C++ 类的继承和多态

文章目录

一、实验内容
二、实验代码
- 1.继承访问权限测试
- 2.友元类继承测试
- 3.多态的综合应用
- - (1)CShape.h
  - (2)main.cpp
  - (3)CShape.cpp
  - (4)运行截图
四、总结
- 1.继承访问权限测试
- 2.继承访问权限测试
- 3.多态的综合应用

一、实验内容

1、继承访问权限测试
设计类A具有public, protected, private等不同属性的成员函数或变量；
类B通过public, protected, private等不同方式继承A，在类B的成员函数中测试访问A的成员函数或变量；
在类B中添加public, protected, private等不同属性的成员函数或变量，在外部测试访问B的各个成员函数或变量；
B以private方式继承A，尝试把A中的部分public成员提升为public。
2、友元类继承测试
设计类A含有私有变量a，在类A中友元给类C；
设计类B继承A，添加私有变量b；在类C中测试访问类B的成员变量a, b;
设计类D继承C，在D的成员函数中测试访问类A的成员变量a，类B的成员变量a, b。
3、多态性综合运用
一般多态性函数：输入输出参数完全一样，在父类中添加virtual；
特殊多态性函数：输入或输出参数在子类中是父类的指针或基类的引用，在子类中对于的是子类的指针或子类的引用；
析构函数的多态性；
多继承，注意什么情况需要虚继承；
设计矢量图，运用多继承设计组合图形，要求具备创建不同类型矢量图、选择图形、移动图形、用不同颜色显示图形（表示选中与否），用vector或数组管理图形。

二、实验代码 1.继承访问权限测试

代码如下（示例）：

#include
using namespace std;
class A
{
public:
	int _a;
protected:
	int _b;
private:
	int _c;
};
class Pub_B :public A
{
public:
	void Pub_Test()
	{
		_a = 10;
		_b = 20;
		//_c不可访问；
	}
	int Pub_pub_b1;
protected:
	int Pub_pro_b1;
private:
	int Pub_pri_b1;
};
class Pro_B :protected A
{
public:
	void Pro_Test()
	{
		_a = 10;
		_b = 20;
		//_c不可访问；
	}
	int Pro_pub_b2;
protected:
	int Pro_pro_b2;
private:
	int Pro_pri_b2;
};
class Pri_B :private A
{
public:
	void Pri_Test()
	{
		_a = 10;
		_b = 20;
		//_c不可访问；
	}
	int Pri_pub_b3;
protected:
	int Pri_pro_b3;
private:
	int Pri_pri_b3;
};
class New_Pri_B :private A
{
public:
	void New_Pri_Test()
	{
		_a = 10;
		_b = 20;
	}
	int npri_pub_b;
	using A::_a;
protected:
	int npri_pro_b;
private:
	int npri_pri_b;
};
void main()
{
	A a;
	a._a;
	//a._b,a._c不可访问
	Pub_B s1;
	s1.Pub_pub_b1;
	s1._a;
	//s1._b;
	//si._c;
	//s1.pub_pro_b2;不可访问
	//s1.pub_pri_b3;不可访问
	Pro_B s2;
	s2.Pro_pub_b2;
	//s2._a;s2._b;s2._c;
	//s2.pro_pro_b2;不可访问
	//s2.pro_pri_b3;不可访问
	Pri_B s3;
	s3.Pri_pub_b3;
	//s3._a;s3._b;s3._c;
	//s3.pri_pro_b2;不可访问
	//s3.pri_pri_b3;不可访问
	New_Pri_B s4;
	s4.npri_pub_b;
	s4._a;
	//s4.pri_pro_b2;
	//s4._b;
}

2.友元类继承测试

代码如下（示例）：

#include
using namespace std;
class A
{
	private: 
		int a=100;
		friend class C;
};
class B :public A
{
	private:
		int b=10;
};
class C
{
	public:
		void test()
		{
			B b1;
			b1.a;
			//b1.b不可访问
		}
};
class D :public C
{
	public:
		void test()
		{
			A a1;
			//a1.a;不可访问
			B b1;
			//b1.a;不可访问
			//b1.b;不可访问
		}
};

