I would like to share signals and possibly slot implementations among different classes, but it seems Qt does not allow this.
Basically I would like to have something like:
class CommonSignals
{
signals:
void mysignal();
};
class A :
public QObject,
public CommonSignals
{
Q_OBJECT
public:
void doSomething()
{
emit mysignal();
}
};
class B :
public QObject,
public CommonSignals
{
Q_OBJECT
public:
B()
{
connect(&a, &A::mysignal, this, &B::mysignal);
}
A a;
};
So that when for some reason A emits a signal B emits the same signal too. This to avoid useless code replication and improve maintainability.
Any ideas?
PS I've tried also with virtual inheritance but I've got classical qmake problems
You cannot do this. QObject cannot be used with multiple inheritance of multiple QObject bases. Only the first inherited class can be a QObject. See:
https://doc.qt.io/qt-5/moc.html#multiple-inheritance-requires-qobject-to-be-first
Since you need both base classes to be a QObject (CommonSignals provides signals, it would need to be a QObject), you're out of luck. Your only option here is using plain old macros:
#define COMMON_SIGNALS \
void signal1(); \
void signal2();
class A: public QObject
{
Q_OBJECT
public:
// ...
signals:
COMMON_SIGNALS
};
class B: public QObject
{
Q_OBJECT
public:
// ...
signals:
COMMON_SIGNALS
};
The core issue with all this is that Qt uses moc to generate the underlying code for signals and slots. However, moc is just a simple preprocessor that doesn't understand most of C++.
You could use Verdigris to get rid of moc:
https://github.com/woboq/verdigris
This allows you to have templated QObject classes, for example. I have not tried it myself and thus don't know if it actually allows multiple inheritance. Might be worth looking into.
Why not just move the inheritance from QObject away from the derived classes A and B and into CommonSignals...
class CommonSignals: public QObject {
Q_OBJECT;
signals:
void mysignal();
};
class A: public CommonSignals {
Q_OBJECT;
public:
void doSomething ()
{
emit mysignal();
}
};
class B: public CommonSignals {
Q_OBJECT;
public:
B ()
{
connect(&a, &A::mysignal, this, &B::mysignal);
}
A a;
};
Will that not work for you?
I have an interface class in C++ that consists of pure virtual member functions, which I want to make parent of all the QWidgets in my app. It has lots of signals and slots, so it must inherit from QObject. The problem is that I can't make my other objects derive from this:
class Interface : public QObject {
public slots:
virtual void slotA() = 0;
signals:
void signalA();
};
Now if I want to create a widget implementing this interface I would need to do something like
class MyWidget : public QWidget, public Interface {
};
This however can't be done because it can't inherit twice from QObject. How can I do this?
Note: QObject Multiple Inheritance does not answer to this as it does not post a satisfatory answer in my opinion
suppose I've the following classes :
class A:public QObject{
Q_OBJECT
...
signals:
void sendData(QString data);
}
class B:public QObject{
Q_OBJECT
public:
A a;
...
public slots:
void onSendData(QString);
signals:
void sendData(QString data);
}
class C:public QObject{
Q_OBJECT
public:
B b;
...
public slots:
void onSendData(QString);
signals:
void sendData(QString data);
}
.
.
.
class MainWindow:public QMainWindow{
Q_OBJECT
public:
LastClass lc;
public slots:
void onSendData(QString);//show data on ui
}
class A digs data and when it finds a special data it must send it to ui(mainwindow) , so it calls sendData signal, then class B which contains an instance of A, grabs the signal and from it's slot sends it to above class ,...
as you can see, it causes a recursive signal/slot send and receiving which I doubt that it is a good design.
Is it a correct way or must I change the design ? ( it's hard to change design in some circumstances though).
(I cannot use inheritance because each class has a different functionality and different functions.)
This is not an obligation to connect the signals to a slot, in your case you can directly connect the signals between them. In the class B for example:
CONNECT(a, SIGNAL(sendData(QString)), this, SIGNAL(sendData(QString)));
Alternatively, you can use events so that you don't have to make this chain (but you won't know for sure who is catching the event then). Documentation here.
