Re: [Interest] QJsonDocument::fromVariant failing with list

[Jason H]

 

Yeah, in coming up with a minimal example, I was a little too far from my example, but it all holds. My actual code was this:

QList<QVariantMap> letters {
                       {{"a",1}},
                       {{"b",2}},
                       {{"c",3}}
};

but can be modified to:

 

QVariantList letters {
                        QVariantMap {{"a",1}},
                        QVariantMap {{"b",2}},
                        QVariantMap {{"c",3}}
 };

 

In which case it works.

 

But I typed out (here)

        QList<QVariantMap> letters {
                        QVariantMap {{"a",1}},
                        QVariantMap {{"b",2}},
                        QVariantMap {{"c",3}}
        };

 

Which is neiher what I acually had but performs the same bad conversion as what I had. I am not sure why the QVariantList vs QList<QVariantMap> have different fidelity  - the one with more fidelity - QList<QVariant> - actually has has less specificity.

 

Sent: Tuesday, June 08, 2021 at 4:42 PM
From: "Jérôme Godbout" <godbo...@amotus.ca>
To: "Jason H" <jh...@gmx.com>, "interestqt-project.org" <interest@qt-project.org>
Subject: Re: [Interest] QJsonDocument::fromVariant failing with list

Most likely the QJson doesn’t known what to do with your QList<> type object. You should put those object into a QVariantList, right now you have a QVariantList that contain a QList as first element. You should convert the Qlist to a QVariant (not sure you want 2 level deep array, but right now you do). You QList<QVariantMap> and QVariantList is QList<QVariant>.

 

Right now if it could do something with the QList you would get this:

{"err":null,"letters":[[{"a": 1},{"b": 2},{"c": 3}]]}

 

Something like this, not sure this is possible declaration but you are looking to get this in the end. Or make a converter that convert your QList to a QVariantList if you still need that QList absolutely.

 

QVariantList letters {
                        QVariantMap {{"a",1}},
                        QVariantMap {{"b",2}},
                        QVariantMap {{"c",3}}
        };

 

Not tested just giving the idea

// can use traits if T is QVariant to simply convert by casting since it’s the same

Template<typename T>

QVariantList Convert<T>(const QList<T>& l)

{

                QVariantList rv;

                Rv.reserve(l.size());

                for(auto i in l)

                {

                                rv.append(QVariant::fromValue(i));

}

return rv;

}

 

 

From: Interest <interest-boun...@qt-project.org> on behalf of Jason H <jh...@gmx.com>
Date: Tuesday, June 8, 2021 at 4:23 PM
To: interestqt-project.org <interest@qt-project.org>
Subject: [Interest] QJsonDocument::fromVariant failing with list

I'm having trouble with QJsonDocument::fromVariant() is not handling arrays:

        QVariant list;

        QList<QVariantMap> letters {
                        QVariantMap {{"a",1}},
                        QVariantMap {{"b",2}},
                        QVariantMap {{"c",3}}
        };

        list.setValue(letters);
        map["err"] = QVariant(); // null
        map["letters"] = list;

        qDebug() << QJsonDocument::fromVariant(map)
                                .toJson(QJsonDocument::Compact);

Output:
{"err":null,"letters":null}

I'm expecting:
{"err":null,"letters":[{"a": 1},{"b": 2},{"c": 3}]}

The debugger (screenshot attached) sees it. Is this a bug or am I doing it wrong?

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