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• F:\projects\Qt_project\5.0.2\games\math_strategy\build-math_strategy-Desktop_Qt_5_12_12_MinGW_64_bit-Debug\debug\moc_Square.cpp:87: ошибка: 'qt_metacast' is not a member of 'QPainter'
  debug\moc_Square.cpp: In member function 'virtual void* Square::qt_metacast(const char*)':
  debug\moc_Square.cpp:87:22: error: 'qt_metacast' is not a member of 'QPainter'
       return QPainter::qt_metacast(_clname);
                        ^~~~~~~~~~~
• F:\projects\Qt_project\5.0.2\games\math_strategy\build-math_strategy-Desktop_Qt_5_12_12_MinGW_64_bit-Debug\debug\moc_Square.cpp:92: ошибка: 'qt_metacall' is not a member of 'QPainter'
  debug\moc_Square.cpp: In member function 'virtual int Square::qt_metacall(QMetaObject::Call, int, void**)':
  debug\moc_Square.cpp:92:21: error: 'qt_metacall' is not a member of 'QPainter'
       _id = QPainter::qt_metacall(_c, _id, _a);
                       ^~~~~~~~~~~
• :-1: ошибка: [Makefile.Debug:8206: debug/moc_Square.o] Error 1

#ifndef SQUARE_H
#define SQUARE_H

#include <QPainter>
#include <QObject>
class QPainter;

class Square : public QPainter, QObject
{
    Q_OBJECT
public:
   Square(QObject* parent = nullptr);
   void drawSquare(QPoint point,QPoint point2,QPoint point3,QPoint point4);
};

#endif // SQUARE_H

#include "Square.h"

Square::Square(QObject* parent) : QPainter(), QObject(parent)
{

}

void Square::drawSquare(QPoint point, QPoint point2, QPoint point3, QPoint point4)
{
    drawPolygon(QPolygon()<< point << point2 << point3 << point4);

}

C++ (Qt)
class Square
    : public QObject // всегда первый
    , public QPainter
{
    Q_OBJECT
public:
   Square(QObject* parent = nullptr);
   void drawSquare(QPoint point,QPoint point2,QPoint point3,QPoint point4);
};