I have a base class, which defines a Qt slot
class Base
{
public:
Base()
{
connect(otherobject, SIGNAL(mySignal), this, SLOT(mySlot));
}
public slots:
virtual void mySlot()
{}
}
Subclass A just implements some other stuff. Subclass B overrides the slot
class SubB : Base
{
public:
SubB() : Base()
{
// Necessary?
connect(otherobject, SIGNAL(mySignal), this, SLOT(mySlot));
}
public slots:
virtual void mySlot() override
{}
}
Does the override of the slot also replace the connection, which was done before in the Bass constructor (I.e. The connect in SubB would be unnecessary)?
It gets better: you don't need any special treatment for the slot in the derived class. There's no need to make it virtual (it already is per C++ semantics), and there's no need to make it a slot again (it already is per Qt semantics). It is incorrect to add the second connection in Derived, it'll simply result in the slot being activated twice per every activation of the signal.
Remember that signals and slots are regular C++ methods, and that slots are invoked from machine-generated code that looks exactly as if you called the slot without specifying the particular class it should be in. Thus a virtual slot acts as you think it should, given the semantics of C++.
The below is sufficient:
class Base : public QObject
{
Q_OBJECT
public:
Base(QObject * src, QObject * parent = 0) : QObject(parent)
{ connect(src, SIGNAL(mySignal), SLOT(mySlot)); }
Q_SLOT virtual void mySlot() {}
};
class Derived : public Base
{
Q_OBJECT
public:
Derived(QObject * src, QObject * parent = 0) : Base(src, parent) {}
void mySlot() Q_DECL_OVERRIDE { ... }
};
You don't need to put the same connect in the subclass constructor. When a SubB object is created, the connect in the Base constructor will connect the right slot (the one overridden in SubB).
I would like to derive all of my widgets from a base class widget that automatically establishes a signal/slot connection between a slot for the class and a (rarely called) signal.
The slot is a virtual function, so that any widgets for which I wish to implement custom functionality can derive from the virtual slot function. In the desired scenario, all my widgets would derive from this base class with the virtual slot, so that by default all of my widget instances would be connected to the desired signal with a slot defined for the object (with default behavior from the base class).
I know that virtual slots are allowed in Qt. However, deriving from two QObject classes is not supported, so that, for example, the following code is disallowed:
class MySignaler : public QObject
{
Q_OBJECT
public:
MySignaler : QObject(null_ptr) {}
signals:
void MySignal();
}
MySignaler signaler;
class MyBaseWidget: public QObject
{
Q_OBJECT
public:
MyBaseWidget() : QObject(null_ptr)
{
connect(&signaler, SIGNAL(MySignal()), this, SLOT(MySlot()));
}
public slots:
virtual void MySlot()
{
// Default behavior here
}
}
// Not allowed!
// Cannot derive from two different QObject-derived base classes.
// How to gain functionality of both QTabWidget and the MyBaseWidget base class?
class MyTabWidget : public QTabWidget, public MyBaseWidget
{
Q_OBJECT
public slots:
void MySlot()
{
// Decide to handle the signal for custom behavior
}
}
As the sample code demonstrates, it seems impossible to gain both the benefits of (in this example) the QTabWidget, and also the automatic connection from the desired signal function to the virtual slot function.
Is there some way, in Qt, to have all my application's widget classes share common base-class slot and connect() functionality while allowing my widgets to nonetheless derive from Qt widget classes such as QTabWidget, QMainWindow, etc.?
Sometimes when inheritance is problematic, one can replace it, or a part of it, with composition.
That's the approach needed in Qt 4: instead of deriving from a QObject, derive from a non-QObject class (MyObjectShared) that carries a helper QObject that is used as a proxy to connect the signal to its slot; the helper forwards that call to the non-QObject class.
In Qt 5, it is not necessary to derive from a QObject at all: signals can be connected to arbitrary functors. The MyObjectShared class remains the same.
Should Qt 4 compatibility be generally useful in other areas of the code, one can use a generic connect function that connects signals to functors in both Qt 4 and Qt 5 (in Qt 4, it would use an implicit helper QObject).
