Invalid use of 'this' in non-member - c++

Question

I have a problem - discussed a lot previously, but all solutions I saw doesn't work here.
What is wrong in this code?
main.cpp:8:19: error: invalid use of ‘this’ in non-member function
#include <QApplication>
#include <QPainter>
#include <math.h>
class QPainter;
int main(int argc, char *argv[]){
QApplication app(argc,argv);
QPainter painter(this);
painter.setPen(QPen(Qt::black,3));
int n=8;
for (int i=0;i<n;++i){
qreal fAngle=2*3.14*i/n;
qreal x = 50 + cos(fAngle)*40;
qreal y = 50 + sin(fAngle)*40;
painter.drawPoint(QPointF(x,y));
}
return app.exec();
}

Answer 1

In the function "main" you are using the reserved keyword "this" which means "address of the current object". main is a function and not a member of any class or object that would have a "this" variable. I know nothing about qt but 5 seconds with google tells me that "QPainter" wants the address of a QPaintDevice.
http://qt-project.org/doc/qt-5.0/qtgui/qpainter.html#QPainter

Answer 2

If you are talking about QPainter painter(this); you are using the this pointer in main. The this pointer is for member functions of an object. Your main function doesn't have a this pointer to use.

Answer 3

In Qt you need to create a window (inheriting from QWidget) before you can see anything on the screen. You then create a QPainter object inside the paintEvent method of your window and there you will have the proper this to pass to QPainter constructor.
You're highly encouraged to follow some tutorial. Qt is fantastic, but it's huge and it's not a library, but a framework (like most other GUIs).
Your code must be structured in a quite specific way.

Answer 4

You just tried to copy some code without understanding what was going on.
It looks like you saw a code like this :
#include <QtGui>
#include <QWidget>
class MyWidget : public QWidget
{
Q_OBJECT
public:
protected:
void paintEvent(QPaintEvent *event)
{
//create a QPainter and pass a pointer to the device.
//A paint device can be a QWidget, a QPixmap or a QImage
QPainter painter(this); // <- Look at this !!
// ^^^^ Allowed
// ...
}
signals:
public slots:
};
int main( int argc, char **argv )
{
QApplication app( argc, argv );
MyWidget myWidget;
myWidget.show();
return app.exec();
}
Here you can see that in the function paintEvent of the class MyWidget we use the this pointer. We can because we use it in a non-static member of the class. Here this is of type MyWidget*.
Look at the standard :
9.3.2 The this pointer [class.this]
In the body of a non-static member function, the keyword this is a prvalue expression whose value is the address of the object for which the function is called. The type of this in a member function of a class X is X*.
So you cannot use the this pointer inside the main function, this is no sense.
And other thing, like it is said in the comment inside the code, you just have to pass a pointer to a device to the QPainter constructor. It can be a QWidget, a QPixmap or a QImage.
Maybe you should read this for a beginning : http://www.cplusplus.com/doc/tutorial/classes/