Issue passing QML object to C++ QVariantMap

Josh

Hi,

I'm getting an error when trying to pass a simple QML object to a C++ function as a QVariantMap.

My CPPClass has a member:

Q_INVOKABLE cppFunc(QVariantMap obj) {}

Over in QML I have a ListModel that has elements like:

{ foo: 1, bar: 2, baz: 3}

I want to pull items from the list model and pass them to cppFunc. Like this:

var obj =listModel.get(0)

cppClass.cppFunc(obj)

Unfortunately, this produces the following runtime error:

Passing incompatible arguments to C++ functions from JavaScript is dangerous and deprecated.

However, if I create a new object based on the obj from the listModel then it works:

var obj =listModel.get(0)
var copyOfObj = {foo: obj.foo, bar: obj.bar, baz: obj.baz}
cppClass.cppFunc(copyOfObj)

According to the debugger obj and copyOfObj are both valid and exactly the same.

Can anyone explain why it's necessary to make this copy of a listModel item before passing it to C++?

Using Qt 5.12.5.

Thanks!

JKSH

Hi @Josh, and welcome!

I'm not sure what's causing this behaviour, but:

What does console.log(typeof obj) and console.log(typeof copyOfObj) show?
The code which produces that error message is at https://code.woboq.org/qt5/qtdeclarative/src/qml/jsruntime/qv4qobjectwrapper.cpp.html#_ZL10CallMethodRK18QQmlObjectOrGadgetiiiPiPN3QV415ExecutionEngineEPNS3_8CallDataEN11QMetaObject4CallE -- Did you also get a message, "Could not convert argument X at Y"?

raven-worx

@Josh
thats because ListModel returns a object type and not a JSON object. Which i assume results in a QJSValue on the C++ side (can easily be checked by changing the signature to take a QVariant and output the passed parameter with qDebug for example)

https://code.qt.io/cgit/qt/qtdeclarative.git/tree/src/qmlmodels/qqmllistmodel.cpp?h=dev#n2550

raven-worx

@Josh
i honestly dont know if it works but try:

var obj = JSON.parse(JSON.stringify(model.get(0)))

Josh

@JKSH Yes, also prints "Could not convert argument 1 at".

Console.log(typeof obj) & console.log(copyOfObj) show both as "object"

qml: object
qml: object

Josh

@raven-worx said in Issue passing QML object to C++ QVariantMap:

@Josh
thats because ListModel returns a object type and not a JSON object. Which i assume results in a QJSValue on the C++ side (can easily be checked by changing the signature to take a QVariant and output the passed parameter with qDebug for example)

https://code.qt.io/cgit/qt/qtdeclarative.git/tree/src/qmlmodels/qqmllistmodel.cpp?h=dev#n2550

The original obj and copyOfObj are both "object" to QML, according to debugger and typeof.

On the C++ side, copyOfObj gets successfully passed into the QVariantMap.

This does work, but it's still generating a copy.

JSON.parse(JSON.stringify(entry)

It seems like it should work without the copy. According to the docs:

The QML engine provides automatic type conversion between QVariantList and JavaScript arrays, and between QVariantMap and JavaScript objects.

and

Similarly, if a C++ type uses a QVariantList or QVariantMap type for a property type or method parameter, the value can be created as a JavaScript array or object in QML, and is automatically converted to a QVariantList or QVariantMap when it is passed to C++.

Am I reading this wrong?

Thanks for your help!

raven-worx

@Josh said in Issue passing QML object to C++ QVariantMap:

It seems like it should work without the copy. According to the docs:
The QML engine provides automatic type conversion between QVariantList and JavaScript arrays, and between QVariantMap and JavaScript objects.

But the returned object is actually of the type QObject.
As i said, try the approach where you change the parameter to a QVariant and check its actual type.

IMHO the docs might be a bit misleading and should read "...and between QVariantMap and JSON objects."

Josh

@raven-worx said in Issue passing QML object to C++ QVariantMap:

@Josh said in Issue passing QML object to C++ QVariantMap:

It seems like it should work without the copy. According to the docs:
The QML engine provides automatic type conversion between QVariantList and JavaScript arrays, and between QVariantMap and JavaScript objects.

But the returned object is actually of the type QObject.
As i said, try the approach where you change the parameter to a QVariant and check its actual type.

IMHO the docs might be a bit misleading and should read "...and between QVariantMap and JSON objects."

You are correct. When passed as a QVariant the original obj is a QObject*, the copy is a QJSValue.

So copying/converting to a QJSValue is the right way to do this?

Yes the docs on this issue are confusing.

Thanks for your help.

raven-worx

@Josh said in Issue passing QML object to C++ QVariantMap:

So copying/converting to a QJSValue is the right way to do this?

this or JSON.parse(JSON.stringify(entry)

Josh

@raven-worx That works for me. Thanks for clarifying this issue for me.