Multiple Inheritance of Interfaces with Slots

[kain]

Hello,

I need some advise on how I can achieve to implement multiple interfaces with Qt.
I got a class which handles communication to different periphiere-cards on an RS485-bus 'PeripherieCommunicator'.
This class includes several interfaces, each declaring the communication to one of these cards in the RS485-bus, for example:

class IPR112Control
{
public:
    virtual ~IPR112Control() {}
public slots:
    virtual signed short ssPR112ReadShortState( STATE * pstrShortState, int addr) = 0;
};
Q_DECLARE_INTERFACE(IPR112Control, "IPR112Control/1.0")

and

class IPR144Control
{
public:
    virtual ~IPR144Control() {}
public slots:
    virtual signed short ssPR144GetFullDeviceInfo( INFO * pstrFullDeviceInfo, int addr ) = 0;
};
Q_DECLARE_INTERFACE(IPR144Control, "IPR144Control/1.0")

My class PeripherieCommunicator implements those 2 interfaces:

class PeripherieCommunicator : public QObject, IPR112Control,  public IPR144Control
{
    Q_OBJECT
    Q_INTERFACES(IPR112Control)
    Q_INTERFACES(IPR144Control)
...
// IPR112Control interface
public slots:
    signed short ssPR112ReadShortState( STATE * pstrShortState, int addr);
// IPR144Control interface
public slots:
    signed short ssPR144GetFullDeviceInfo( INFO * pstrFullDeviceInfo, int addr ) = 0;
}

The Problem occurs when I try to connect to those slots using the Interface, i.e.:

MotorControl::MotorControl(IPR112Control *pcom, QObject *parent) : QObject(parent)
{
    this->pcom = pcom;
    connect(this, &MotorControl::readShortState, this->pcom, &IPR112Control::ssPR112ReadShortState, Qt::ConnectionType::BlockingQueuedConnection);

The problem seems to be that my Interface IPR112Control (and other interfaces) does not inherit from QObject but when I change them to inherit QObject I get errors in my PeripherieCommunicator class since the reference to QObject is ambigous then.
Is there any advice on how to fix this problem in a proper way? Of course I could just inject the pointer to 'PeripherieCommunicator' in the constructor of 'MotorControl' but this would make the class less testable. The other way I see is to merge all Interfaces into one single interface but this does not seem the right way either...

[VRonin]

Try changing it to connect(this, &MotorControl::readShortState, this, std::bind(&IPR112Control::ssPR112ReadShortState, this->pcom, std::placeholders::_1, std::placeholders::_2), Qt::ConnectionType::BlockingQueuedConnection);

The only problem now is that you need to make sure this->pcom does not get destroyed while there's still a pending slot to be executed and this was destroyed.

If that's a problem then save the return of the connect: pcomConnection = QObject::connect(this, &MotorControl::readShortState, this->pcom, std::bind(&IPR112Control::ssPR112ReadShortState,std::placeholders::_1,std::placeholders::_2), Qt::ConnectionType::BlockingQueuedConnection); and then call QObject::disconnect(pcomConnection); before the deletion of this->pcom and replace the deletion with a delayed one:

auto pcomToDelete = this->pcom;
QTimer::singleShot(0,[pcomToDelete]()->void{delete pcomToDelete;});

[Christian Ehrlicher]

Wouldn't a simple cast to IPR112Control help here?

IPR112Control *ifc = pcom;
connect(this, &MotorControl::readShortState, ifc, &IPR112Control::ssPR112ReadShortState, Qt::ConnectionType::BlockingQueuedConnection);

[kain]

@Christian-Ehrlicher no, since pcom gets already injected as IPR112Control. That connect would also fail since IPR112Control does not inherit from QObject.

[VRonin]

@Christian-Ehrlicher the problem is the 3rd argument (receiver/context), not the slot, I think

[kain]

Yes, but i already hat pcom of type IPR112Control but you were right with the 3rd argument.
I did solve the problem now with a dynamic cast and using the old connection-syntax

connect(this, SIGNAL(readShortState(STATE, int)), dynamic_cast<QObject*>(this->pcom), SLOT(ssPR112ReadShortState(STATE, int)));

This way I lose the compile time checking but this seems to be the smallest tradeoff.

[VRonin]

@kain said in Multiple Inheritance of Interfaces with Slots:

This way I lose the compile time checking but this seems to be the smallest tradeoff.

My suggestion above didn't work? I'm surprised dynamic_cast works, it should only accept reinterpret_cast there...

[kshegunov]

@Christian-Ehrlicher said in Multiple Inheritance of Interfaces with Slots:

Wouldn't a simple cast to IPR112Control help here?

Nope. This will fail the static assertions in QObject::connect.

@VRonin said in Multiple Inheritance of Interfaces with Slots:

Try changing it to connect(this, &MotorControl::readShortState, this, std::bind(&IPR112Control::ssPR112ReadShortState, this->pcom, std::placeholders::_1, std::placeholders::_2), Qt::ConnectionType::BlockingQueuedConnection);

Correct, however second point is moot. In fact you should set the proper context (this is incorrect here):

QObject * context = qobject_cast<QObject *>(pcom);
QObject::connect(this, &MotorControl::readShortState, context, std::bind(&IPR112Control::ssPR112ReadShortState, pcom, std::placeholders::_1, std::placeholders::_2), Qt::ConnectionType::BlockingQueuedConnection);

Now the connection is broken cleanly whenever context (which is pcom) gets destroyed. No additional work's needed.

I'm surprised dynamic_cast works, it should only accept reinterpret_cast there...

Why? That's the whole reason dynamic_cast exists in the first place - to runtime cast polymorphic types.

[Christian Ehrlicher]

@kshegunov said in Multiple Inheritance of Interfaces with Slots:

Nope. This will fail the static assertions in QObject::connect.

Then just use a lambda :)