QObject::connect() with enum parameters - c++

I'm trying to use QObject::connect() with enum parameters. I got this message...
QObject::connect: Incompatible sender/receiver arguments
Calendar::calendarExceptionThrown(Calendar*,ExceptionType)
--> CalendarDBView::handleCalendarException(Calendar*,Calendar::ExceptionType)
The connect code is:
connect(cal, SIGNAL(calendarExceptionThrown(Calendar*,ExceptionType)),
this, SLOT(handleCalendarException(Calendar*,Calendar::ExceptionType)));
From what I've read, I figured I needed to add this to the class definition of Calendar:
Q_ENUMS(ExceptionType)
I added this to main.cpp:
qRegisterMetaType<Calendar::ExceptionType>("ExceptionType");
I'm still getting the warning message. This related thread didn't solve my issue. What am I doing wrong?

I believe if you simply change ExceptionType to Calendar::ExceptionType in the SIGNAL part of your connect, it will fix the problem. moc expects the strings for both the SIGNAL and SLOT to match.

You need to be very consistent with your signal and slots declarations, and with what you put in the SIGNAL and SLOT macros. moc isn't a full C++ compiler, it's just a relatively smart parser. But ultimately the signal and slot names are just strings.
For example, this "works":
#include <QtCore>
class A: public QObject {
Q_OBJECT
public:
enum MyEnum { e0 };
public:
A(QObject *parent=0): QObject(parent) {
connect(this, SIGNAL(fire(A::MyEnum)), this, SLOT(foo(A::MyEnum)));
}
public slots:
void foo(A::MyEnum) {
qDebug() << "In slot A::foo()";
}
signals:
void fire(A::MyEnum);
public:
void test() { emit fire(e0); }
};
It would also work if you remove all the A:: qualifiers for MyEnum. But it will fail if you leave some in but remove others.
So qualify all the names if these signals/slots need to be visible/accessible outside your class.

Related

enums and enum classes in Qt Queued Slots

Following advice from Qt documentation and this question's answers, I have code structured like so:
emulator.h:
class Emulator : public QObject
{
Q_OBJECT
public:
enum HaltCause {
Breakpoint,
ReadWatch,
WriteWatch,
UserHalted,
Reset,
SingleStep,
};
Q_ENUM(HaltCause)
...
signals:
void emulationHalted(HaltCause cause);
...
};
My MainWindow class has a matching slot:
private slots:
...
void onEmulationHalted(Emulator::HaltCause cause);
In mainwindow.cpp, the constructor contains:
...
qRegisterMetaType<Emulator::HaltCause>();
...
and in a method invoked later on in the MainWindow class:
...
connect(m_emulator, &Emulator::emulationHalted, this, &MainWindow::onEmulationHalted);
...
The Emulator class is threaded, so the connection between its signal and MainWindow's slot is queued.
Regardless of seemingly following all the guidelines for getting something into Qt's meta-object system, I still get this debug message when the Emulator's thread emits the signal:
QObject::connect: Cannot queue arguments of type 'HaltCause'
(Make sure 'HaltCause' is registered using qRegisterMetaType().)
I've tried this with Enumeration::HaltCause being both a simple enum and a strongly-typed enum class. Both yield the same results.
What am I missing?
Turns out the unqualified HaltCause used in the declaration of Emulator's slot confused the meta-object system. The resolution was to change
signals:
void EmulationHalted(HaltCause cause);
to
signals:
void EmulationHalted(Emulator::HaltCause cause);
in the Emulator declaration (emulator.h).

Why does adding a function argument cause a SLOT() to be unrecognised?