3.多态的综合应用 (1)CShape.h

代码如下（示例）：

#ifndef CSHAPE_H
#define CSHAPE_H
#include
#include
#include
using namespace std;

class CPoint;
class CRect;
class CShape
{
public:
	CShape();
	CShape(const CShape& shape);
	virtual ~CShape();
	virtual double GetArea() const;
	virtual bool ptIn(const CPoint& pt) const;
	virtual bool InRect(const CRect& rc) const;
	virtual void Draw() const;
	virtual void DrawColor();
	virtual CShape* Clone() const;
	virtual CShape& Move(int nOffsetX, int nOffsetY);

protected:
	string m_sName;
};

class CPoint :public CShape {
public:
	int m_nPosX;
	int m_nPosY;
	CPoint() {
		m_nPosX = 0;
		m_nPosY = 0;
	}
	CPoint(int nPosX, int nPosY);
	CPoint(const CPoint& pt);
	virtual ~CPoint();
	double GetArea() const;
	bool ptIn(const CPoint& pt) const;
	bool InRect(const CRect& rc) const;
	void Draw() const;
	void DrawColor();
	CPoint* Clone() const;
	CPoint& Move(int nOffsetX, int nOffsetY);
};
class CTriangle :virtual public CShape {
public:
	CTriangle() {}
	CTriangle(const CPoint& pt1, const CPoint& pt2, const CPoint& pt3);
	CTriangle(const CTriangle& rc);
	CTriangle(const CPoint& pt);
	virtual ~CTriangle();
	double GetArea() const;
	bool ptIn(const CPoint& pt) const;
	bool InRect(const CRect& rc) const;
	void Draw() const;
	void DrawColor();
	CShape* Clone() const;
	CShape& Move(int nOffsetX, int nOffsetY);
	CPoint m_pts[3];
};

class CRect :virtual public CShape {
public:
	CRect() {}
	CRect(CPoint pt1, CPoint pt2);
	CRect(const CRect& rc);
	CRect(CPoint pt1);
	virtual ~CRect();
	double GetArea() const;
	bool ptIn(const CPoint& pt) const;
	bool InRect(const CRect& rc) const;
	void Draw() const;
	void DrawColor();
	CShape* Clone() const;
	CShape& Move(int nOffsetX, int nOffsetY);
	CPoint m_ptLT;
	CPoint m_ptBR;
};

class Comgraphics :public CRect, public CTriangle {
public:
	Comgraphics(const CRect& pt1);
	Comgraphics(const Comgraphics& rc);
	Comgraphics(const CPoint pt1);
	virtual ~Comgraphics();
	double GetArea() const;
	bool ptIn(const CPoint& pt) const;
	bool InRect(const CRect& rc) const;
	void Draw() const;
	void DrawColor();
	CShape* Clone() const;
	CShape& Move(int nOffsetX, int nOffsetY);
	CPoint m_pt1;
	CPoint m_pt2;

};
#endif

(2)main.cpp

代码如下（示例）：

#include
#include "graphics.h"
#include
#include "CShape.h"
using namespace std;

int main()
{
	//图形画布基础设置
	initgraph(640, 480);
	setbkcolor(WHITE);
	delay_ms(0);
	setcolor(BLACK);
	setfont(20, 0, "楷体");
	setbkmode(TRANSPARENT);
	//enter+左击-->新建矩形");
	//enter+右击-->新建三角形");
	//enter+滚轮中间-->新建组合图形

	//ctrl+左击-->复制图形");
	//ctrl+右击-->粘贴图形");

	vectorshapes;
	vectorshapestmp;

	shapes.push_back(new CTriangle(CPoint(320, 320), CPoint(250, 340), CPoint(340, 450)));
	//shapes.push_back(new CTriangle(CPoint(10, 10), CPoint(150, 10), CPoint(150, 150)));
	shapes.push_back(new CRect(CPoint(200, 200), CPoint(300, 300)));
	shapes.push_back(new Comgraphics(CRect(CPoint(250, 50))));


	//移动
	bool move_flag = false;
	bool copy_flag = false;
	bool redraw = true;
	//鼠标点击时记录它的坐标
	int clickX, clickY;
	int copyX, copyY;
	int checkedid = -1;
	int copyid = -1;

	for (; is_run(); delay_fps(60)) {
		while (mousemsg()) {
			mouse_msg msg = getmouse();

			//判断鼠标的移动
			if (msg.is_move()) {
				if (checkedid != -1) {
					if (move_flag) {
						shapes[checkedid]->Move(msg.x - clickX, msg.y - clickY);
					}
				}
				clickX = msg.x;
				clickY = msg.y;
				redraw = true;
			}