// https://github.com/KubaO/stackoverflown/tree/master/questions/main.cpp
#include <QtCore>
#include <functional>
#include <type_traits>
class MySignaler : public QObject {
Q_OBJECT
public:
Q_SIGNAL void mySignal();
} signaler;
#if QT_VERSION < 0x050000
class MyObjectShared;
class MyObjectHelper : public QObject {
Q_OBJECT
MyObjectShared *m_object;
void (MyObjectShared::*m_slot)();
public:
MyObjectHelper(MyObjectShared *object, void (MyObjectShared::*slot)())
: m_object(object), m_slot(slot) {
QObject::connect(&signaler, SIGNAL(mySignal()), this, SLOT(slot()));
}
Q_SLOT void slot() { (m_object->*m_slot)(); }
};
#endif
class MyObjectShared {
Q_DISABLE_COPY(MyObjectShared)
#if QT_VERSION < 0x050000
MyObjectHelper helper;
public:
template <typename Derived>
MyObjectShared(Derived *derived) : helper(derived, &MyObjectShared::mySlot) {}
#else
public:
template <typename Derived, typename = typename std::enable_if<
std::is_base_of<MyObjectShared, Derived>::value>::type>
MyObjectShared(Derived *derived) {
QObject::connect(&signaler, &MySignaler::mySignal,
std::bind(&MyObjectShared::mySlot, derived));
}
#endif
bool baseSlotCalled = false;
virtual void mySlot() { baseSlotCalled = true; }
};
class MyObject : public QObject, public MyObjectShared {
Q_OBJECT
public:
MyObject(QObject *parent = nullptr) : QObject(parent), MyObjectShared(this) {}
// optional, needed only in this immediately derived class if you want the slot to be a
// real slot instrumented by Qt
#ifdef Q_MOC_RUN
void mySlot();
#endif
};
class MyDerived : public MyObject {
public:
bool derivedSlotCalled = false;
void mySlot() override { derivedSlotCalled = true; }
};
void test1() {
MyObject base;
MyDerived derived;
Q_ASSERT(!base.baseSlotCalled);
Q_ASSERT(!derived.baseSlotCalled && !derived.derivedSlotCalled);
signaler.mySignal();
Q_ASSERT(base.baseSlotCalled);
Q_ASSERT(!derived.baseSlotCalled && derived.derivedSlotCalled);
}
int main(int argc, char *argv[]) {
test1();
QCoreApplication app(argc, argv);
test1();
return 0;
}
#include "main.moc"
To share some code between two QObjects, you could have the QObject as a member of the class,an interposing non-object class that uses generic class that's parametrized only on the base type. The generic class can have slots and signals. They must be made visible to moc only in the immediately derived class - and not in any further derived ones.
Alas, you generally cannot connect any of the generic class's signals or slots in the constructor of the class, since at that point the derived class isn't constructed yet, and its metadata isn't available - from Qt's perspective, the signals and slots don't exist as such. So the Qt 4-style runtime-checked connect will fail.
The compile-time-checked connect will not even compile, because the this pointer it works on has an incorrect compile-time type, and you know nothing about the type of the derived class.
A workaround for Qt-4 style connect only is to have a doConnections method that the derived constructor has to call, where the connections are made.
Thus, let's make the generic class parametric on the base and the derived class as well - the latter is known as the Curiously Recurring Template Pattern, or CRTP for short.
Now you have access to the derived class's type, and can use a helper function to convert this to a pointer to the derived class, and use it in the Qt 5-style compile-time-checked connects.
The Qt 4-style runtime checked connect still needs to be invoked from doConnections. So,if you use Qt 5, that's not an issue. You shouldn't be using Qt 4-style connect in Qt 5 code anyway.
The slots require slightly different treatment depending on whether the class immediately derived from the generic class overrides them or not.
If a slot is virtual and has an implementation in the immediately derived class, you should expose it to moc in the normal fashion - using a slots section or the Q_SLOT macro.
If a slot doesn't have an implementation in the immediately derived class (whether virtual or not), its implementation in the generic class should be made visible to moc only, but not to the compiler - you don't wish to override it, after all. Thus the slot declarations are wrapped in #ifdef Q_MOC_RUN block that is only active when moc is reading the code. The generated code will refer to the generic implementations of the slots.
As we wish to make sure this indeed works, we'll add some booleans to track whether the slots were invoked.
