QMetaType casting to desired type

Redman

Hello,

I want to cast an object I receive over network to the specific class that represents that objects structure. The receiver object, before casting, is a generic networking class. I want to achieve that using QMetaType.

Right now I have something like this

      // Check for cast compatibility
      QString className = "ExampleClass"
      QMetaType targetType = QMetaType::fromName(className.toUtf8());
      QMetaType sourceTarget = QMetaType::fromType<NetworkClass>();
      bool canConvert = QMetaType::canConvert(sourceTarget , targetType);
      if (!type.isValid() || !canConvert) {
         qWarning() << "targetType not valid or can not convert to targetType";
         break;
      }

# untill here all works fine

???

What to do after that. I tried a lot of things. All QMetaType functions return void* which I cant do nothing with.

I could write down every possible qobject_cast in a switch statement but I want to keep this as dynamic as possible

jeremy_k

How is the object transmitted? I had presumed this was for POD classes until the mention of qobject_cast.

As far as doing something with the reconstructed object, what's the goal? Ignoring the transmission question, if the object is QObject derived, the QMetaObject::invoke* and property functions can be used.

If the goal is a symbol referring to a derived type, perhaps a template function accepting a parameter pack of types could return a std::variant or similar and save a little typing.

Asperamanca

@Redman
I guess you are missing this code:

MyType myTypedValue = {};
QMetaType::convert(sourceTarget, targetType, &myTypedValue);

jeremy_k

@Asperamanca said in QMetaType casting to desired type:

@Redman
I guess you are missing this code:

I saw that, and it surprised me that a sequence of bytes delivered over a network would successfully convert to a QObject.

Redman

@jeremy_k I'm talking QtRemoteObjects. I intentionally left that piece of information out because every time I mention them in this forum my posts never get answered.

So what exactly is happening?

I have a class ExampleClass

class ExampleClass : public ExampleClassSimpleSource {
   Q_OBJECT
     .
     .
     .

Which gets remoted via

bool rslt = myRegistry.enableRemoting(&obj);

somewhere in my code.

Retrieving that remoted object via

     QSharedPointer<ExampleClassReplica> rep;
     rep.reset(node.data()->acquire<ExampleClassReplica>(id));

Now, obviously I want to have more classes I can remote than this ExampleClass. But for now I have this solution.

// Enum of my classes which are used as remote objects
      RO ro = static_cast<T>(
                  QMetaEnum::fromType<RO>().keyToValue(entry.second.typeName.toLocal8Bit()));
      switch (ro) {
         case RoReceiver::ExampleClass: {
            QSharedPointer<ExampleClassReplica> rep;
            rep.reset(node.data()->acquire<ExampleClassReplica>(id));

            // Helps us keep track of the remote objects
            connect(rep.data(), SIGNAL(removeChanged(bool)),
                m_instanceTest.data(), SLOT(recRemoveChanged(bool)));
            connect(rep.data(), &QRemoteObjectReplica::initialized,
                m_instanceTest.data(), &RoReceiverTest::recInitChanged);

            // Keep track of available remote objects
            rep.data()->setObjectName(id);
            m_instanceTest.data()->m_nodes[node].insert(id, rep);
            Q_EMIT m_instanceTest.data()->echoRecRemoteObjectTest(rep);
            return;
         } break;
      }

This switch statement is gonna get bigger and bigger aswell as be error prone.
Is there any way to make this piece of code dynamic?

Asperamanca

@jeremy_k said in QMetaType casting to desired type:

@Asperamanca said in QMetaType casting to desired type:

@Redman
I guess you are missing this code:

I saw that, and it surprised me that a sequence of bytes delivered over a network would successfully convert to a QObject.

Anything in a computer is really just a sequence of bytes (or bits). Just a matter on how you view it...

Asperamanca

@Redman Looks like you should be able to at least put most of the case content into a template function, so your switch should look like

switch(ro) {
   case RoReceiver::ExampleClass: {
      createRemoteObject<ExampleClassReplica>(node, id, m_instanceTest);
      return;
   }
   // ...more cases

Redman

@Asperamanca That is correct.
What makes me wonder is that I qRegisterMetaType<ExampleClassReplica>(); which makes it usable with QMetaType. The way I understood QMetaType is that it provides reflection functionality. By calling QMetaType::create/convert I expected to get the correct type. But all it returns is a void* which I can not cast without explicitly knowing the type during compile time

Asperamanca

@Redman
I have to admit, I have never created objects in such a fashion myself.
But maybe the following works:

QVariant v;
QMetaType::convert(sourceTarget, targetType, v.data());
// Maybe also needed:
v.convert(targetType);

If that works, and the contained type is a QObject type, you should be able to generically access properties, enumerations and suchlike.

jeremy_k

@Asperamanca said in QMetaType casting to desired type:

@jeremy_k said in QMetaType casting to desired type:

@Asperamanca said in QMetaType casting to desired type:

@Redman
I guess you are missing this code:

I saw that, and it surprised me that a sequence of bytes delivered over a network would successfully convert to a QObject.

Anything in a computer is really just a sequence of bytes (or bits). Just a matter on how you view it...

Structure within the data that includes pointers makes treating an object as just a sequence of bytes a little more complicated.

jeremy_k

@Redman said in QMetaType casting to desired type:

@jeremy_k I'm talking QtRemoteObjects. I intentionally left that piece of information out because every time I mention them in this forum my posts never get answered.

That's an important detail. It sounds like a useful project. I personally have remained minimally interested due to the project's stated lack of interest in compatibility with non-QRO implementations. I'm not lucky enough to work in a distributed environment where all nodes run a compatible Qt application.

This switch statement is gonna get bigger and bigger aswell as be error prone.
Is there any way to make this piece of code dynamic?

I was imagining:

template <class Type, class... Others> std::any convert(void *obj) {
    QMetaType fromType = QMetaType::fromType<NetworkClass>();
    QMetaType toType = QMetaType::fromType<Type>();
    std::any ret;
    if QMetaType::canConvert(fromType, toType) {
        QMetaType::convert(obj, fromType, &ret, toType);
        return ret;
    }
    else
        return convert<Others...>(obj);
}

template <> convert(void *obj) {
    return std::any(nullptr);
}

NetworkClass *obj = getObject();
std::any convertedObject = convert<Type1, Type2, Type3>(obj);
if (std::any_cast<void *>(convertedObject) != nullptr)
    ...

Not expected to compile as is
Not runtime dynamic
The user of the std::any will need to know what it wants to convert it to

I'm not sure this improves the situation.