errors when using Q_OBJECT and slots

**kja** · 13th November 2010, 00:47

Hi,
I'm new to qt programming and I'm trying to make some custom signals and slots but I keep getting errors when I try to do some simple examples.

I can get this program to run by commenting out Q_OBJECT, but if I leave it in i get errors, any idea why that is?

Qt Code:

Switch view

#include <QtGui>
#include <QApplication>
#include <QVBoxLayout>
#include <QLineEdit>
#include <QPushButton>
#include "qobjectdefs.h"
 
class myCanvas : public QWidget{
  Q_OBJECT   //this program runs fine if i comment out Q_OBJECT
public:
  myCanvas (QWidget * parent = 0);
public Q_SLOTS:
  void copyText(const QString & text);
};
 
myCanvas::myCanvas(QWidget *parent):QWidget(parent){
  QLineEdit * lineedit = new QLineEdit;
  QPushButton * pushbutton = new QPushButton("Enter");
 
  QVBoxLayout * layout = new QVBoxLayout;
  layout->addWidget(lineedit);
  layout->addWidget(pushbutton);
  setLayout(layout);
}
 
int main (int argc, char * argv[])
{
  QApplication app (argc, argv);
  myCanvas bob;
  bob.show();
  return app.exec();
}

#include <QtGui>
#include <QApplication>
#include <QVBoxLayout>
#include <QLineEdit>
#include <QPushButton>
#include "qobjectdefs.h"

class myCanvas : public QWidget{
  Q_OBJECT   //this program runs fine if i comment out Q_OBJECT
public:
  myCanvas (QWidget * parent = 0);
public Q_SLOTS:
  void copyText(const QString & text);
};

myCanvas::myCanvas(QWidget *parent):QWidget(parent){
  QLineEdit * lineedit = new QLineEdit;
  QPushButton * pushbutton = new QPushButton("Enter");

  QVBoxLayout * layout = new QVBoxLayout;
  layout->addWidget(lineedit);
  layout->addWidget(pushbutton);
  setLayout(layout);
}

int main (int argc, char * argv[])
{
  QApplication app (argc, argv);
  myCanvas bob;
  bob.show();
  return app.exec();
}

To copy to clipboard, switch view to plain text mode

errors with Q_OBJECT uncommented:

Qt Code:

Switch view

1>Linking...
1>connectWorking.obj : error LNK2001: unresolved external symbol "public: virtual struct QMetaObject const * __thiscall myCanvas::metaObject(void)const " (?metaObject@myCanvas@@UBEPBUQMetaObject@@XZ)
1>connectWorking.obj : error LNK2001: unresolved external symbol "public: virtual void * __thiscall myCanvas::qt_metacast(char const *)" (?qt_metacast@myCanvas@@UAEPAXPBD@Z)
1>connectWorking.obj : error LNK2001: unresolved external symbol "public: virtual int __thiscall myCanvas::qt_metacall(enum QMetaObject::Call,int,void * *)" (?qt_metacall@myCanvas@@UAEHW4Call@QMetaObject@@HPAPAX@Z)
1>...\Debug\funcLoadData.exe : fatal error LNK1120: 3 unresolved externals
1>funcLoadData - 4 error(s), 0 warning(s)
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

1>Linking...
1>connectWorking.obj : error LNK2001: unresolved external symbol "public: virtual struct QMetaObject const * __thiscall myCanvas::metaObject(void)const " (?metaObject@myCanvas@@UBEPBUQMetaObject@@XZ)
1>connectWorking.obj : error LNK2001: unresolved external symbol "public: virtual void * __thiscall myCanvas::qt_metacast(char const *)" (?qt_metacast@myCanvas@@UAEPAXPBD@Z)
1>connectWorking.obj : error LNK2001: unresolved external symbol "public: virtual int __thiscall myCanvas::qt_metacall(enum QMetaObject::Call,int,void * *)" (?qt_metacall@myCanvas@@UAEHW4Call@QMetaObject@@HPAPAX@Z)
1>...\Debug\funcLoadData.exe : fatal error LNK1120: 3 unresolved externals
1>funcLoadData - 4 error(s), 0 warning(s)
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

To copy to clipboard, switch view to plain text mode

**Timoteo** · 13th November 2010, 00:58

Looks like you didn't implement your copyText() slot.