I have a class as follows:
handler.h:
#ifndef HANDLER_H
#define HANDLER_H
#include <QObject>
class handler : public QObject
{
Q_OBJECT
public:
explicit handler(QObject *parent = nullptr);
~handler();
public slots:
void returnHandler(int input);
};
#endif // HANDLER_H
handler.cpp:
#include "handler.h"
#include "otherclass.h"
handler::handler(QObject *parent) : QObject(parent)
{
}
handler::~handler()
{
}
void handler::returnHandler(int input)
{
otherclass *otherclassPointer = otherclass::getInstance();
otherclassPointer->returnFunction(input);
}
As shown, this is a very simple class, which aims to receive an input and pass the input to a function in an external class ('otherclass'). In my main application ('main.cpp'), I create a QThread, and call the returnHandler slot when the QThread is started, as follows:
main.cpp:
QThread* newThread = new QThread();
handler* handlerPointer = new handler();
handlerPointer->moveToThread(newThread);
connect(newThread, SIGNAL(started()), handlerPointer, SLOT(returnHandler(someInput)));
newThread->start();
The issue I'm having is this:
I'm currently get the following error:
QObject::connect: No such slot handler::returnHandler(someInput) in ../app/main.cpp:100
However, if I remove the int input from the handler class (both the .h and .cpp files), the SLOT() is recognized and called successfully when the QThread is started.
Why does adding an argument cause the slot to no longer be recognized?
EDIT: Following some very informative and appreciated comments/answers below, I've modified the approach as follows:
Create a signal in the handler class, which matches the parameters of the returnHandler slot. E.g. void handlerSignal(int).
Used the handlerSignal() SIGNAL instead of the QThread::started() signal in the connect().
Emit the handlerSignal() once the QThread is started.
`
QThread* newThread = new QThread();
handler* handlerPointer = new handler();
handlerPointer->moveToThread(newThread);
connect(handlerPointer, SIGNAL(handlerSignal(int)), handlerPointer, SLOT(returnHandler(int)));
newThread->start();
emit handlerPointer->handlerSignal(someInput);
Thanks!
Two things:
Qt expects the signal and the slot to have the same parameter types.
In SLOT(), you have to provide types, and not names for the parameters.SLOT(returnHandler(int)) instead of SLOT(returnHandler(someInput))
Qt uses the signals and slots's names and argument list to identify them. I your case, Qt looks for a slot named 'returnHandler' and having only one parameter, from type 'someInput'.
connect takes strings as the identification of the signal & slot to connect. The macros SIGNAL and SLOT stringise their arguments (using the preprocessor functionality for that). The argument to SIGNAL or SLOT must therefore be the function name, with parameter types in the parentheses. You cannot do argument binding with them.
If you need to connect to a nullary signal, you need a nullary slot.

QObject::connect: signal not found

I have a MainWindow class which is declared in mainwindow.h and defined in mainwindow.cpp respectively like this:
In mainwindow.h:
class MainWindow : public QMainWindow
{
Q_OBJECT
public:
MainWindow(QWidget *parent = 0);
...
void addNewTab(QString fullFilePath, QString textString="");
public slots:
void disableMenuItem();
...
private:
...
};
In mainwindow.cpp:
MainWindow::MainWindow(QWidget *parent)
: QMainWindow(parent)
{
...
connect(this, &MainWindow::addNewTab, this, &MainWindow::disableMenuItem);
...
}
void MainWindow::addNewTab(QString fullFilePath, QString textString)
{
...
}
void MainWindow::disableMenuItem()
{
...
}
Everything compiles and run fine except for the following message on the console:
QObject::connect: signal not found in MainWindow
The message come from the connect call in the constructor above. What does that message mean in my case, and where am I doing wrong?
As drescherjm and Learner mentioned, you forgot to add a signals: section to your header file, and declare your signal within it.
Qt connects signals to slots at runtime, not at compile time, so mis-connected signals and slots cannot be detected until the program is actually run; that's why this problem is reported when it is.
Qt uses the moc preprocessor to turn signals and slots into standard c++, so that's why the signals: and slots: sections of your header will not cause problems when compiling.
Signals are fully defined by moc, so you do not need to define them in your .cpp file, but they still need to be in the header so moc knows to create them.
EDIT:
It appears that you are trying to use a signal with the name of one of your class functions. I don't think that's going to work. the documentation for the new signal/slot syntax indicates that you can connect TO anything, it doesn't have to be a slot, but I believe you still need to define your signal as a signal.

