[Solved] QJsonDocument parsing issue Qt 5.3 vs. Qt 5.4 on OSX using C++11

[Manromen]

Hi there,

I have problems parsing json since I upgraded to Qt 5.4.

Here is an example:
@#include <QCoreApplication>

#include <QJsonDocument>
#include <QJsonObject>
#include <QJsonArray>

#include <QDebug>

int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);

char jsString[] {
    "{\"results\":[{\"id\":1,\"title\":\"Test1\"},{\"id\":2,\"title\":\""
    "Test2\"},{\"id\":3,\"title\":\"Test3\"},{\"id\":4,\"title\":\"Test4\"}]}"
};

QJsonParseError *error { nullptr };
// parse bytes to json
QJsonDocument doc { QJsonDocument::fromJson(jsString, error) };

if (error) {
    qDebug() << "error parsing json:" << error->errorString();
} else {

    QJsonObject rootObj { doc.object() };
    QJsonArray results { rootObj.value("results").toArray() };

    qDebug() << "results.count:" << results.count();

    for (QJsonValue v : results) {
        qDebug() << "v:" << v.toObject().value("title").toString();
    }
}
return a.exec(&#41;;

}@

If I run this using Qt 5.3 all is fine. The output is:
@results.count: 4
v: "Test1"
v: "Test2"
v: "Test3"
v: "Test4"@

If I run this using Qt 5.4 I get this:
@results.count: 1
v: ""@

I run this on Mac OS X Yosemite 64-Bit with the clang compiler.

Has anyone an idea whats wrong?

Cheers,
Manromen

[SGaist]

Hi and welcome to devnet,

Can't really comment on this, the only thing I can say is that it works fine when not using C++11

[Manromen]

Hi,

thank you for your Response.

I replaced
@QJsonArray results { rootObj.value("results").toArray() };@
with:
@QJsonArray results = rootObj.value("results").toArray();@

So now it works ....

[SGaist]

Sill, it's a bit strange that it worked with Qt 5.3 and not 5.4

Did you change anything else in between ? (e.g. compiler)

[Manromen]

I just switched between the Kits:
Desktop Qt 5.3 clang 64bit
and
Desktop Qt 5.4.0 clang 64bit

Both Kits use the Clang (x86 64bit in /usr/bin).
Only difference I can see is that the Qt 5.3 Kit has no Debugger.

[SGaist]

Then I'd recommend checking the "bug report system":http://bugreports.qt.io to see if it's something known. If not please consider opening a new report providing a minimal compilable example to reproduce the behavior

[JKSH]

The Qt 5.4 behaviour is the correct, "documented":http://doc.qt.io/qt-5/qjsonarray.html#QJsonArray-2 one.

rootObj.value("results").toArray() returns a QJsonArray containing 4 elements. However, by using the initializer-list, this array is first converted into a single QJsonValue. Then, this single QJsonValue is stored in the outer array. That's why the outer array reports having 1 element.

Do this and you will see that you have a 2D array (more precisely, it's an array-within-an-array):
@
QJsonArray results{ rootObj.value("results").toArray() };
qDebug() << results;
@

Do this and you will see that you have a 1D array:
@
QJsonArray results = rootObj.value("results").toArray();
qDebug() << results;
@

[Manromen]

Hi JKSH,

makes absolutely sense. Thank you very much for this great explanation!

Cheers,
Manromen

[JKSH]

You're welcome :)

[SGaist]

Okay… Just saw that my offline doc was from the wrong Qt version :D
Thanks for the explanation JKSH