**kja** · 13th November 2010, 01:31

Thanks for the fast response.

I'm sorry if this is obvious, but even if i only put

Qt Code:

Switch view

class myCanvas : public QWidget{
  Q_OBJECT
public:
  myCanvas (QWidget * parent = 0);
};

class myCanvas : public QWidget{
  Q_OBJECT
public:
  myCanvas (QWidget * parent = 0);
};

To copy to clipboard, switch view to plain text mode

and don't try to make my own slot, simply using a default slot like clear()

Qt Code:

Switch view

connect(pushbutton, SIGNAL(clicked()), lineedit, SLOT(clear()));

connect(pushbutton, SIGNAL(clicked()), lineedit, SLOT(clear()));

To copy to clipboard, switch view to plain text mode

I still get the same three errors.

Are these errors coming because Q_OBJECT needs me to have my own slots or should it be able to exist in the class without any custom slots?

thanks

**Timoteo** · 13th November 2010, 02:17

Hmm. Is all of your code in connectWorking.cpp? Moc has to see the slots declared in a header (with the Q_OBJECT macro in the class) in order for moc to generate the proper code.

**kja** · 13th November 2010, 17:35

Yes I did have all of my code in connectWorking.cpp. So I moved it like you said and put the class with Q_OBJECT into a header file then included it in my .cpp but I still get those pesky three errors.

header file:

[CODE
#ifndef SLOTS_H
#define SLOTS_H

#include <QtGui>
#include <QApplication>
#include <QVBoxLayout>
#include <QLineEdit>
#include <QPushButton>

class myCanvas : public QWidget{
Q_OBJECT
QLineEdit *lineedit;
QPushButton *pushbutton;
public:
myCanvas (QWidget * parent = 0);
public Q_SLOTS:
void slotButtonClicked();
};

#endif SLOTS_H[/CODE]

.cpp file:

Qt Code:

Switch view

#include "slots.h"
 
myCanvas::myCanvas(QWidget *parent):QWidget(parent){
  lineedit = new QLineEdit(this);
  pushbutton = new QPushButton("Enter", this);
 
  connect(pushbutton, SIGNAL(clicked()), this, SLOT(slotButtonClicked()));
  QVBoxLayout * layout = new QVBoxLayout;
  layout->addWidget(lineedit);
  layout->addWidget(pushbutton);
  setLayout(layout);
}
void myCanvas::slotButtonClicked()
{
  QString theText = lineedit->text();
}
 
int main (int argc, char * argv[])
{
  QApplication app (argc, argv);
  myCanvas bob;
  bob.show();
  return app.exec();
}

#include "slots.h"

myCanvas::myCanvas(QWidget *parent):QWidget(parent){
  lineedit = new QLineEdit(this);
  pushbutton = new QPushButton("Enter", this);
  
  connect(pushbutton, SIGNAL(clicked()), this, SLOT(slotButtonClicked()));
  QVBoxLayout * layout = new QVBoxLayout;
  layout->addWidget(lineedit);
  layout->addWidget(pushbutton);
  setLayout(layout);
}
void myCanvas::slotButtonClicked()
{
  QString theText = lineedit->text();
}

int main (int argc, char * argv[])
{
  QApplication app (argc, argv);
  myCanvas bob;
  bob.show();
  return app.exec();
}

To copy to clipboard, switch view to plain text mode

Some additional information:
So now that I implemented my slot, if I comment out Q_OBJECT I get an error in the output window

"Object::connect: No such slot QWidget::slotButtonClicked() int c:\...\connectworking.cpp:114"

thanks for helping me out.