			// 判断鼠标左键
			else if (msg.is_left()) {
				// 判断鼠标左键是否按下
				if (msg.is_down()) {
					clickX = msg.x;
					clickY = msg.y;

					CPoint pt = CPoint(clickX, clickY);
					int isIn = 0;
					for (int i = 0; i < shapes.size(); i++) {
						if (shapes[i]->ptIn(pt)) {
							isIn = 1;
							//如果鼠标在图形区域内就设置移动的flag为true
							move_flag = true;
							checkedid = i;
							redraw = true;
							break;
						}
					}
					if (isIn == 0)
						checkedid = -1;
				}
				else {
					move_flag = false;
				}
			}
		}
		// 重新绘图
		if (redraw) {
			redraw = false;
			cleardevice();
			for (int i = 0; i < shapes.size(); i++) {
				if (i == checkedid)
					shapes[i]->DrawColor();
				else
					shapes[i]->Draw();
			}
		}

		while (kbmsg()) {
			key_msg msgk = getkey();
			if (msgk.key == key_enter && msgk.msg == key_msg_down) {
				mouse_msg msgm = getmouse();
				if (msgm.is_left()) {
					// 判断鼠标左键是否按下
					if (msgm.is_down()) {
						shapes.push_back(new CRect(CPoint(msgm.x, msgm.y)));
						redraw = true;
					}
				}
				if (msgm.is_right()) {
					// 判断鼠标右键是否按下
					if (msgm.is_down()) {
						shapes.push_back(new CTriangle(CPoint(msgm.x, msgm.y)));
						redraw = true;
					}
				}
				if (msgm.is_mid()) {
					CRect r1 = CRect(CPoint(msgm.x, msgm.y));
					// 判断鼠标中键是否按下
					if (msgm.is_down()) {
						shapes.push_back(new Comgraphics(r1));
						redraw = true;
					}
				}
			}
			if (msgk.key == key_control && msgk.msg == key_msg_down) {
				mouse_msg msgm = getmouse();
				if (msgm.is_left()) {
					// 判断鼠标左键是否按下
					if (msgm.is_down()) {
						copyX = msgm.x;
						copyY = msgm.y;
						CPoint pt = CPoint(copyX, copyY);
						for (int i = 0; i < shapes.size(); i++) {
							if (shapes[i]->ptIn(pt)) {
								//如果鼠标在图形区域内就设置移动的flag为true
								copy_flag = true;
								copyid = i;
								break;
							}
						}
					}
				}
				if (msgm.is_right()) {
					// 判断鼠标右键是否按下
					if (msgm.is_down()) {
						if (copy_flag == true) {
							shapes.push_back(&(shapes[copyid]->Clone())->Move(msgm.x - copyX, msgm.y - copyY));
							redraw = true;
						}
					}
				}

			}
		}
	}
	closegraph();
	return 0;
}

(3)CShape.cpp

代码如下（示例）：

#include "CShape.h"
#include "graphics.h"
#include 
using namespace std;
//CShape
CShape::CShape()
{
}
CShape::CShape(const CShape& shape) {
	m_sName = shape.m_sName;
}
CShape::~CShape()
{
}
double CShape::GetArea() const {
	return 0;
}
bool CShape::ptIn(const CPoint& pt) const {
	return false;
}
bool CShape::InRect(const CRect& rc) const {
	return false;
}
void CShape::Draw() const
{
}
void CShape::DrawColor()
{
}
CShape* CShape::Clone() const {
	return new CShape(*this);
}
CShape& CShape::Move(int nOffsetX, int nOffsetY) {
	return *this;
}

//CPoint
CPoint::CPoint(int nPosX, int nPosY) {
	m_nPosX = nPosX;
	m_nPosY = nPosY;
}
CPoint::CPoint(const CPoint& pt) {
	m_nPosX = pt.m_nPosX;
	m_nPosY = pt.m_nPosY;
}
CPoint::~CPoint() {
	//cout << "CPoint::~CPoint()n";
}
double CPoint::GetArea() const {
	return 0;
}
bool CPoint::ptIn(const CPoint& pt) const {
	return false;
}
bool CPoint::InRect(const CRect& rc) const {
	return rc.ptIn(*this);
}
void CPoint::Draw() const {
	circle(m_nPosX, m_nPosY, 2);
}
void CPoint::DrawColor()
{
}
CPoint* CPoint::Clone() const {
	return new CPoint(*this);
}
CPoint& CPoint::Move(int nOffsetX, int nOffsetY) {
	m_nPosX += nOffsetX;
	m_nPosY += nOffsetY;
	return *this;
}

