Call invokeMethod passing argument by (no const) reference - Q_ARG(Type&, value)

nico88desmo

Dear all,

just a simple question: suppose having a slot defined in this way:

class Sample : public QObject {
   Q_OBJECT
   ...
public slots:
   int populateBytes(QByteArray& bytes);
   ...
}

I'm calling that method using QMetaObject::invokeMethod() cause its object instance is living in another thread.
I'm using a BlockingQueuedConnection, cause I need to wait method results; the calling function is similar to this one:

void testFunction() {
   int result;
   QByteArray testBytes;

   QMetaObject::invokeMethod(
      &objInstance,
      "populateBytes",
      Qt::BlockingQueuedConnection,
      Q_RETURN_ARG(result, int),
      Q_ARG(QByteArray&, testBytes)  // <--- is it correct???
   );
   return result;
}

From Q_ARG documentation, I expect that isn't possible passing a parameter using a no const value. Instead it seems working.

Going a bit deeper, I see that Q_ARG is define as follow:
#define Q_ARG(type, data) QArgument<type >(#type, data)

where:

template <class T>
class QArgument: public QGenericArgument
{
public:
    inline QArgument(const char *aName, const T &aData)
        : QGenericArgument(aName, static_cast<const void *>(&aData)) // <--- const void* !!!
        {}
};
template <class T>
class QArgument<T &>: public QGenericArgument
{
public:
    inline QArgument(const char *aName, T &aData)
        : QGenericArgument(aName, static_cast<const void *>(&aData)) // <--- const void* !!!
        {}
};

So, in both solution, aData will be use as a const void* parameter.

Am I "forcing" some const value to be no const?
In another way, is it safe passing a variable by reference, instead of const reference in a QMetaObject::invokeMethod(..., Q_ARG(Type&, value),..) call?

I'm using Qt 6.2.7

Thanks all.

Christian Ehrlicher

You're aware that QMetaObject::invokeMethod() does not block so you can't return anything if it is async. Again: a slot does not return anything - your design is broken, fix it.

Christian Ehrlicher

A slot can never have a (non-const) reference since a slot might be executed asynchronously.
A slot is doing something but never bringing something back so your populateBytes() is not a slot at all.

nico88desmo

Generally I'm agree to you @Christian-Ehrlicher.

But, in case I'm using that slot only via Qt::DirectConnection or Qt::BlockingQueueConnection, is not possible using a (non-const) reference?

From some tests, I see that testBytes is correctly populated using Qt::BlockingQueueConnection, but I'm not sure if this is an exceptional case or not.

nico88desmo

Or, maybe, a better solution is passing a QByteArray*: in this case, it's my responsibity to assure that allocate space is keeping available in async call..

Christian Ehrlicher

You're aware that QMetaObject::invokeMethod() does not block so you can't return anything if it is async. Again: a slot does not return anything - your design is broken, fix it.

nico88desmo

@Christian-Ehrlicher Ok, i'm going to use a signal/slot like solution.

It's both more clean and safety.