JSON parsing with Qt - c++

Question

Hei , I have this text in a JSON :
( without the returns all in one line)
[
{
"ERROR":false,
"USERNAME":"Benutzer",
"FORMAT":"HUMAN",
"LATITUDE_MIN":84,
"LATITUDE_MAX":36,
"LONGITUDE_MIN":5,
"LONGITUDE_MAX":20,
"RECORDS":203
},
[
{
"MMSI":233434540,
"TIME":"2014-10-09 06:19:06 GMT",
"LONGITUDE":8.86037,
"LATITUDE":54.12666,
"COG":347,
"SOG":0,
"HEADING":236,
"NAVSTAT":0,
"IMO":0,
"NAME":"HELGOLAND",
"CALLSIGN":"DK6068",
"TYPE":90,
"A":20,
"B":15,
"C":4,
"D":4,
"DRAUGHT":2,
"DEST":"BREMERHAVEN",
"ETA":"00-00 00:00"
},
{
"MMSI":319072300,
"TIME":"2014-10-09 06:08:53 GMT",
"LONGITUDE":9.71578,
"LATITUDE":54.31949,
"COG":343.6,
"SOG":0,
"HEADING":197,
"NAVSTAT":5,
"IMO":1012189,
"NAME":"M.Y. ESTER III",
"CALLSIGN":"ZGED3",
"TYPE":37,
"A":31,
"B":35,
"C":7,
"D":6,
"DRAUGHT":3.5,
"DEST":"SCHACT AUDORF",
"ETA":"09-16 08:00"
}
// many more lines but the Json IS VALID.
]
]
I would parse it and put that in a MYSQL table.
Not all, only name and MMSI first.
But this don't view anything in my consle because its dont jump in the foreach:
bool ok = true;
// my json data is in reply & ok is a boolean
QVariantList result = parser.parse(reply, &ok).toList();
foreach(QVariant record, result) {
QVariantMap map = record.toMap();
qDebug() << map.value("NAME");
}
What's wrong ?
When i debug, i only see that it doesn't jump in the foreach.
I use the QJson libary : QJson::Parser parser; But please anyone can tell me what i do wrong?

Answer 1

Your code looks like you are iterating over top level array, while the data you are looking for is in the nested array, which is effectively the second item of the top level array. So, you need to iterate over items in the inner array.
The following code works for me with your sample JSON:
QVariantList result = parser.parse(reply, &ok).toList().at(1).toList();
foreach (const QVariant &item, result) {
QVariantMap map = item.toMap();
qDebug() << map["NAME"].toString();
qDebug() << map["MMSI"].toLongLong();
}

Answer 2

If you are using Qt5 or above, you can make use of the awesome features provided by QJsonDocument, QJsonObject and QJsonArray.
I have copied your json into a file named test.txt in my D-drive and the code below works fine.
QJsonDocument jsonDoc;
QByteArray temp;
QFile file("D://test.txt");
if (file.open(QIODevice::ReadOnly | QIODevice::Text))
{
temp = file.readAll();
}
jsonDoc = QJsonDocument::fromJson(temp);
QJsonArray jsonArray = jsonDoc.array().at(1).toArray(); //Since you are interested in the json array which is the second item and not the first json object with error
for(int i =0; i < jsonArray.size(); ++i)
{
QJsonObject jsonObj = jsonArray.at(i).toObject();
int mmsi = jsonObj.find("MMSI").value().toInt();
QString name = jsonObj.find("NAME").value().toString();
qDebug() << mmsi;
qDebug() << name;
}
If you have to stick to Qt4, you can try using the qjson4 library which tries to mimic the behavior of the 'QJsonDocument' which is part of Qt5.