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

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:

I would say it's an issue with the MOC: it only runs on header files. But the Q_OBJECT macro needs the MOC to work. If you remove the Q_OBJECT macro, everything will be fine — Xatyrian
– Xatyrian, Commented Apr 11, 2018 at 9:04
Yes. remove Q_OBJECT, the code runs well. But if I remove it, I cannot use the connect(), which is a useful function I need — JosanSun
– JosanSun, Commented Apr 11, 2018 at 9:07
Declarations should be in the .hpp or .h file, not in the .cpp. — Michael Schm.
– Michael Schm., Commented Apr 11, 2018 at 9:23

talamaki · Accepted Answer · 2018-04-11 09:18:58Z

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.

p-a-o-l-o · Accepted Answer · 2018-04-11 09:14:10Z

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).

Simon · Accepted Answer · 2018-04-11 09:28:09Z

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

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