the simplest custom slot

[tommy]

Hi all,

I'm trying to figure out if I don't know how to make a custom slot or my compiler (Dev-C++) is not configured correctly to compile a program with a custom slot (everything compiles fine when I use a predefined slot). I'd be extremely thankful to you if you could either:
1) send me the simplest fully functioning code (using just one *.cpp file) that uses a custom slot,
or
2) take a look at my code and suggest why this is not compiling and I get this error:
[Linker error] undefined reference to `vtable for MyExit'

Here's my 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();
};
 
MyExit::MyExit(QObject *parent):QObject(parent) 
   {
   }
 
   void MyExit::myExitSlot() 
       {
       qApp->quit();
       }
 
 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()), qApp, 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();
};
  
MyExit::MyExit(QObject *parent):QObject(parent) 
   {
   }
      
   void MyExit::myExitSlot() 
       {
       qApp->quit();
       }

 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()), qApp, SLOT(myExitSlot()));
     window.show();
     return a.exec();
 }

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Thanks a lot in advance. Tommy

[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

[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.

[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.

[tommy]

Thanks Jacek,

You made a lot of things much more clear for me, I'm beginning to understand the basics now.
I was able to use qmake just as you suggested but did not get it to work with Dev-C++. I'm thinking of abandoning Dev-C++ and trying command line (just get plain mingw). How do you include the makefile when you compile from command line? What's the syntax?

Thankfully,
Tommy

[jacek]

I'm thinking of abandoning Dev-C++ and trying command line (just get plain mingw).

I'm sure you can find some tutorials about using Dev-C++ for Qt applications on google.

[mchara]

Hi,
I'm using qt on dev-cpp, so i had similar problem
I'm not using qmake(it says it can't find FORCE target), but i didn't searched the reason of this error, because i'm using multiple makefiles to have effect of several projects in one solution (dev-cpp works on projects only).
You need to set use custom makefile in projects options, add a tatget *.moc, with 2 commands:
1 moc your *.h to *.moc(or moc_*.cpp)
2 compile generated file

or the simpler way makefile that calls only qmake & make on file generated by, unfortunately i can't test this because of mentioned qmake error.

[jacek]

I'm not using qmake(it says it can't find FORCE target), but i didn't searched the reason of this error

What is the value of the TARGET variable in your .pro file? If it isn't set, what is the name of the .pro file?

You need to set use custom makefile in projects options, add a tatget *.moc, with 2 commands:
1 moc your *.h to *.moc(or moc_*.cpp)
2 compile generated file

What about all of those magic macros Qt needs?

[mchara]

What is the value of the TARGET variable in your .pro file? If it isn't set, what is the name of the .pro file?

Doesn't matter, i have the same error for every makefile generated by qmake, i found that
FORCE: is an empty target used to force something to be build even if there's no dependencies so it shall be empty definition.
It is in deed in generated makefiles' but i suppose that problem might be in missing tab & newline after FORCE: - some make's are sensitive for tabs instead of spaces etc., i think make is one of them.

What about all of those magic macros Qt needs?

What macros?
DO you mean those defines at the beginning

-DUNICODE -DQT_LARGEFILE_SUPPORT -DQT_DLL -DQT_NO_DEBUG -DQT_CORE_LIB -DQT_GUI_LIB -DQT_THREAD_SUPPORT -DQT_NEEDS_QMAIN

-DUNICODE -DQT_LARGEFILE_SUPPORT -DQT_DLL -DQT_NO_DEBUG -DQT_CORE_LIB -DQT_GUI_LIB -DQT_THREAD_SUPPORT -DQT_NEEDS_QMAIN

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, i've copied them from first makefile found in qt's exapmles & worked fine.

Those are always the same view defines(& view more depending on used modules -DQT_*_LIB) so it's not much work.

I thought that maybe custom makefie that would have defined only qmake -project; qmake &make on newly generated makefile would solve the problem if we would find what's wrong with FORCE error... can anyone check is this common problem on mingw or just mine?

Oh, one more thing : make sure to not use --short-double or --expensive-optimizations because using one of them causes QPainter::drawPixmap()/drawImage() stops working... it's really strange but i've checked it several times & it really happens.