was intended to be easy to use.

So I tried jsoncpp from the gSOAP tools on the JSON doc shown in one of the answers above and this is the code generated with jsoncpp to construct a JSON object in C++ which is then written in JSON format to std::cout:

value x(ctx);x["appDesc"]["description"] = "SomeDescription";x["appDesc"]["message"] = "SomeMessage";x["appName"]["description"] = "Home";x["appName"]["message"] = "Welcome";x["appName"]["imp"][0] = "awesome";x["appName"]["imp"][1] = "best";x["appName"]["imp"][2] = "good";std::cout << x << std::endl;

and this is the code generated by jsoncpp to parse JSON from std::cin and extract its values (replace USE_VAL as needed):

value x(ctx);std::cin >> x;if (x.soap->error)  exit(EXIT_FAILURE); // error parsing JSON#define USE_VAL(path, val) std::cout << path << " = " << val << std::endlif (x.has("appDesc")){  if (x["appDesc"].has("description"))    USE_VAL("$.appDesc.description", x["appDesc"]["description"]);  if (x["appDesc"].has("message"))    USE_VAL("$.appDesc.message", x["appDesc"]["message"]);}if (x.has("appName")){  if (x["appName"].has("description"))    USE_VAL("$.appName.description", x["appName"]["description"]);  if (x["appName"].has("message"))    USE_VAL("$.appName.message", x["appName"]["message"]);  if (x["appName"].has("imp"))  {    for (int i2 = 0; i2 < x["appName"]["imp"].size(); i2++)      USE_VAL("$.appName.imp[]", x["appName"]["imp"][i2]);  }}

This code uses the JSON C++ API of gSOAP 2.8.28. I don't expect people to change libraries, but I think this comparison helps to put JSON C++ libraries in perspective.

JSON under QT is actually quite pleasant - I was surprised. This is an example of how to create a JSON output with some structure.

Forgive me for not explaining what the fields all mean - it is a Ham Radio processing output script.

This is the QT C++ Code

void CabrilloReader::JsonOutputMapper(){  QFile file(QDir::homePath() + "/1.json");  if(!file.open(QIODevice::ReadWrite)) {      qDebug() << "File open error";    } else {      qDebug() <<"JSONTest2 File open!";    }  // Clear the original content in the file  file.resize(0);  // Add a value using QJsonArray and write to the file  QJsonArray jsonArray;  for(int i = 0; i < 10; i++) {      QJsonObject jsonObject;      CabrilloRecord *rec= QSOs.at(i);      jsonObject.insert("Date", rec->getWhen().toString());      jsonObject.insert("Band", rec->getBand().toStr());      QJsonObject jsonSenderLatObject;      jsonSenderLatObject.insert("Lat",rec->getSender()->fLat);      jsonSenderLatObject.insert("Lon",rec->getSender()->fLon);      jsonSenderLatObject.insert("Sender",rec->getSender_call());      QJsonObject jsonReceiverLatObject;      jsonReceiverLatObject.insert("Lat",rec->getReceiver()->fLat);      jsonReceiverLatObject.insert("Lon",rec->getReceiver()->fLon);      jsonReceiverLatObject.insert("Receiver",rec->getReceiver_call());      jsonObject.insert("Receiver",jsonReceiverLatObject);      jsonObject.insert("Sender",jsonSenderLatObject);      jsonArray.append(jsonObject);      QThread::sleep(2);    }  QJsonObject jsonObject;  jsonObject.insert("number", jsonArray.size());  jsonArray.append(jsonObject);  QJsonDocument jsonDoc;  jsonDoc.setArray(jsonArray);  file.write(jsonDoc.toJson());  file.close();  qDebug() << "Write to file";}

It takes an internal QT Structure (a List of pointers to a CabrilloRecord object ... which you just ignore) and extracts some fields. These fields are then output in a nested JSON format which looks like this.

[    {        "Band": "20",        "Date": "Sat Jul 10 12:00:00 2021",        "Receiver": {            "Lat": 36.400001525878906,            "Lon": 138.3800048828125,            "Receiver": "8N3HQ       "        },        "Sender": {            "Lat": 13,            "Lon": 122,            "Sender": "DX0HQ       "        }    },    {        "Band": "20",        "Date": "Sat Jul 10 12:01:00 2021",        "Receiver": {            "Lat": 36.400001525878906,            "Lon": 138.3800048828125,            "Receiver": "JA1CJP      "        },        "Sender": {            "Lat": 13,            "Lon": 122,            "Sender": "DX0HQ       "        }    }]

I hope this speeds up someone else's progression on this topic.