Pure virtual interface for signal and slots leads to amibigiuos connect method

SiGa

I want to create a pure virtual interface which is then inherited by other classes, so that these child classes can be accessed through the interface without exposing all contents.
I used QObject as the parent for my interface to have access to signal and slot keywords.
However this leads to the interface overloading the connect in the livetab, which is also a child of QObject (through QWidget).

What is the correct parent for my interface so that I can define signals and slots, without overloading the connect method?
Is such a solution possible?

As an alternative solution I could specify the base class before the connect call, but would like to avoid it.

...
QWidget::connect(
        startstopBtn, &QPushButton::clicked, this, &LiveTab::startStopClicked);
...

servercommandinterface.hpp

#pragma once
#include <QObject>

class IServerCommand : public QObject
{
    Q_OBJECT
    
public:
    virtual ~IServerCommand(){};

signals:
    virtual void requestServerCommandConnection() = 0;

public slots:
    virtual void handleServerCommandResponse() = 0;
};

livetab.hpp

class LiveTab : public QWidget, IServerCommand
{
    Q_OBJECT

public:
    LiveTab(QWidget *parent = nullptr);
...
};

JonB

@SiGa

class IServerCommand : public QObject

class LiveTab : public QWidget, IServerCommand

You are dual-inheriting from QObject (QWidget obviously inherits it too), I thought that was not permitted (moc can't handle it for signals/slots)?

Maybe you are already aware of this. I just don't think you will be able to derive from anything inheriting QObject as an addition to QWidget etc. You might encapsulate an IServerCommand object. Maybe someone else has a pattern for what you are trying to achieve.

Christian Ehrlicher

@SiGa said in Pure virtual interface for signal and slots leads to amibigiuos connect method:

signals:
virtual void requestServerCommandConnection() = 0;

A signal can not be pure virtual

jeremy_k

@JonB said in Pure virtual interface for signal and slots leads to amibigiuos connect method:

@SiGa
class IServerCommand : public QObject

class LiveTab : public QWidget, IServerCommand
You are dual-inheriting from QObject (QWidget obviously inherits it too), I thought that was not permitted (moc can't handle it for signals/slots)?

It's the C++ diamond inheritance issue rather than, or perhaps in addition to moc's limitation.

jeremy_k

@SiGa said in Pure virtual interface for signal and slots leads to amibigiuos connect method:

...
QWidget::connect(
        startstopBtn, &QPushButton::clicked, this, &LiveTab::startStopClicked);
...

The versions of QObject::connect that take a sender object are static. The code might as well invoke it that way.

SiGa

I tried declaring pure virtual signal and slots without inheriting from QObject but including it.

#pragma once
#include <QObject>

class IServerCommand 
{

public:
    virtual ~IServerCommand(){};

signals:
    virtual void requestServerCommandConnection() = 0;

public slots:
    virtual void handleServerCommandResponse() = 0;
};

This seems to work. What could go wrong if I do it this way?
If I later get a QObject* and want to cast it to IServerCommand*, will this cause problems if I connect the signal and slots of the interface?

jeremy_k

@SiGa said in Pure virtual interface for signal and slots leads to amibigiuos connect method:

I tried declaring pure virtual signal and slots without inheriting from QObject but including it.

If you're not already familiar, a browse through https://code.qt.io/cgit/qt/qtbase.git/tree/src/corelib/kernel/qtmetamacros.h might be enlightening. The signals and slots keywords are there for moc and documentation. If the class isn't QObject-derived, they're nearly no-ops*.

Q_SIGNALS changes the access mode to public, allowing calls to QObject::connect to work from non-friend classes.

This seems to work. What could go wrong if I do it this way?

What is a virtual signal intended it to mean? Wrong implies a corresponding definition of right.

J.Hilk

@SiGa said in Pure virtual interface for signal and slots leads to amibigiuos connect method:

This seems to work. What could go wrong if I do it this way?

Everything!

virtual slots are fine, you control declaration and implementation. Signals on the other hand you only control the declaration and moc, the meta object compiler, will automatically generate the implementation in the moc_*.cpp files it generates. You simply can't make it virtual.

moc also requieres QObject inheritance and Q_OBJECT macro to correctly parse and interpret your class, if you're using signals. You can also use the more lightwhight Q_GADGET macro if you don't wan't/can't inherit from QObject but still want access to some of the Q_OBJECT features

SiGa

@jeremy_k
With the pure virtual interface I intend to force the inheriting class to implement/have these.
With "Right" I mean that I can connect the child signals&slots and they are correctly triggered.
Is such a mechanism even possible with moc generation?
Does moc generate for each *.cpp files, or does it respect inheritance?

jeremy_k

@SiGa said in Pure virtual interface for signal and slots leads to amibigiuos connect method:

@jeremy_k
With the pure virtual interface I intend to force the inheriting class to implement/have these.

moc implements signals based on their signature. If it did support virtual signals, it would be creating the same implementation for each derived class.
The code is essentially:

void signal(Types... args) {
   for (auto slot: connected_slots)
      slot(args);
}

What's the point of an identical override of that?

With "Right" I mean that I can connect the child signals&slots and they are correctly triggered.

Right and correct are synonyms in this situation. To avoid a circular discussion, please use code examples.

Is such a mechanism even possible with moc generation?

