the simplest custom slot

[jacek]

If you put Q_OBJECT macro inside a .cpp file, you have to add #include "filename.moc" at the end to let qmake know that it has to invoke moc on that file (normally qmake looks for Q_OBJECT only in header files).

[jml]

I'm not sure you can call connect from the main() routine; I believe it should be invoked from an object derived from QObject. Try moving your class declaration to a .h flie, and do the connect from the MyExit constructor.

[wysota]

I'm not sure you can call connect from the main() routine; I believe it should be invoked from an object derived from QObject.

No, you can call connect() from everywhere you want. It's a static method of QObject, so you just need to prefix it with QObject:: if you use it outside any QObject, just like the thread author did.

[spud]

The slot seems find, You need to connect to it, though.

MyExit myExit;
QObject::connect(b1, SIGNAL(clicked()), &myExit, SLOT(myExitSlot()));

MyExit myExit;
QObject::connect(b1, SIGNAL(clicked()), &myExit, SLOT(myExitSlot()));

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Right now your connecting to a QApplication object, which doesn't have the slot myExitSlot()

[tommy]

Thanks to all who responded to me. You have given me lots of ideas and helped me a lot.
For example I did not know that custom slot classes should be in the *.h file with mingw compiler. I had absolutely no idea that you could include "filename.moc" file if your custom class is in the *.cpp file. I did not get this to work yet but it it good to know that such an option exists. Thanks for pointing out that I had the slot connected all wrong, too.
So I guess I have to start using *.h files for classes and I will. But at the same time I'm very curious if my compiler has some kind of a problem. I'd appreciate it a lot if somone was able to run the below code through their compiler and let me know what kind of error messages other compilers gave. Or perhaps it'll work with other compilers no problem.
Thanks a lot.
Here's the code:

#include <QApplication>
 #include <QPushButton>
 #include <QHBoxLayout>
 
  using namespace std;
 
class MyExit : public QObject
{ 
    Q_OBJECT
 
    public:
       MyExit (QObject *parent = 0);
    public slots:
        void myExitSlot();
};
 
 
 
   void MyExit::myExitSlot()
    {
    qApp->quit();
    }
 
   MyExit::MyExit(QObject *parent):QObject(parent)
     {      
     }
 
 
 int main(int argc, char *argv[])
 {
     QApplication a(argc, argv);
     QWidget window;
     QHBoxLayout* mainLayout = new QHBoxLayout(&window);
     QPushButton *b1 = new QPushButton("Exit");
 
     mainLayout->addWidget(b1);
 
     QObject::connect(b1, SIGNAL(clicked()), &MyExit, SLOT(myExitSlot()));
 
     window.show();
     return a.exec();
 }

#include <QApplication>
 #include <QPushButton>
 #include <QHBoxLayout>
 
  using namespace std;

class MyExit : public QObject
{ 
    Q_OBJECT
     
    public:
       MyExit (QObject *parent = 0);
    public slots:
        void myExitSlot();
};
  

      
   void MyExit::myExitSlot()
    {
    qApp->quit();
    }

   MyExit::MyExit(QObject *parent):QObject(parent)
     {      
     }


 int main(int argc, char *argv[])
 {
     QApplication a(argc, argv);
     QWidget window;
     QHBoxLayout* mainLayout = new QHBoxLayout(&window);
     QPushButton *b1 = new QPushButton("Exit");
    
     mainLayout->addWidget(b1);
   
     QObject::connect(b1, SIGNAL(clicked()), &MyExit, SLOT(myExitSlot()));
 
     window.show();
     return a.exec();
 }

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[jpn]

Looks like no instance of MyExit is being created. Take a look at spud's post.

[tommy]

Thanks jpn.

Unfortunately I don't exactly understand how to do what I'm supposed to do: create that instance or make the connection (I just started with Qt a few days ago). Do you mind showing me the lines.
Thanks a lot.

Tommy

[jacek]

Unfortunately I don't exactly understand how to do what I'm supposed to do: create that instance or make the connection

You can create connections only between objects, but MyExit is a class. You have to create an object of this class (just like you did with QWidget or QPushButton).

[tommy]

Great. I created the instance required but the code still won't compile. And it won't compile when I put the new class in a *.h file either.
Does anyone have any more ideas?
Thankfully - Tommy.

Code:

#include <QApplication>
#include <QPushButton>
#include <QHBoxLayout>
 
using namespace std;
 
class MyExit : public QObject
{ 
Q_OBJECT
 
public:
MyExit (QObject *parent = 0);
public slots:
void myExitSlot();
};
 
 
 
void MyExit::myExitSlot()
{
qApp->quit();
}
 
MyExit::MyExit(QObject *parent):QObject(parent)
{ 
}
 
 
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
MyExit Exit;
QWidget window;
QHBoxLayout* mainLayout = new QHBoxLayout(&window);
QPushButton *b1 = new QPushButton("Exit");
 
mainLayout->addWidget(b1);
 
QObject::connect(b1, SIGNAL(clicked()), &Exit, SLOT(myExitSlot()));
 
window.show();
return a.exec();
}

#include <QApplication>
#include <QPushButton>
#include <QHBoxLayout>

using namespace std;

class MyExit : public QObject
{ 
Q_OBJECT

public:
MyExit (QObject *parent = 0);
public slots:
void myExitSlot();
};



void MyExit::myExitSlot()
{
qApp->quit();
}

MyExit::MyExit(QObject *parent):QObject(parent)
{ 
}


int main(int argc, char *argv[])
{
QApplication a(argc, argv);
MyExit Exit;
QWidget window;
QHBoxLayout* mainLayout = new QHBoxLayout(&window);
QPushButton *b1 = new QPushButton("Exit");

mainLayout->addWidget(b1);

QObject::connect(b1, SIGNAL(clicked()), &Exit, SLOT(myExitSlot()));

window.show();
return a.exec();
}

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[jacek]

The code is OK. Do you use qmake to compile it?

[tommy]

hi Jacek

Thanks for confirming my code. No, I do not use qmake. I'm going to need
another piece of help here if you don't mind because I do not know how to use qmake.
My compiler is Dev-C++ (mingw). I guess I have to put "qmake" somewhere in the
project options sections? Or is qmake a standalone program you run before or when you compile? (instead of your compiler?).
Thanks to you and the other helpful people my problem is now narrowed down to
compiler/linker problem and most likely qmake.

Tommy

[jacek]

My compiler is Dev-C++ (mingw). I guess I have to put "qmake" somewhere in the
project options sections? Or is qmake a standalone program you run before or when you compile? (instead of your compiler?).

qmake is a tool that generates makefiles from .pro files, which are much simplier. To generate an initial .pro file you have to run "qmake -project" in the directory where your project is. Then you run "qmake" or "qmake filename.pro" to generate makefile. I don't use Dev-C++, but AFAIR you can make it use a custom makefile, so you just have to point it to the makefile generated by qmake.

[rvb13]

The code is OK. Do you use qmake to compile it?

Hi Jacek,

I was having a look at Tommy's code and I am a bit confused. In this MyExitSlot() implementation, I see he is calling qapp.quit(); and I dont see a QApplication object named as qapp. Shouldnt he be declaring one in the private section of MyExit ? I think thats the reason why the compiler is complaining because there is no reference to the method in the vtable, since there is no object defined. Right ? Please correct me if I am wrong.

Thanks
- Rahul

[jpn]

See qApp in QApplication docs:

A global pointer referring to the unique application object. It is equivalent to the pointer returned by the QCoreApplication::instance() function except that, in GUI applications, it is a pointer to a QApplication instance.