No matching function for QObject::connect - c++

I'm writing a program that send an UDP frame every 10 mS. Here's how my program is supposed to work :
I've got a client class :
//Constructor
clientSupervision::clientSupervision()
{
}
void clientSupervision::sendDataUDP(){
//Create a frame and send it
...
}
void clientSupervision::sendDataUDPTimer(int timer){
QTimer *tempsEnvoieTrameSupervision = new QTimer();//Create a timer
tempsEnvoieTrameSupervision->setInterval(timer);//Set the interval
//Mise en place des connections
QObject::connect (tempsEnvoieTrameSupervision,SIGNAL (timeout()),this, SLOT (envoiTrameSupervision())); //Connect the timer to the function
tempsEnvoieTrameSupervision->start();// Start the timer
}
//Call sendDataUDP
void clientSupervision::envoiTrameSupervision(){
std::cout << "Envoi de la trame de supervision";
sendDataUDP();
}
My header file of clienSupervision.h :
#ifndef CLIENTSUPERVISION_H
#define CLIENTSUPERVISION_H
#include <winsock2.h> // pour les fonctions socket
#include <cstdio> // Pour les Sprintf
#include "StructureSupervision.h"
#include "utilitaireudp.h"
#include <QTimer>
#include <QObject>
#include <iostream>
class clientSupervision
{
Q_OBJECT
public:
clientSupervision();
void sendDataUDP();
void sendDataUDPTimer(int timer);
public slots:
void envoiTrameSupervision();
};
#endif // CLIENTSUPERVISION_H
Then I use this in my main :
int main(int argc, char *argv[])
{
clientSupervision c;
c.sendDataUDPTimer(10);
QCoreApplication a(argc, argv);
return a.exec();
}
I've got the error :
no matching function for call to 'QObject::connect(QTimer*&, const char*, clientSupervision* const, const char*)
I don't understand why the connect function can't find a matching function.
What should I change?

There can be several reasons for the issue in general:
You do not inherit QObject.
You do not have the Q_OBJECT macro in your class.
You do not define the method as slot in your header file where the class is declared.
Your issue is the first which can be seen here:
class clientSupervision
You should change your code to:
class clientSupervision : public QObject
// ^^^^^^^^^^^^^^^^
Of course, the constructor implementation and signature would need to change, too, as follows:
explicit clientSupervision(QObject *parent = Q_NULL_PTR) : QObject(parent) { ... }
In addition, you seem to leak your QTimer instance as it does not get the parent as a parameter to the constructor.
Furthermore, the QObject:: scope is needless in your code as your class ought to inherit QObject directly or indirectly either way.
Even more, I would highly encourage you to utilize the new signal-slot syntax.

Another possible cause of this error is trying to connect to a slot which is overloaded. For example, this well cause the same error
QObject::connect(this,
&MazeWidget::MyUpdate,
this,
&QWidget::update,
Qt::QueuedConnection);
But not if you explicitely cast:
QObject::connect(this,
&MazeWidget::MyUpdate,
this,
static_cast<void (QWidget::*)()>(&QWidget::update),
Qt::QueuedConnection);

Here's another one that snuck up on me: The class of the slot object had been forward declared in the header, but not defined in the implementation by including its header.

If none of the answers above worked, check if you have assigned correct object to the signals and the slots. You will get this error when the signals and slots are valid and refer to one object but the object assigned to that signal and slot is different.

Related

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.

