I tried putting it in a an empty function but it doesnt work.
Here is my code for reference. Thank you and looking forward for a discussion.

//paint_circle.h

//paint_circle.h
#ifndef PAINT_CIRCLE_H
#define PAINT_CIRCLE_H

#include <QMainWindow>
#include <QPainter>
#include <QPaintEvent>

namespace Ui {
class paint_circle;
}

class paint_circle : public QMainWindow
{
    Q_OBJECT

public:
    explicit paint_circle(QWidget *parent = 0);
    ~paint_circle();

    QSize sizeHint() const
    {
        return rect.size();
    }

protected:
    void paintEvent(QPaintEvent* event);

private:
    Ui::paint_circle *ui;
    QRect rect;

    void paint_test();

};

#endif // PAINT_CIRCLE_H

//paint_circle.cpp

//paint_circle.cpp
#include "paint_circle.h"
#include "ui_paint_circle.h"

paint_circle::paint_circle(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::paint_circle)
{
    ui->setupUi(this);
    rect = QRect(0, 0, this->width(), this->height());

    paint_test();
}

paint_circle::~paint_circle()
{
    delete ui;
}

void paint_circle::paint_test(){
    //I WANT TO TRANSFER LAYER 1 BACKGROUND DRAWING HERE.
}

void paint_circle::paintEvent(QPaintEvent* event)
{
    //BACKGROUND DRAWING (LAYER 1) 
    QRectF rectangle(0, 0, this->width(), this->height());
    QPainter b_color(this);
    b_color.setPen(QPen(Qt::white,0 ));
    b_color.fillRect(event->rect(), Qt::transparent);
    b_color.setBrush(QColor(Qt::red));
    b_color.drawRoundedRect(rectangle, 20.0, 10.0);

    //TO REMAIN HERE (LAYER 2)
    QPainter p(this);
    p.setRenderHint(QPainter::Antialiasing, true);
    p.setPen(QPen(Qt::white, 0));
    p.setBrush(Qt::black);
    p.drawEllipse(QRect((this->width()/2)-15,(this->height()/2)-15,30,30));
    p.rotate(90); // some random rotation number.
}