//CTriangle
CTriangle::CTriangle(const CTriangle& tri) {
	for (int i = 0; i < 3; i++) {
		m_pts[i] = tri.m_pts[i];
	}
}
CTriangle::~CTriangle() {
	//cout << "CTriangle::~CTriangle()n";
}
CTriangle::CTriangle(const CPoint& pt1, const CPoint& pt2, const CPoint& pt3) {
	m_pts[0] = pt1;
	m_pts[1] = pt2;
	m_pts[2] = pt3;
}
CTriangle::CTriangle(const CPoint& pt)
{
	CPoint* pt1 = new CPoint(pt.m_nPosX + 100, pt.m_nPosY + 90);
	CPoint* pt2 = new CPoint(pt.m_nPosX, pt.m_nPosY + 90);
	m_pts[0] = pt;
	m_pts[1] = *pt1;
	m_pts[2] = *pt2;
}

CShape& CTriangle::Move(int nOffsetX, int nOffsetY) {
	for (int i = 0; i < 3; i++) {
		m_pts[i].Move(nOffsetX, nOffsetY);
	}
	return *this;
}
double CTriangle::GetArea() const {
	int x1, y1, x2, y2, x3, y3;
	x1 = m_pts[0].m_nPosX;
	y1 = m_pts[0].m_nPosY;
	x2 = m_pts[1].m_nPosX;
	y2 = m_pts[1].m_nPosY;
	x3 = m_pts[2].m_nPosX;
	y3 = m_pts[2].m_nPosY;

	double bottomLine = sqrt(pow(x1 - x2, 2) + pow(y1 - y2, 2));
	double verticalLine1 = abs((y1 - y2) * x3 - (x1 - x2) * y3 + (x1 - x2) * y2 - (y1 - y2) * x2);
	double verticalLine2 = sqrt(pow(y1 - y2, 2) + pow(x1 - x2, 2));
	double verticalLine = verticalLine1 / verticalLine2;

	return (verticalLine * bottomLine) / 2.0;
}
bool CTriangle::ptIn(const CPoint& pt) const {
	CTriangle c1 = CTriangle(m_pts[0], m_pts[1], pt);
	CTriangle c2 = CTriangle(m_pts[1], m_pts[2], pt);
	CTriangle c3 = CTriangle(m_pts[2], m_pts[0], pt);

	double totalArea = c1.GetArea() + c2.GetArea() + c3.GetArea();

	if (totalArea == this->GetArea())
		return true;
	else
		return false;
}
bool CTriangle::InRect(const CRect& rc) const {
	return rc.ptIn(m_pts[0]) && rc.ptIn(m_pts[1]) && rc.ptIn(m_pts[2]);
}
void CTriangle::Draw() const {
	int poly[8] = { m_pts[0].m_nPosX ,m_pts[0].m_nPosY,m_pts[1].m_nPosX,m_pts[1].m_nPosY,
				m_pts[2].m_nPosX,m_pts[2].m_nPosY, m_pts[0].m_nPosX ,m_pts[0].m_nPosY };
	setfillcolor(EGERGB(0xFF, 0xFF, 0xFF));
	fillpoly(4, poly);
}
void CTriangle::DrawColor() {
	int poly[8] = { m_pts[0].m_nPosX ,m_pts[0].m_nPosY,m_pts[1].m_nPosX,m_pts[1].m_nPosY,
				m_pts[2].m_nPosX,m_pts[2].m_nPosY, m_pts[0].m_nPosX ,m_pts[0].m_nPosY };
	setfillcolor(EGERGB(0xFF, 0xA5, 0x00));
	fillpoly(4, poly);
}
CShape* CTriangle::Clone() const {
	return new CTriangle(*this);
}

