This is the sequel of my previous article explaining the implementation details of the signals and slots. In the Part 1, we have seen the general principle and how it works with the old syntax.In this blog post, we will see the implementation details behind the new function pointer based syntax in Qt5. Qt Development General and Desktop SOLVED Inheriting virtual slots and use the new QT5 connect flavour SOLVED Inheriting virtual slots and use the new QT5 connect flavour. This topic has been deleted. Only users with topic management privileges can see it. Jensen82 last edited. If i create a class from a base class with virtual slots, the slots never get called with the new.

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Hello.

I want to write a small tool and like to separate the business logic from the GUI so I can easily make and use different GUIs.

Therefore I've made a class which contains the business logic called AppCore. The GUI class(es) are called MainWindow.

My idea is to pass the constructor of the AppCore a reference to the GUI so the constructor of the AppCore can connect the AppCore singals to the GUI slots and vice versa.

But to make it flexible I don't want to have the actual GUI as type of the parameter but an Interface (pure virtual class) called IGui which the GUI inherits from.

@
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
AppCore *appCore;
MainWindow w;

appCore = new AppCore(&w);
w.show();

return a.exec();
}

class IGui
{
public slots:
virtual void slot() = 0;

};

class MainWindow : public QMainWindow, public IGui
{
Q_OBJECT

};

class AppCore : public QObject
{
Q_OBJECT

}

AppCore::AppCore(IGui *gui, QObject *parent) :
QObject(parent)
{
connect(gui, SIGNAL(signal()),
this, SLOT(onSingal()));

But when I try to compile I get errors:
[quote]
AppCore.cpp: In constructor 'AppCore::AppCore(IGui*, QObject*)':
AppCore.cpp:9: error: no matching function for call to 'AppCore::connect(IGui*&, const char [10], AppCore* const, const char [12])'
c:QtSDKDesktopQt4.8.1mingwincludeQtCore/qobject.h:204: note: candidates are: static bool QObject::connect(const QObject*, const char*, const QObject*, const char*, Qt::ConnectionType)
c:QtSDKDesktopQt4.8.1mingwincludeQtCore/qobject.h:217: note: static bool QObject::connect(const QObject*, const QMetaMethod&, const QObject*, const QMetaMethod&, Qt::ConnectionType)
c:QtSDKDesktopQt4.8.1mingwincludeQtCore/qobject.h:337: note: bool QObject::connect(const QObject*, const char*, const char*, Qt::ConnectionType) const
AppCore.cpp:12: error: no matching function for call to 'AppCore::connect(AppCore* const, const char [10], IGui*&, const char [8])'
c:QtSDKDesktopQt4.8.1mingwincludeQtCore/qobject.h:204: note: candidates are: static bool QObject::connect(const QObject*, const char*, const QObject*, const char*, Qt::ConnectionType)
c:QtSDKDesktopQt4.8.1mingwincludeQtCore/qobject.h:217: note: static bool QObject::connect(const QObject*, const QMetaMethod&, const QObject*, const QMetaMethod&, Qt::ConnectionType)
c:QtSDKDesktopQt4.8.1mingwincludeQtCore/qobject.h:337: note: bool QObject::connect(const QObject*, const char*, const char*, Qt::ConnectionType) const[/quote]

If I let IGui derive from QObject I get this errors:
[quote]
releasemoc_MainWindow.cpp: In static member function 'static void MainWindow::qt_static_metacall(QObject*, QMetaObject::Call, int, void**)':
releasemoc_MainWindow.cpp:52: error: 'QObject' is an ambiguous base of 'MainWindow'
releasemoc_MainWindow.cpp: In member function 'virtual const QMetaObject* MainWindow::metaObject() const':
releasemoc_MainWindow.cpp:78: error: 'QObject' is an ambiguous base of 'MainWindow'
releasemoc_MainWindow.cpp:78: error: 'QObject' is an ambiguous base of 'MainWindow'
releasemoc_MainWindow.cpp: In member function 'virtual int MainWindow::qt_metacall(QMetaObject::Call, int, void**)':
releasemoc_MainWindow.cpp:98: error: 'QObject' is an ambiguous base of 'MainWindow'
releasemoc_MainWindow.cpp: In member function 'virtual void MainWindow::signal()':
releasemoc_MainWindow.cpp:107: error: 'QObject' is an ambiguous base of 'MainWindow'
[/quote]

How do I have to do it right?

Introduction

Remember old X-Window call-back system? Generally it isn't type safe and flexible. There are many problems with them. Qt offer new event-handling system - signal-slot connections. Imagine alarm clock. When alarm is ringing, signal is sending (emitting). And you're handling it as a slot.

Every QObject class may have as many signals of slots as you want.
You can emit signal only from that class, where signal is.
You can connect signal with another signal (make chains of signals);
Every signal and slot can have unlimited count of connections with other.
ATTENTION! You can't set default value in slot attributes. e.g.

Connection

You can connect signal with this template:

You have to wrap const char * signal and const char * method into SIGNAL () and SLOT() macros.

Qt Signal Slot Example

And you also can disconnect signal-slot:

Deeper

C++ Signals And Slots

Widgets emit signals when events occur. For example, a button will emit a 'clicked' signal when it is clicked. A developer can choose to connect to a signal by creating a function (a 'slot') and calling the function to relate the signal to the slot. Qt's signals and slots mechanism does not require classes to have knowledge of each other, which makes it much easier to develop highly reusable classes. Since signals and slots are type-safe, type errors are reported as warnings and do not cause crashes to occur.For example, if a Quit button's signal is connected to the application's slot, a user's click on Quit makes the application terminate. In code, this is written as

Connections can be added or removed at any time during the execution of a Qt application, they can be set up so that they are executed when a signal is emitted or queued for later execution, and they can be made between objects in different threads. https://newsurf.amebaownd.com/posts/18264526.

The signals and slots mechanism is implemented in standard C++. The implementation uses the C++ preprocessor and moc, the Meta-Object Compiler, included with Qt. Code generation is performed automatically by Qt's build system. Developers never have to edit or even look at the generated code.

Qt Virtual Signals And Slots

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Virtual Signal And Slot Qt