Why cannot write a QWidget class in the main.cpp? - c++

Question

Update:
If I delete Q_OBJECT, and I do not use SLOT SINGAL, just use connect() like this:
connect(this, &QWidget::destroyed, this, &QWidget::myslot),
my code will run well without any warnings and errors.
I want to write a little code to instruct some classes, so I try to simplify my code. But I encountered some strange things. I cannot write a simple widget in my main.cpp. If I write the widget in mywidget.cpp and mywidget.h, the program runs well. If I want to write the widget in the main.cpp, what should I do?
This is my code.
#include <QApplication>
#include <QWidget>
class Widget : public QWidget
{
Q_OBJECT
public:
Widget(QWidget *parent = 0);
~Widget();
};
Widget::Widget(QWidget *parent)
: QWidget(parent)
{
}
Widget::~Widget()
{
}
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
Widget w;
w.show();
return a.exec();
}
And the error information is:

Answer 1

Add following line to the end of main.cpp and rerun qmake:
#include "main.moc"
That will invoke moc tool for your main.cpp. It generates meta-object function definitions for your Widget class resolving your linker errors when you rebuild.

Answer 2

As stated in documentation:
Whenever qmake is run, it parses the project's header files and
generates make rules to invoke moc for those files that contain a
Q_OBJECT macro.
Thus, put the class declaration in a header file (e.g. widget.h) and the class definition in a source file with the same name (e.g. widget.cpp).

Answer 3

If you still want to make it work with signals you can remove the Q_OBJECT macro and use the QObject::connect()
for example:
#include <QApplication>
#include <QWidget>
#include <iostream>
class Widget : public QWidget
{
//Q_OBJECT
public:
Widget(QWidget *parent = 0);
~Widget();
};
Widget::Widget(QWidget *parent) : QWidget(parent)
{
// say bey
QObject::connect(this,&Widget::destroyed,
[](){std::cout<<"bye"<<std::endl;});
}
Widget::~Widget()
{
}
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
Widget w;
w.show();
return a.exec();
}
I test it on Qt5.9.4