With my current understanding of the problem, I'm going to say 'no', with the caveat that I don't think it is necessary.

Does moc generate for each *.cpp files, or does it respect inheritance?

moc is a relatively simplistic code generator. Given a file (.cpp or .h makes no difference) that mentions a signal or slot section, it generates code for signals and lookup tables for both. If moc never sees the file, no code is generated. If it finds no relevant keywords, no code is generated.

SiGa

@jeremy_k
The versions of QObject::connect that take a sender object are static. The code might as well invoke it that way.

Do you mean that with specific calls to connect this might work?

jeremy_k

@SiGa said in Pure virtual interface for signal and slots leads to amibigiuos connect method:

@jeremy_k
The versions of QObject::connect that take a sender object are static. The code might as well invoke it that way.

Do you mean that with specific calls to connect this might work?

I only use QObject::connect(&sender, &SenderClass::signal, ...). I never use
&SenderClass::connect(...), or any other superclass of SenderClass.

Asperamanca

How about composition instead of inheritance?

class AbstractServerCommand : public QObject
{
  Q_OBJECT
public:
  virtual void ~AbstractServerCommand ( ) = default;

signals:
  void requestServerCommandConnection();
public slots:
  void handleServerCommandResponse(); // Calls handleServerCommandResponseImpl
private:
  void handleServerCommandResponseImpl() = 0;
};

class IServerCommandProvider
{
public:
  virtual void ~IServerCommandProvider() = default;

  virtual AbstractServerCommand& getServerCommandInterface() = 0;
};

class LiveTabServerResponse : public AbstractServerCommand 
{
  Q_OBJECT
public:
  ~LiveTabServerResponse() override = default;
private:
  void handleServerCommandResponseImpl() override;
};

class LiveTab : public QWidget, IServerCommandProvider
{
  Q_OBJECT
public:
  ~LiveTab() override = default;

  // Co-variant overload
  LiveTabServerResponse& getServerCommandInterface() override; // Returns reference to m_ServerCommandInterface

private:
  LiveTabServerResponse m_ServerCommandInterface;

Now you should have everything covered:

If you derive from IServerCommandProvider, it forces you to provide access to a AbstractServerCommand
The AbstractServerCommand defines the signal, and allows you to do a custom implementation of the slot
If you need the same implementation for AbstractServerCommand in different classes, you only need to write it once
If LiveTabServerResponse needs to call something in LiveTab, there are multiple ways to solve this. One way is to pass a std::function to the constructor of LiveTabServerResponse that e.g. should be called whenever the slot is called.

jeremy_k

@Asperamanca said in Pure virtual interface for signal and slots leads to amibigiuos connect method:

public slots:
  void handleServerCommandResponse(); // Calls handleServerCommandResponseImpl
private:
  void handleServerCommandResponseImpl() = 0;

This is an unnecessary indirection. Virtual slots are fine. They are not implemented by moc.
Qt uses virtual slots within public apis.
https://doc.qt.io/qt-6/qabstractitemview.html#reset

void QAbstractItemView::reset() [virtual slot]

SimonSchroeder

I don't see a way to get signals into IServerCommand. To solve the diamond problem (from the point-of-view of C++, not necessarily Qt) the common parent (i.e. QObject) would need to virtual. I expect you do not want to change (and recompile) the Qt source so that QWidget uses virtual inheritance. This would leave the solution proposed by @Asperamanca.

If there were a way to do this, the signals of IServerCommand don't have to be virtual. moc will implement them for you and subclasses will just inherit their implementation. The implementation would not change for subclasses anyway. You could still connect using the name of the subclass.

You have to be careful with connecting to your slots of IServerCommand and their subclasses. Under no circumstance should you write

QObject::connect(request, &Request::responseAvailable, command, &IServerCommand::handleServerCommandResponse);

If you write it like this C++ will not respect your virtual keyword (since you explicitly specified the implementation of which class to use). I hate to say this, but here you should use the old connect syntax (this is what it was meant for):

QObject::connect(request, SIGNAL(responseAvailable()), command, SLOT(handleServerCommandResponse()));

If you want to stick to the new connect syntax, you are back to the other approach suggested by @Asperamanca:

class IServerCommand : public QObject
{
  //...
public slots:
  void handleServerCommandResponse(); // calls handleServerCommandResponseImpl
private:
  virtual void handleServerCommandResponseImpl() = 0;
}

In any case class LiveTab : public QWidget, IServerCommand is not a good idea as it is mixing concerns. A widget should do widget stuff and a command should do command stuff. The widget can know about the command and display its results. Think of the good old MVC pattern (though with Qt it is quite usable to mix View and Controller, but the Model should be kept separate).

Asperamanca

@SimonSchroeder Thanks, you said all I wanted to say here, in more detail

SiGa

Thank you all for the suggestions.
In the end I implemented it as a QObject, which becomes a child of the parent, which contains the interface.
My mainframe then finds all instances of my ServerInterface through the QMetaMethod System, and connects everything accordingly
Surely not the purest C++ solution to this problem, but it seemed most logical to me.

I don't know which reply I should mark as answer since I did not try out the suggestion from @Asperamanca

Asperamanca

@SiGa There usually is more than one answer :-)

J.Hilk

@SiGa you can just use the topic tools and set the whole topic to solved