SICP ：讨论分层及封装性的又一极好例子。

.h文件

#ifndef WIDGET_H
#define WIDGET_H

#include <QWidget>

QT_BEGIN_NAMESPACE
namespace Ui { class Widget; }
QT_END_NAMESPACE

class Widget : public QWidget
{
    Q_OBJECT

public:
    Widget(QWidget *parent = nullptr);
    ~Widget();


    void Draw_Element(QPainter *p, QPoint point, float height);
    void Draw_Level_One(QPainter *p, QPoint point, float height);
    void Draw_Level_Two(QPainter *p, QPoint point, float height);
    void Draw_Level_Third(QPainter *p, QPoint point, float height);
    void Draw_Level_Four(QPainter *p, QPoint point, float height);
    void Draw_Level_Five(QPainter *p, QPoint point, float height);
private:
    Ui::Widget *ui;

    // QWidget interface
protected:

    virtual void paintEvent(QPaintEvent *event) override;

};
#endif // WIDGET_H

.cpp文件

#include "widget.h"
#include "ui_widget.h"

#include <QPaintEvent>
#include <QPainter>
#include <QPoint>
#include <QtMath>


Widget::Widget(QWidget *parent)
    : QWidget(parent)
    , ui(new Ui::Widget)
{
    ui->setupUi(this);
}

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

void Widget::Draw_Element(QPainter* p,QPoint point,float height)
{
    // first point
    QPoint point_1 = point;

    // second point
    QPoint point_2 = QPoint(point_1.x() - (2 * height) / qSqrt(3),point_1.y() + height);

    // third point
    QPoint point_3 = QPoint(point_1.x() + (2 * height) / qSqrt(3),point_1.y() + height);

    p->drawLine(point_1,point_2);
    p->drawLine(point_2,point_3);
    p->drawLine(point_3,point_1);
}

void Widget::Draw_Level_One(QPainter* p,QPoint point,float height)
{
    // draw around triangle
    Draw_Element(p,point,height);

    // draw internal triangle
    QPoint point_1 = QPoint(point.x() - height / qSqrt(3),point.y() + height / 2);

    QPoint point_2 = QPoint(point.x() + height / qSqrt(3),point.y() + height / 2);

    QPoint point_3 = QPoint(point.x(),point.y() + height);

    p->drawLine(point_1,point_2);
    p->drawLine(point_2,point_3);
    p->drawLine(point_3,point_2);
    p->drawLine(point_3,point_1);

}

void Widget::Draw_Level_Two(QPainter* p,QPoint point,float height)
{
    // draw around triangle
    Draw_Element(p,point,height);

    // draw internal triangle
    QPoint point_1 = QPoint(point.x() - height / qSqrt(3),point.y() + height / 2);

    QPoint point_2 = QPoint(point.x() + height / qSqrt(3),point.y() + height / 2);

    QPoint point_3 = QPoint(point.x(),point.y() + height);

    // first triangle
    Draw_Level_One(p,point,height / 2);

    // second triangle
    Draw_Level_One(p,point_1,height / 2);

    // third triangle
    Draw_Level_One(p,point_2,height / 2);

}

void Widget::Draw_Level_Third(QPainter* p,QPoint point,float height)
{
    // draw around triangle
    Draw_Element(p,point,height);

    // draw internal triangle
    QPoint point_1 = QPoint(point.x() - height / qSqrt(3),point.y() + height / 2);

    QPoint point_2 = QPoint(point.x() + height / qSqrt(3),point.y() + height / 2);

    QPoint point_3 = QPoint(point.x(),point.y() + height);

    // first triangle
    Draw_Level_Two(p,point,height / 2);

    // second triangle
    Draw_Level_Two(p,point_1,height / 2);

    // third triangle
    Draw_Level_Two(p,point_2,height / 2);
}

void Widget::Draw_Level_Four(QPainter* p,QPoint point,float height)
{
    // draw around triangle
    Draw_Element(p,point,height);

    // draw internal triangle
    QPoint point_1 = QPoint(point.x() - height / qSqrt(3),point.y() + height / 2);

    QPoint point_2 = QPoint(point.x() + height / qSqrt(3),point.y() + height / 2);

    QPoint point_3 = QPoint(point.x(),point.y() + height);

    // first triangle
    Draw_Level_Third(p,point,height / 2);

    // second triangle
    Draw_Level_Third(p,point_1,height / 2);

    // third triangle
    Draw_Level_Third(p,point_2,height / 2);
}

void Widget::Draw_Level_Five(QPainter* p,QPoint point,float height)
{
    // draw around triangle
    Draw_Element(p,point,height);

    // draw internal triangle
    QPoint point_1 = QPoint(point.x() - height / qSqrt(3),point.y() + height / 2);

    QPoint point_2 = QPoint(point.x() + height / qSqrt(3),point.y() + height / 2);

    QPoint point_3 = QPoint(point.x(),point.y() + height);

    // first triangle
    Draw_Level_Four(p,point,height / 2);

    // second triangle
    Draw_Level_Four(p,point_1,height / 2);

    // third triangle
    Draw_Level_Four(p,point_2,height / 2);
}
void Widget::paintEvent(QPaintEvent *event)
{
    /*
     * left: 10px element 10px level1 10px level2
     * */

    QPainter painter(this);
    painter.setPen(Qt::blue);
    painter.setRenderHint(QPainter::Antialiasing, true);

    float element_hight = 100;
    int init_point_x = 120;
    float init_point_y = 40;

    // element
    Draw_Element(&painter,QPoint(init_point_x,init_point_y),element_hight);

    // level 1
    init_point_x = 360;
    Draw_Level_One(&painter,QPoint(init_point_x,init_point_y),element_hight);

    // level 2
    init_point_x = 600;
    Draw_Level_Two(&painter,QPoint(init_point_x,init_point_y),element_hight);

    // level 3
    init_point_x = 850;
    Draw_Level_Third(&painter,QPoint(init_point_x,init_point_y),element_hight);


    // level 4
    init_point_x = 1100;
    Draw_Level_Four(&painter,QPoint(init_point_x,init_point_y),element_hight);

    // level 5
    init_point_x = 1350;
    Draw_Level_Five(&painter,QPoint(init_point_x,init_point_y),element_hight);
}