// main.cpp
#include <QtWidgets>
template <class Base, class Derived> class MyGenericView : public Base {
inline Derived* dthis() { return static_cast<Derived*>(this); }
public:
bool slot1Invoked, slot2Invoked, baseSlot3Invoked;
MyGenericView(QWidget * parent = 0) : Base(parent),
slot1Invoked(false), slot2Invoked(false), baseSlot3Invoked(false)
{
QObject::connect(dthis(), &Derived::mySignal, dthis(), &Derived::mySlot2); // Qt 5 style
QObject::connect(dthis(), &Derived::mySignal, dthis(), &Derived::mySlot3);
}
void doConnections() {
Q_ASSERT(qobject_cast<Derived*>(this)); // we must be of correct type at this point
QObject::connect(this, SIGNAL(mySignal()), SLOT(mySlot1())); // Qt 4 style
}
void mySlot1() { slot1Invoked = true; }
void mySlot2() { slot2Invoked = true; }
virtual void mySlot3() { baseSlot3Invoked = true; }
void emitMySignal() {
emit dthis()->mySignal();
}
};
The generic class is very simple to use. Remember to wrap any non-virtual overridden slots in a moc-only guard!
Also recall the general rule that applies to all Qt code: if you have a slot, it should be declared to moc only once. So, if you had a class that further derives from MyTreeWidget or MyTableWidget, you don't want a Q_SLOT or slots macro in front of any necessarily virtual slot overrides. If present, it'll subtly break things. But you definitely want Q_DECL_OVERRIDE.
If you're on Qt 4, remember to call doConnections, otherwise the method is unnecessary.
The particular choice of QTreeWidget and QTableWidget is completely arbitrary, meaningless, and shouldn't be taken to mean that such use makes any sense (it likely doesn't).
class MyTreeWidget : public MyGenericView<QTreeWidget, MyTreeWidget> {
Q_OBJECT
public:
bool slot3Invoked;
MyTreeWidget(QWidget * parent = 0) : MyGenericView(parent), slot3Invoked(false) { doConnections(); }
Q_SIGNAL void mySignal();
#ifdef Q_MOC_RUN // for slots not overridden here
Q_SLOT void mySlot1();
Q_SLOT void mySlot2();
#endif
// visible to the C++ compiler since we override it
Q_SLOT void mySlot3() Q_DECL_OVERRIDE { slot3Invoked = true; }
};
class LaterTreeWidget : public MyTreeWidget {
Q_OBJECT
public:
void mySlot3() Q_DECL_OVERRIDE { } // no Q_SLOT macro - it's already a slot!
};
class MyTableWidget : public MyGenericView<QTreeWidget, MyTableWidget> {
Q_OBJECT
public:
MyTableWidget(QWidget * parent = 0) : MyGenericView(parent) { doConnections(); }
Q_SIGNAL void mySignal();
#ifdef Q_MOC_RUN
Q_SLOT void mySlot1();
Q_SLOT void mySlot2();
Q_SLOT void mySlot3(); // for MOC only since we don't override it
#endif
};
Finally, this little test case shows that it indeed works as desired.
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
MyTreeWidget tree;
MyTableWidget table;
Q_ASSERT(!tree.slot1Invoked && !tree.slot2Invoked && !tree.slot3Invoked);
emit tree.mySignal();
Q_ASSERT(tree.slot1Invoked && tree.slot2Invoked && tree.slot3Invoked);
Q_ASSERT(!table.slot1Invoked && !table.slot2Invoked && !table.baseSlot3Invoked);
emit table.mySignal();
Q_ASSERT(table.slot1Invoked && table.slot2Invoked && table.baseSlot3Invoked);
return 0;
}
#include "main.moc"
This approach gives you the following:
The common code class derives from the base class, and can thus easily invoke or override the behavior of the base class. In this particular example, you can reimplement the QAbstractItemView methods etc.
There is full support for signals and slots. Even though the signals and slots are declared as such in the metadata of the derived class, you can still use them in the generic class.
In this situation you may make use of composition rather than multiple inheritance. Something like this:
class MySignaler : public QObject
{
Q_OBJECT
public:
MySignaler : QObject(NULL) {}
signals:
void MySignal();
}
MySignaler signaler;
class MyBaseWidgetContainer: public QWidget
{
Q_OBJECT
public:
MyBaseWidgetContainer() : QObject(NULL), widget(NULL)
{
connect(&signaler, SIGNAL(MySignal()), this, SLOT(MySlot()));
}
public slots:
virtual void MySlot()
{
// Default behavior here
}
private:
QWidget *widget;
}
class MyTabWidgetContainer : public MyBaseWidgetContainer
{
Q_OBJECT
public:
MyTabWidgetContainer() {
widget = new QTabWidget(this);
QLayout *layout = new QBoxLayout(this);
layout->addWidget(widget);
}
public slots:
void MySlot()
{
// Decide to handle the signal for custom behavior
}
}