How to Write and read in JSON file using QT - json

Question

I write this code and it append my json file with all data again.
After this content of my file become twice.
QFile file("C:/Users/Vizexperts/Documents/QT projects/Json_File_Read_Write/new 1.json");
file.open(QIODevice::ReadWrite|QIODevice::Text);
QString dataString=file.readAll();
QJsonDocument doc= QJsonDocument::fromJson(dataString.toUtf8());
QJsonObject dataobject =doc.object();
qDebug()<<(dataobject)["D"].toString();
QJsonValue valueA=dataobject.value(QString("A"));
QJsonObject valueAobject=valueA.toObject();
QJsonValue valueA1=valueAobject["A1"];
qDebug()<<valueA1.toString();
ui->label->setText(valueA1.toString());
dataobject["D"]="a";
doc.setObject(dataobject);
file.write(doc.toJson());
file.close();

Answer 1

There is no easy way to edit a single value in a JSON file and then writing it back.
The easiest (in terms of prone to errors) way is to
Open file
Load text into JSON object
Edit data
Write JSON object back to file
When you open a file for writing and want to overwrite the existing data in it, you need to also use the QIODevice::Truncate flag. This truncates the text inside your JSON file and whatever you write to that file erases the previous content.
If you want, you can open the file twice (and close it accordingly) - once when you read from it (so no writing access to it) and once when you write to it. This way you can handle the given operation's specific errors that may occur. If error handling is not something you want to bother yourself with, just add the extra flag and you are good to go.

Answer 2

It appears you are rewriting the JSONdoc to file.
file.write(doc.toJson());
As far as I know there is no way to just update the file with the change you did in the code. The easy way is to clear the file and rewrite the whole object again.
A bigger challenge would be to parse the file again, compare the JSON objects, find the cursor position where you need to write the changes and update the file. Depending on the size of the JSON object, this seems over-engineered.