//CRect
CRect::CRect(CPoint pt1, CPoint pt2) {
	m_ptLT = CPoint(min(pt1.m_nPosX, pt2.m_nPosX), min(pt1.m_nPosY, pt2.m_nPosY));
	m_ptBR = CPoint(max(pt1.m_nPosX, pt2.m_nPosX), max(pt1.m_nPosY, pt2.m_nPosY));
}
CRect::CRect(const CRect& rc) {
	m_ptLT = rc.m_ptLT;
	m_ptBR = rc.m_ptBR;
}
CRect::CRect(CPoint pt1)
{
	m_ptLT = CPoint(pt1.m_nPosX, pt1.m_nPosY);
	m_ptBR = CPoint(pt1.m_nPosX + 100, pt1.m_nPosY + 100);
}
CRect::~CRect() {
	// cout << "CRect::CRect()n";
}
double CRect::GetArea() const {
	return (m_ptBR.m_nPosX - m_ptLT.m_nPosX) * (m_ptBR.m_nPosY - m_ptLT.m_nPosY);
}
bool CRect::ptIn(const CPoint& pt) const {
	return (pt.m_nPosX >= m_ptLT.m_nPosX && pt.m_nPosX <= m_ptBR.m_nPosX) &&
		(pt.m_nPosY >= m_ptLT.m_nPosY && pt.m_nPosY <= m_ptBR.m_nPosY);
}
bool CRect::InRect(const CRect& rc) const {
	return rc.ptIn(m_ptLT) && rc.ptIn(m_ptBR);
}
void CRect::Draw() const {
	// 存储n个顶点的x,y坐标
	int pts[10] = { m_ptLT.m_nPosX,m_ptLT.m_nPosY,m_ptBR.m_nPosX,m_ptLT.m_nPosY,
	m_ptBR.m_nPosX,m_ptBR.m_nPosY,m_ptLT.m_nPosX,m_ptBR.m_nPosY,m_ptLT.m_nPosX,m_ptLT.m_nPosY };
	// 绘制n个顶点的多边形，第一个参数必须要传入n+1，pts最后一个顶点坐标和第一个相同
	//drawpoly(5, pts);
	setfillcolor(EGERGB(0xFF, 0xFF, 0xFF));
	fillpoly(5, pts);
}
void CRect::DrawColor() {
	int pts[10] = { m_ptLT.m_nPosX,m_ptLT.m_nPosY,m_ptBR.m_nPosX,m_ptLT.m_nPosY,
	m_ptBR.m_nPosX,m_ptBR.m_nPosY,m_ptLT.m_nPosX,m_ptBR.m_nPosY,m_ptLT.m_nPosX,m_ptLT.m_nPosY };
	// 绘制n个顶点的多边形，第一个参数必须要传入n+1，pts最后一个顶点坐标和第一个相同
	setfillcolor(EGERGB(0xFF, 0xA5, 0x00));
	fillpoly(5, pts);
}
CShape* CRect::Clone() const {
	return new CRect(*this);
}
CShape& CRect::Move(int nOffsetX, int nOffsetY) {
	m_ptLT.Move(nOffsetX, nOffsetY);
	m_ptBR.Move(nOffsetX, nOffsetY);
	return *this;

}
//Comgraphics
Comgraphics::Comgraphics(const CRect& pt1) {

	m_pt1.m_nPosX = pt1.m_ptBR.m_nPosX;
	m_pt1.m_nPosY = pt1.m_ptLT.m_nPosY + (pt1.m_ptBR.m_nPosY - pt1.m_ptLT.m_nPosY) / 2;
	m_pt2.m_nPosX = pt1.m_ptLT.m_nPosX + (pt1.m_ptBR.m_nPosX - pt1.m_ptLT.m_nPosX) / 2;
	m_pt2.m_nPosY = pt1.m_ptBR.m_nPosY;
	m_ptLT = pt1.m_ptLT;
	m_ptBR = pt1.m_ptBR;

}
Comgraphics::Comgraphics(const Comgraphics& rc) {
	m_pt1 = rc.m_pt1;
	m_pt2 = rc.m_pt2;
	m_ptBR = rc.m_ptBR;
	m_ptLT = rc.m_ptLT;
}
Comgraphics::Comgraphics(const CPoint pt1) {
	m_ptLT = CPoint(pt1.m_nPosX, pt1.m_nPosY);
	m_ptBR = CPoint(pt1.m_nPosX + 60, pt1.m_nPosY + 80);
}
Comgraphics::~Comgraphics() {
	cout << "Comgraphics::~Comgraphics()" << endl;

