undefined reference to function send - c++

Question

I'm a very beginnner at C++ /Qt programming. I've made this simple dialog box that check the QLineEdit, if the text entered is "bob" should enable the OK button.
I can't get it to compile successfully, it gives me:
dialog.cpp|31|undefined reference to `Dialogmio::send()'
What am I doing wrong?
This is dialog.h:
//dialog.h
#ifndef DIALOG_H_INCLUDED
#define DIALOG_H_INCLUDED
#include <QDialog>
class QPushButton;
class QLineEdit;
class Dialogmio : public QWidget
{
public:
Dialogmio(QWidget *parent =0);
signals:
void send ();
public slots:
void recip(QString &text);
private:
QLineEdit *linedit;
QPushButton *buttonOK;
};
#endif
This is dialog.cpp:
//dialog.cpp
#include <QtGui>
#include "dialog.h"
Dialogmio::Dialogmio(QWidget *parent)
: QWidget(parent)
{
linedit = new QLineEdit();
buttonOK = new QPushButton("OK");
buttonOK->setEnabled(FALSE);
connect( linedit, SIGNAL( textChanged(const QString &) ), this, SLOT( recip(const QString &) ));
connect (this,SIGNAL( send()), this, SLOT( buttonOK->setEnabled(true)) );
QHBoxLayout *layout = new QHBoxLayout();
layout->addWidget(linedit);
layout->addWidget(buttonOK);
setLayout(layout);
}
void Dialogmio::recip(QString &text)
{
QString a = linedit->text();
if (a == "bob"){
emit send(); //here it gives me the error
}
}
This is main.cpp:
#include <QApplication>
#include "dialog.h"
int main(int argc, char* argv[])
{
QApplication app(argc, argv);
Dialogmio *dialog = new Dialogmio;
dialog->show();
return app.exec();
}
I've inserted the Q_OBJECT macro as suggested, now I get one more errors on line 7:
dialog.cpp|7|undefined reference to `vtable for Dialogmio'|

Answer 1

You start by including the Qt file for QDialog, but then go on to inherit from QWidget. While inheriting from QWidget is not a problem, was your intention to actually inherit from QDialog(?), in which case you should define your class this way: -
class Dialogmio : public QDialog
{
Q_OBJECT
public:
Dialog(QWidget* parent);
private slots:
void aSlotFunction();
}
The signals and slots mechanism are C++ extensions that are unique to Qt and in order for a class to use it, the class must include the Q_OBJECT macro, which adds all the necessary functionality. During the build stages, Qt parses the header and creates the code required for the extensions, including run-time type information, the dynamic property system and of-course the signals and slots.
As you state you're using codeblocks as the IDE, if it doesn't automatically run qmake before building, you'll need to do that whenever you add any signals or slots to a class in order for the moc (meta-object-compiler) to see them.
Another thing is that the call to connect signals and slots is wrong: -
connect (this,SIGNAL( send()), this, SLOT( buttonOK->setEnabled(true)) );
The parameter in the SLOT macro takes a slot function, so you need to create a slot and connect it to the send signal: -
connect(this, SIGNAL(send()), this, SLOT(aSlotFunction());
Inside the aSlotFunction, you can then call the set enabled for the button: -
void Dialogmio::aSlotFunction()
{
buttonOK->setEnabled(true);
}
If you're using Qt 5, there's an easier syntax of handling the connection: -
connect(this, &Dialogmio::send, this, &Dialogmio::aSlotFunction);
As this syntax accepts pointers to the functions that will be called, they don't actually have to be declared as slots to work. In addition, you do not provide the arguments, so if they change, you won't have to update the connect calls too.