Clickable QLabel class "multiply defined" functions

I'm having trouble moving from VC++ to Qt-style programming with slots/signals. I wanted to create a button with an image that, when clicked, changed to another image, and when released, changed back to its original. I created a class called ClickableQLabel that inherits from QLabel, but it's telling me that certain functions are re-defined. Specifically, the ones that are emitted.
MainProgram.cpp
#include "MainProgram.h"
#include <QApplication>
int main(int argc, char *argv[]) {
QApplication a(argc, argv);
CodeVault w;
w.show();
return a.exec();
}
MainProgram.h
#ifndef MAINPROGRAM_H
#define MAINPROGRAM_H
#include <QMainWindow>
#include "clickableqlabel.h"
namespace Ui {
class MainProgram;
}
class MainProgram : public QMainWindow
{
Q_OBJECT
public:
explicit MainProgram(QWidget *parent = 0);
~MainProgram();
private:
Ui::MainProgram *ui;
};
#endif // MAINPROGRAM_H
clickableqlabel.h
#ifndef CLICKABLEQLABEL_H
#define CLICKABLEQLABEL_H
#include <QLabel>
#include <QMouseEvent>
using namespace Qt;
class ClickableQLabel : public QLabel {
Q_OBJECT
QPixmap pushed;
QPixmap unpushed;
public:
ClickableQLabel(QWidget *parent = 0);
void mousePressEvent(QMouseEvent *eve );
void mouseReleaseEvent(QMouseEvent *eve );
void setPushed(QPixmap &p);
void setUnpushed(QPixmap &p);
signals:
void leftButtonPressed(ClickableQLabel* sender);
void leftButtonReleased(ClickableQLabel* sender);
};
#endif // CLICKABLEQLABEL_H
clickableqlabel.cpp
#include "clickableqlabel.h"
ClickableQLabel::ClickableQLabel(QWidget *parent) : QLabel(parent){
// set up qlabel
}
void ClickableQLabel::setPushed(QPixmap &p){
pushed = p;
}
void ClickableQLabel::setUnpushed(QPixmap &p){
unpushed = p;
}
void ClickableQLabel::leftButtonPressed(ClickableQLabel* sender){
if(!pushed.isNull())
sender->setPixmap(pushed.scaledToWidth(sender->width()));
}
void ClickableQLabel::leftButtonReleased(ClickableQLabel* sender){
if(!unpushed.isNull())
sender->setPixmap(unpushed.scaledToWidth(sender->width()));
}
void ClickableQLabel::mousePressEvent(QMouseEvent *eve ){
if(eve->button() == Qt::LeftButton){
emit leftButtonPressed(this);
}
}
void ClickableQLabel::mouseReleaseEvent(QMouseEvent *eve ){
if(eve->button() == Qt::LeftButton){
emit leftButtonReleased(this);
}
}
What I receive is the following 3 errors:
moc_clickableqlabel.obj:-1: error: LNK2005: "public: void __cdecl ClickableQLabel::leftButtonPressed(class ClickableQLabel *)" (?leftButtonPressed#ClickableQLabel##QEAAXPEAV1##Z) already defined in clickableqlabel.obj
moc_clickableqlabel.obj:-1: error: LNK2005: "public: void __cdecl ClickableQLabel::leftButtonReleased(class ClickableQLabel *)" (?leftButtonReleased#ClickableQLabel##QEAAXPEAV1##Z) already defined in clickableqlabel.obj
debug\CodeVault.exe:-1: error: LNK1169: one or more multiply defined symbols found
The two functions that are causing the errors are the two signals in the clickableqlabel.h file. How am I supposed to set up the connect function and where?
You shouldn't provide an implementation for your signals. You just declare signals in the class header. the Qt moc provides an implementation that is responsible to call the slots connected to the signal when it is emitted, see this. Because you are providing an implementation and Qt moc is providing another one, you end up with two different implementations and the linker complains.
so, to make some piece of code execute when a signal is emitted, you can put it in some slot that is connected to that signal, or you can execute it manually before emitting the signal. . .
Another thing to note is that your signals have a parameter named sender. normally there is no need to do this, QObject::sender() provides similar functionality.
How am I supposed to set up the connect function and where?
When you instantiate your ClickableQLabel in your MainProgram window for example (You can do this by using ClickableQLabel in a .ui form file), you can connect its signals to slots of the MainProgram, like this.
mainprogram.h
#ifndef MAINPROGRAM_H
#define MAINPROGRAM_H
#include <QMainWindow>
#include "clickableqlabel.h"
namespace Ui {
class MainProgram;
}
class MainProgram : public QMainWindow
{
Q_OBJECT
public:
explicit MainProgram(QWidget *parent = 0);
~MainProgram();
public slots:
void labelPressed();
void labelReleased();
private:
Ui::MainProgram *ui;
};
#endif // MAINPROGRAM_H
then in the constructor connect the ClickableQLabel's signals to the MainProgram's slots , something like this:
connect(clickableLabel, SIGNAL(leftButtonPressed()), this, SLOT(labelPressed()));
where clickableLabel is your ClickableQLabel object.
You should not explicitly define signal "functions" - they are simply emitted (as you do in the mousePressEvent() and mouseReleaseEvent() functions).
If you want some action performed (such as setPixmap), you would either do that in the slot functions that are connected to those signals, or perhaps directly in the mousePressEvent and mouseReleaseEvent functions.
Unrelated, but you may also need to do something like qRegisterMetaType<ClickableQLabel>() to use that data type with the signal/slot mechanism.
You don't need to subclass QLabel to achieve this: put an onLabelButtonClicked slot (method) in your MainWindow class, which is connected to the QPushButton clicked() signal, and which calls setText / setIcon as required.
If you want to re-use this clickable element you can of course encapsulate it in a class, but subclassing and handling raw-events is only required in Qt when defining new kinds of widget; if you're simply composing standard functionality (being clicked, adjusting appearance) then slots on your main window / dialog class are usually sufficient.

