Qt save strings to JSON - c++

Question

Im trying to save my waypoints into .json file, but it is always saving the wrong string.. which I dont know where it is coming.
This is my code:
for (int i = 1; i < path->GetPointCount(); i++) {
Vector lonlatalt;
path->GetPointLocation(i).GetGeodetic(&lonlatalt);
sstr << "waypoint";
sstr << i;
sstr << " ";
sstr << lonlatalt.x;
sstr << " ";
sstr << lonlatalt.y;
sstr << " 0.0";
sstr << std::endl;
std::string alt = " 0.0";
QJsonObject json;
json["waypoint"+i] = lonlatalt.x, lonlatalt.y, alt;
QJsonDocument json_doc(json);
QString json_string = json_doc.toJson();
QString filename = "./tempwpf";
QFile save_file(filename);
if(!save_file.open(QIODevice::WriteOnly)){
qDebug() << "failed to open save file";
}
save_file.write(json_string.toLocal8Bit());
save_file.close();
}
The output of json file should be:
{
"waypoint0": "10.05456, 51.02453, 0.0",
"waypoint1": "10.05456, 51.02453, 0.0",
"waypoint2": "10.05456, 51.02453, 0.0",
"waypoint3": "10.05456, 51.02453, 0.0",
"waypoint4": "10.05456, 51.02453, 0.0",
"waypoint5": "10.05456, 51.02453, 0.0",
"waypoint6": "10.05456, 51.02453, 0.0"
}

Answer 1

json["waypoint"+i] = lonlatalt.x, lonlatalt.y, alt;
There's not a single right thing in this line.
"waypoint"+i
"waypoint" is a string literal, i.e. an array of char, which decays to a pointer to its first element; now, if you sum an integer to a pointer, you get an incremented pointer, i.e. a pointer that points i elements after the first.
Hence, here you are effectively taking a substring of "waypoint" starting at the i-th character. "waypoint"+1 => "aypoint"; "waypoint"+2 => "ypoint" and so on.
Given that you are using Qt and QString anyhow to put this into the JSON document, you can simply use the Qt string formatting methods; so, on the left hand side you can do:
json[QString("waypoint%1").arg(i)]
On the right hand side, it's all wrong again; here you are mistakingly using the comma operator, which evaluates the expressions at both its sides and discards the left one; so when you write
lonlatalt.x, lonlatalt.y, alt
you are actually writing
(lonlatalt.x, lonlatalt.y), alt
which boils down to plain alt.
Now, if you want to format this as a comma-separated string, you can again use the QString methods:
json[QString("waypoint%1").arg(i)] = QString("%1, %2, 0.0").arg(lonlatalt.x, 0, 'f', 5).arg(lonlatalt.y, 0, 'f', 5)
Finally, you are creating/saving a new QJsonDocument at each iteration, so every time you create a file with just one waypoint and overwrite the previous one. You should move the creation/save outside of the loop:
QJsonObject json;
goes before the for
json["waypoint"+i] = lonlatalt.x, lonlatalt.y, alt;
remains inside, and all the rest outside. Notice that even the code to save is suboptimal: json_doc.toJson() returns a QByteArray already in UTF-8 (which is generally used as the "canonical" encoding for JSON); you are reconverting it to a QString and then converting it back to an 8 bit encoding (which is lossy and most probably unintended).
Also, you check if the file was opened successfully, and write on it even in case of error - that's definitely a bad idea.
Finally, close() is not needed - it's performed automatically when the QFile goes out of scope.
To sum it up:
QFile save_file("./tempwpf");
if(save_file.open(QIODevice::WriteOnly)){
QJsonDocument json_doc(json);
save_file.write(json_string.toJson());
} else {
qDebug() << "failed to open save file";
}