**tbscope** · 13th November 2010, 17:50

Just use "public slots:" instead of "public Q_SLOTS:"
And clean the project completely and rebuild it.

**kja** · 13th November 2010, 17:54

when I do that I what look like syntax errors:

Qt Code:

Switch view

public slots:
    void slotButtonClicked();

public slots:
    void slotButtonClicked();

To copy to clipboard, switch view to plain text mode

Qt Code:

Switch view

1>------ Rebuild All started: Project: funcLoadData, Configuration: Debug Win32 ------
1>Deleting intermediate and output files for project 'funcLoadData', configuration 'Debug|Win32'
1>Compiling...
1>connectWorking.cpp
1>c:\...\slots.h(16) : error C2146: syntax error : missing ':' before identifier 'slots'
1>c:\...\slots.h(17) : error C2062: type 'void' unexpected
1>c:\...\slots.h(17) : error C2238: unexpected token(s) preceding ';'
1>c:\...\connectworking.cpp(120) : error C2039: 'slotButtonClicked' : is not a member of 'myCanvas'
1>        c:\...slots.h(10) : see declaration of 'myCanvas'
1>c:\...\connectworking.cpp(122) : error C2065: 'lineedit' : undeclared identifier
1>c:\...\connectworking.cpp(122) : error C2227: left of '->text' must point to class/struct/union/generic type
1>        type is ''unknown-type''
1>Build log was saved at "file://c:\...\BuildLog.htm"
1>funcLoadData - 6 error(s), 0 warning(s)
========== Rebuild All: 0 succeeded, 1 failed, 0 skipped ==========

1>------ Rebuild All started: Project: funcLoadData, Configuration: Debug Win32 ------
1>Deleting intermediate and output files for project 'funcLoadData', configuration 'Debug|Win32'
1>Compiling...
1>connectWorking.cpp
1>c:\...\slots.h(16) : error C2146: syntax error : missing ':' before identifier 'slots'
1>c:\...\slots.h(17) : error C2062: type 'void' unexpected
1>c:\...\slots.h(17) : error C2238: unexpected token(s) preceding ';'
1>c:\...\connectworking.cpp(120) : error C2039: 'slotButtonClicked' : is not a member of 'myCanvas'
1>        c:\...slots.h(10) : see declaration of 'myCanvas'
1>c:\...\connectworking.cpp(122) : error C2065: 'lineedit' : undeclared identifier
1>c:\...\connectworking.cpp(122) : error C2227: left of '->text' must point to class/struct/union/generic type
1>        type is ''unknown-type''
1>Build log was saved at "file://c:\...\BuildLog.htm"
1>funcLoadData - 6 error(s), 0 warning(s)
========== Rebuild All: 0 succeeded, 1 failed, 0 skipped ==========

To copy to clipboard, switch view to plain text mode

**Timoteo** · 13th November 2010, 19:13

I just constructed a project out of your code, and it compiles, links and executes as expected. I did make some minor adjustments (some formatting and streamlining the includes for a bit faster build). Here's what I have:

Qt Code:

Switch view

#ifndef SLOTS_H
#define SLOTS_H
 
#include <QWidget>
class QLineEdit;
class QPushButton;
 
class myCanvas : public QWidget
{
    Q_OBJECT
    QLineEdit *lineedit;
    QPushButton *pushbutton;
public:
    myCanvas (QWidget * parent = 0);
public slots:
    void slotButtonClicked();
};
 
 
#endif  //SLOTS_H

#ifndef SLOTS_H
#define SLOTS_H

#include <QWidget>
class QLineEdit;
class QPushButton;

class myCanvas : public QWidget
{
    Q_OBJECT
    QLineEdit *lineedit;
    QPushButton *pushbutton;
public:
    myCanvas (QWidget * parent = 0);
public slots:
    void slotButtonClicked();
};


#endif  //SLOTS_H

To copy to clipboard, switch view to plain text mode

Qt Code:

Switch view

/#include "slots.h"
#include <QPushButton>
#include <QLineEdit>
#include <QVBoxLayout>
#include <QApplication>
myCanvas::myCanvas(QWidget *parent):QWidget(parent){
  lineedit = new QLineEdit(this);
  pushbutton = new QPushButton("Enter", this);
 
  connect(pushbutton, SIGNAL(clicked()), this, SLOT(slotButtonClicked()));
  QVBoxLayout * layout = new QVBoxLayout;
  layout->addWidget(lineedit);
  layout->addWidget(pushbutton);
  setLayout(layout);
}
void myCanvas::slotButtonClicked()
{
  QString theText = lineedit->text();
}
 
int main (int argc, char * argv[])
{
  QApplication app (argc, argv);
  myCanvas bob;
  bob.show();
  return app.exec();
}

/#include "slots.h"
#include <QPushButton>
#include <QLineEdit>
#include <QVBoxLayout>
#include <QApplication>
myCanvas::myCanvas(QWidget *parent):QWidget(parent){
  lineedit = new QLineEdit(this);
  pushbutton = new QPushButton("Enter", this);

  connect(pushbutton, SIGNAL(clicked()), this, SLOT(slotButtonClicked()));
  QVBoxLayout * layout = new QVBoxLayout;
  layout->addWidget(lineedit);
  layout->addWidget(pushbutton);
  setLayout(layout);
}
void myCanvas::slotButtonClicked()
{
  QString theText = lineedit->text();
}

int main (int argc, char * argv[])
{
  QApplication app (argc, argv);
  myCanvas bob;
  bob.show();
  return app.exec();
}

To copy to clipboard, switch view to plain text mode

It worked before and after the alterations. What does your .pro look like?

Qt Code:

Switch view

TEMPLATE = app
CONFIG += qt
QT += gui
 
SOURCES += \
    connect.cpp
 
HEADERS += \
    slots.h

TEMPLATE = app
CONFIG += qt
QT += gui

SOURCES += \
    connect.cpp

HEADERS += \
    slots.h

To copy to clipboard, switch view to plain text mode

**kja** · 15th November 2010, 20:23

my .pro file is the same as yours

Qt Code:

Switch view

TEMPLATE = app
CONFIG += qt
QT += gui
 
SOURCES += \
	connectWorking.cpp
 
HEADERS += \
	slots.h

TEMPLATE = app
CONFIG += qt
QT += gui

SOURCES += \
	connectWorking.cpp

HEADERS += \
	slots.h

To copy to clipboard, switch view to plain text mode

I just don't understand why I get syntax errors when I try to declare public slots...

All of the examples that came with Qt use "public slots:" and Q_OBJECT and they all run fine. Could the problem have something to do with my project settings or something like that?

Thanks for all of your help

**kja** · 16th November 2010, 00:23

Yea! I got it!

the problem was that my .moc file was not being generated because I didn't have the correct properties for my .h file.

I needed to edit the command line and additional dependencies.

Thanks for all your help Timoteo!

Thread: errors when using Q_OBJECT and slots

Thread Tools

Search Thread

Display

errors when using Q_OBJECT and slots

Re: errors when using Q_OBJECT and slots

Re: errors when using Q_OBJECT and slots

Re: errors when using Q_OBJECT and slots

Re: errors when using Q_OBJECT and slots

Re: errors when using Q_OBJECT and slots

Re: errors when using Q_OBJECT and slots

Re: errors when using Q_OBJECT and slots

The following user says thank you to Timoteo for this useful post:

Re: errors when using Q_OBJECT and slots

Re: errors when using Q_OBJECT and slots

Similar Threads

Linker errors with Q_OBJECT

Q_OBJECT macro and link errors

Creating slots with Q_OBJECT

When do I need Q_OBJECT?

Q_object

Tags for this Thread

Bookmarks

Bookmarks

Posting Permissions