How to find out from the slot which signal has called this slot?

I mean if I have many different signals which are connected to the same slot. I saw this question but can't understand the link in the answer. Can you give me simple example?
I think you can use this method:
[protected] int QObject::​senderSignalIndex() const
From Qt documentation:
Returns the meta-method index of the signal that called the currently executing slot, which is a member of the class returned by sender(). If called outside of a slot activated by a signal, -1 is returned.
For signals with default parameters, this function will always return the index with all parameters, regardless of which was used with connect(). For example, the signal destroyed(QObject *obj = 0) will have two different indexes (with and without the parameter), but this function will always return the index with a parameter. This does not apply when overloading signals with different parameters.
Warning: This function violates the object-oriented principle of modularity. However, getting access to the signal index might be useful when many signals are connected to a single slot.
Warning: The return value of this function is not valid when the slot is called via a Qt::DirectConnection from a thread different from this object's thread. Do not use this function in this type of scenario.
This function was introduced in Qt 4.8.
Here is a small example that I created for you that demonstrates how it works:
#include <QTimer>
#include <QMetaObject>
#include <QMetaMethod>
#include <QCoreApplication>
#include <QDebug>
#include <QObject>
class Foo : public QObject
{
Q_OBJECT
public slots:
void mySlot() {
QMetaMethod metaMethod = sender()->metaObject()->method(senderSignalIndex());
qDebug() << metaMethod.name();
qDebug() << metaMethod.methodSignature();
qApp->quit();
}
};
#include "main.moc"
int main(int argc, char **argv)
{
QCoreApplication coreApplication(argc, argv);
QTimer timer;
Foo foo;
QObject::connect(&timer, &QTimer::timeout, &foo, &Foo::mySlot);
timer.setSingleShot(true);
timer.start(1000);
return coreApplication.exec();
}
main.pro
TEMPLATE = app
TARGET = main
QT = core
CONFIG += c++11
SOURCES += main.cpp
Build and Run
qmake && make && ./main
Output
"timeout"
"timeout()"

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.

How can i call a my function in a qtslot?

I'm trying to call my function as a function slot in Qt,But i don't know how to go about it.
it seems the following approach is wrong :
Update:
According to an answer i updated my source code,but still something is apparently wrong with it.Trying to compile this snippet of code causes these errors:
C2515:' no appropriate default constructor is available.'
And
C2665: QObject::connect':none of the 3 overloads could convert all the
arguments.'
respectively in Visual studio 2010.
#include <QtGui/QApplication>
#include <QPushButton>
#include <QObject>
#include <QMessageBox>
class myclass;
int main(int argc,char *argv[])
{
QApplication a(argc,argv);
QPushButton btnshowmessage("show");
myclass *my=new myclass();
QObject::connect(&btnshowmessage,SIGNAL(clicked()),my,SLOT(warningmessage()));
btnshowmessage.show();
return a.exec();
}
//////////////////////////////////////////////////////////////////////////
class myclass: public QObject
{
Q_OBJECT
public:myclass(){}
public slots:
void warningmessage()
{
QMessageBox::warning(0,"Warning","Test Message!",QMessageBox::Ok);
}
};
You use signals and slots to connect one Object's signal to another Object's slot. Every signal or slot should be inside a class which must be also derived from QObject class and contain the Q_OBJECT macro.
So to make your code work, put the slot into some class of yours:
class MySlotClass:public QObject
{
Q_OBJECT
public slots:
void MyFunction()
{
QMessageBox::warning(0,"WarningTest","This is a waring text message",QMessageBox::Ok);
}
}
and connect like this:
MySlotClass m = new MySlotClass();
Qobject::connect(&btnShowaMessageBox,SIGNAL(clicked()), &m ,SLOT(MyFunction()));
Currently Qt does not allow connection of signals to functions that are not declared as slots on some QObject derivative. I believe Qt5 may offer this possibility, but connect will have different syntax to allow this.
basically your slot function must be in a QObject derived class and declared in a
public slots:
section.
Read the documentation on signals/slots.