}
double Comgraphics::GetArea()  const {
	return 0.0;
}
bool Comgraphics::ptIn(const CPoint& pt) const {
	return (pt.m_nPosX >= m_ptLT.m_nPosX && pt.m_nPosX <= m_ptBR.m_nPosX) &&
		(pt.m_nPosY >= m_ptLT.m_nPosY && pt.m_nPosY <= m_ptBR.m_nPosY);
}
bool Comgraphics::InRect(const CRect& rc) const const {
	return rc.ptIn(m_ptLT) && rc.ptIn(m_ptBR);
}
void Comgraphics::Draw() const {
	// 存储n个顶点的x,y坐标
	int pts[10] = { m_ptLT.m_nPosX,m_ptLT.m_nPosY,m_ptBR.m_nPosX,m_ptLT.m_nPosY,
	m_ptBR.m_nPosX,m_ptBR.m_nPosY,m_ptLT.m_nPosX,m_ptBR.m_nPosY,m_ptLT.m_nPosX,m_ptLT.m_nPosY };
	// 绘制n个顶点的多边形，第一个参数必须要传入n+1，pts最后一个顶点坐标和第一个相同
	//drawpoly(5, pts);
	setfillcolor(GREEN);
	fillpoly(5, pts);
	line(m_pt1.m_nPosX, m_pt1.m_nPosY, m_pt2.m_nPosX, m_pt2.m_nPosY);
	line(m_ptLT.m_nPosX, m_ptLT.m_nPosY, m_pt2.m_nPosX, m_pt2.m_nPosY);
	line(m_pt1.m_nPosX, m_pt1.m_nPosY, m_ptLT.m_nPosX, m_ptLT.m_nPosY);
}
void Comgraphics::DrawColor() {
	// 存储n个顶点的x,y坐标
	int pts[10] = { m_ptLT.m_nPosX,m_ptLT.m_nPosY,m_ptBR.m_nPosX,m_ptLT.m_nPosY,
	m_ptBR.m_nPosX,m_ptBR.m_nPosY,m_ptLT.m_nPosX,m_ptBR.m_nPosY,m_ptLT.m_nPosX,m_ptLT.m_nPosY };
	// 绘制n个顶点的多边形，第一个参数必须要传入n+1，pts最后一个顶点坐标和第一个相同
	setfillcolor(YELLOW);
	fillpoly(5, pts);
	line(m_pt1.m_nPosX, m_pt1.m_nPosY, m_pt2.m_nPosX, m_pt2.m_nPosY);
	line(m_ptLT.m_nPosX, m_ptLT.m_nPosY, m_pt2.m_nPosX, m_pt2.m_nPosY);
	line(m_pt1.m_nPosX, m_pt1.m_nPosY, m_ptLT.m_nPosX, m_ptLT.m_nPosY);
}
CShape* Comgraphics::Clone() const {
	return new Comgraphics(*(this));
}

CShape& Comgraphics::Move(int nOffsetX, int nOffsetY) {
	m_ptLT.Move(nOffsetX, nOffsetY);
	m_ptBR.Move(nOffsetX, nOffsetY);
	m_pt1.Move(nOffsetX, nOffsetY);
	m_pt2.Move(nOffsetX, nOffsetY);
	return *this;
}

(4)运行截图

四、总结 1.继承访问权限测试

如果想让继承而来的数据成员作为public或者protected成员，可以使用{using A::_a; }进行重新声明。重新声明的数据成员的访问权限由using之前的访问说明符来决定如：

public:
	void New_Pri_Test()
	{
		_a = 10;
		_b = 20;
	}
	int npri_pub_b;
	using A::_a;

那么此时a的访问权限是public；

2.继承访问权限测试

（1）当类Y被说明为类X的友元时，类Y的所有成员函数都成为类X的友元函数，这就意味着作为友元类Y中的所有成员函数都可以访问类X中的所有成员（包括私有成员）。
（2）设计了A中有私有变量a，在A类中设置友元类C；设计B类继承A类并添加私有变量b。在C类中测试时，访问不到成员变量b，可以访问到成员变量a。
（3）设计D类继承C。在D类的成员函数中是访问不到A类的变量a，B类的a，b。
（4）

3.多态的综合应用

（1）多态按字面的意思就是多种形态。当类之间存在层次结构，并且类之间是通过继承关联时，就会用到多态。
（2）虚函数：
虚函数的作用：使属于不同派生类的不同对象产生不同的行为，从而实现运行时的多态
（4）特殊多态性含义为函数的参数是父类的指针（引用）或者子类的指针（引用）（即在运行过程中实现虚函数的绑定（运行时多态））
（5）虚继承：要使某个公共基类在派生类中只产生一个子对象，必须将该基类声明为虚继承，使这个基类成为虚基类；虚继承是在继承时使用关键字virtual将被继承的类声明为虚基类，使公共基类的成员在重复继承的派生类中只产生一个拷贝。

C++ 类的继承和多态

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