How connect a signal from C++ to QML function with parameter

I want to connect a signal which is created by a C++ QObject to a function in a QML item. The signal is the "activated" signal from a QSystemTrayIcon and its argument is ActivationReason (an enum value).
Unfortunately it seems I can't connect a signal with this signature to a slot which seems to only be able to receive QVariant.
In the QML file
function trayIconClicked(reason) { ... }
In the C++ code
QObject::connect(trayIcon, SIGNAL(activated(QSystemTrayIcon::ActivationReason)), rootObject, SLOT(trayIconClicked(QVariant)));
And this is what I get
QObject::connect: Incompatible sender/receiver arguments
QSystemTrayIcon::activated(QSystemTrayIcon::ActivationReason) --> ApplicationWindow_QMLTYPE_11_QML_60::trayIconClicked(QVariant)
If I completely drop the argument on the slot side, the event calls the function. But then I have no idea what ActivationReason triggred it.
A full working version of the code above is here, except that I can't hand in the reason parameter https://github.com/sturmf/qt_samples/tree/master/trayicon
I now subclassed the QSystemTrayicon and added a slot which emits another signal without the parameter.
#ifndef TRAYICON_H
#define TRAYICON_H
#include <iostream>
#include <QObject>
#include <QSystemTrayIcon>
class TrayIcon : public QSystemTrayIcon
{
Q_OBJECT
public:
explicit TrayIcon(QObject *parent = 0) : QSystemTrayIcon(parent){}
signals:
void triggered();
public slots:
void trayIconActivated(QSystemTrayIcon::ActivationReason reason) {
std::cout << "activated" << std::endl;
if (reason == QSystemTrayIcon::Trigger) {
std::cout << "tiggered" << std::endl;
emit triggered();
}
}
};
#endif // TRAYICON_H
In Qt5 you can connect to an anonymous function, by the way.
QObject::connect(trayIcon
, SIGNAL(activated(QSystemTrayIcon::ActivationReason))
, [=](QSystemTrayIcon::ActivationReason r){
rootObject->trayIconClicked(r);
});

Error with QObject::connect()

I am trying to run QTimer and have it warn me when timeouting. To do so, I use slot and signal to link the two.
The guy.h:
#ifndef GUY_H
#define GUY_H
#include <QGraphicsItem>
#include <QTimer>
#include <QObject>
class Guy : public QGraphicsItem
{
public:
Guy(int x, int y);
void timerStart();
public slots:
void onTimeOutTimer();
[...]
QTimer *timer;
}
#endif // GUY_H
The guy.cpp:
#include "guy.h"
#include <QTimer>
#include <QObject>
#include <stdio.h>
#include <iostream>
Guy::Guy(int x, int y)
{
timer = new QTimer();
}
void Guy::timerStart()
{
QObject::connect(timer, SIGNAL(timeout()), this, SLOT(onTimeOutTimer()));
this->timer->setInterval(1000);
this->timer->start();
std::cout << "starting timer" << std::endl;
}
void Guy::onTimeOutTimer()
{
std::cout << "check" << std::endl;
}
But as an ouput, I get this error:
No matching function for call to 'QObject::connect(QTimer*&, const char*, Guy* const, const char*)'
As I undertsand it is that QTimer is no QObject required as first input of the function connect(), but the documentation specifies QTimer inherits from QObject.
I have no clue here.
You will need to inherit from QObject, too, to get this working as signals and slots are availabl for QObjects. QGraphicsItem does not inherit QObject, not even indirectly.
Not only that, you will also need to add the Q_OBJECT macro as follows:
class Guy : public QObject, public QGraphicsItem
{
Q_OBJECT
...
}
or even better because QGraphicsObject inherits QObject and QGraphicsItem.
...
#include <QGraphicsObject>
...
class Guy : public QGraphicsQObject
{
Q_OBJECT
...
}
Also, if you make this change, I suggest to change the QObject::connect to connect as you do not need to indicate the QObject:: scope then.
On a side note, including stdio.h does not seem to make sense here.
Furthermore, allocating the QTimer instance on the heap looks like wasteful to me. It is not only leaking the memory, but also adds additional complexity. Even if you allocate it on the heap, you should pass this as the parent and use the initializer list or C++11 style initialization. Also, if you allocate it on the heap, you could use forward declaration in the header.
If the slot is not used outside the class, you should also make that private.
It is probably also a bad idea to make the timer member public. Hopefully you do not do that.
You can inherit from QGraphicsObject which provides a base class for all graphics items that require signals, slots and inherits QGraphicsItem and QObject. Also add Q_OBJECT macro in your class declaration.
If you use new Qt5 style connects as in
QObject::connect(timer, &QTimer::timeout, this, &Guy::onTimeOutTimer)
The onTimeOutTimer function does not need to be marked as slot, and Guy could stay a non-QObject. Much slimmer and less macros involved.