How to retrieve integer value from JSON data in Qt - c++

Question

How can I retrieve the integer value from the QJsonValue object? Suppose that I have the following JSON data:
{
"res": 1,
"message": "Common error"
}
I need to extract the "res" value from this data, so I've tried to use the following code:
QJsonDocument d = QJsonDocument::fromJson(some_json_data.toUtf8());
QJsonObject root_object = d.object();
QJsonValue res = root_object.value("res");
But I've found that QJsonValue class doesn't have a member function toInt or something like this (there's toDouble, toString, etc only). What can I do in such a situation? What is the best way to extract integer values via QjsonValue class?

Answer 1

(tl;dr: a one-liner solution at the end of answer.)
First a comprehensive way giving you full control. Below code assumes int is enough range for your integers, but could be expanded to work for most of the range of int64_t (but better test the boundary cases to get it fully correct):
QJsonValue res = root_object.value("res");
int result = 0;
double tmp = res.toDouble();
if (tmp >= std::numeric_limits<int>::min() && // value is not too small
tmp <= std::numeric_limits<int>::max() && // value is not too big
std::floor(tmp) == tmp // value does not have decimals, if it's not ok
) {
result = std::floor(tmp); // let's be specific about rounding, if decimals are ok
} else {
// error handling if you are not fine with 0 as default value
}
A shorter way using QVariant, and as an example also getting result into a larger integer type, if you just want to let Qt do it's thing. I'm not sure how it it handles integer values which are too big for a double to handle accurately, so again if that is important, better test.
QJsonValue res = root_object.value("res");
QVariant tmp = res.toVariant();
bool ok = false;
qlonglong result = tmp.toLongLong(&ok);
if (!ok) {
// error handling if you are not fine with 0 as default value
}
or same as error-ignoring one-liner, change integer type as appropriate:
qlonglong result = root_object.value("res").toVariant().toLongLong();

Answer 2

int QJsonValue::toInt(int defaultValue = 0) const
Converts the value to an int and returns it.
If type() is not Double or the value is not a whole number, the
defaultValue will be returned.
QByteArray JSettings::read_file(QString path)
{
QFile file;
file.setFileName(path);
if (!file.open(QIODevice::ReadOnly | QIODevice::Text))
return QByteArray();
QByteArray barray = file.readAll();
file.close();
return barray;
}
QVariantMap JSettings::to_map(QByteArray json)
{
QJsonDocument jdoc = QJsonDocument::fromJson(json);
QVariantMap vmap = jdoc.toVariant().toMap();
// qDebug() << vmap["res"].toInt();
return vmap;
}
QJsonObject JSettings::to_object(QByteArray json)
{
QJsonDocument jdoc = QJsonDocument::fromJson(json);
QJsonObject obj = jdoc.object();
// qDebug() << obj["res"].toInt();
return obj;
}
Qt JSON portal: http://doc.qt.io/qt-5/json.html

Answer 3

QJsonDocument d = QJsonDocument::fromJson(some_json_data.toUtf8());
QJsonObject root_object = d.object();
QJsonValue res = root_object.value("res");
int i = res.toString().toInt();
this is working, QString to Int