1.理解多态

（一）多态的概念

1、根据实际的对象类型决定函数调用的具体目标；
2、同样的调用语句在实际运行时有多种不同的表现形态体现，这就是多态，同一语句具有多种形态。函数的重载、运算符的重载都是多态现象。
3.在C++中，多态性表现形式之一是：具有不同功能的函数可以用同一个函数名，这样就可以实现用一个函数名调用不同内容的函数。
4.从系统实现的角度来看，多态性分为两类：静多态性和动多态性，静多态性是通过函数重载实现的，动多态性是通过虚函数实现的。

（二）虚函数概念

C++中的虚函数就是用来解决动态多态问题的，所谓虚函数，就是在基类声明函数是虚拟的，并不是实际存在的，然后在派生类中才正式定义此函数，在程序运行期间，用指针指向某一派生类对象，这样就能调用指针指向的派生类对象中的函数，而不对调用其他派生类中的函数。

（三）多态的意义

1、在程序运行中展现出动态的特性；
2、函数重写必须多态实现，否则没有任何意义；
3、多态是面向对象组件化程序设计的基础特性。

2.编程加深对多态的理解

（一）一般多态性

新建CAnimal类，Move()是其内部虚函数，新建CCat类、CEagle类、COwl类继承CAnimal类，并重写其内部的函数Move()的实现方法以实现多态的效果。
代码：

#include <iostream>  
using namespace std;
class CAnimal
{
public:CAnimal() {m_nLegs = 8;}CAnimal(int nLeg) {m_nLegs = nLeg;}void Move() {cout << "animal has many legs" << endl;};
protected:int m_nLegs;
};class CCat : virtual public CAnimal {
public:CCat() {m_nLegs = 4;}CCat(int nLegs) {m_nLegs = nLegs;}void Move() {cout << "cat has four legs" << endl;};
};class CEagle :virtual public CAnimal {
public:CEagle() {m_nLegs = 2;}CEagle(int nLegs) {m_nLegs = nLegs;}void Move() {cout << "eagle has two legs" << endl;};
};class COwl : public CCat, public CEagle
{
public://无参构造函数COwl() {m_nLegs = 2;}COwl(int nLegs) {m_nLegs = nLegs;}void Move() {cout << "owl has two legs" << endl;}
};void TestAnimal() {CAnimal* Animal[4];Animal[0] = new CAnimal;Animal[1] = new CCat;Animal[2] = new CEagle;Animal[3] = new COwl;for(int i = 0; i < 4; i++){Animal[i]->Move();}
}
int main()
{TestAnimal();return 0;
}

实验截图：

可以看到输出全部一样。

（二）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
代码：

 void callMove(CAnimal* a) {a->Move();
}void TestAnimal()
{CAnimal animal;CCat cat;CEagle eagle;COwl owl;callMove(&animal);callMove(&cat);callMove(&eagle);callMove(&owl);
}int main()
{TestAnimal();return 0;
}

运行截图：

可以看到Move函数输出的都是一样的。
为了让每个子类能够输出自己类中重写的函数Move,则我们只需要在每个子类的父类Move 函数前加上virtual。即让父类的Move函数变为虚函数，告诉编译器它是虚函数，要进行晚绑定。
更改后的运行截图：

（三）析构函数的多态性

代码：

#include <iostream>
using namespace std;class Base {
public:Base() { cout << "Base constructor... \n"; }~Base() { cout << "Base destructor... \n"; }virtual void fun() const { cout << "Base functoin...\n"; }
};class Derived :public Base {
public:Derived() {p = new int(0);cout << "Derived Constructor...\n";}~Derived() {cout << "Derived destructor...\n";delete p;}void fun() const { cout << "Derived function...\n"; }
private:int* p;
};int main() {Base* pd = new Derived;pd->fun();delete pd;return 0;
}

运行截图：

可以看到当指向基类的指针被释放时，派生类的析构函数其实没有被调用，造成在派生类中申请的空间没有被释放，从而导致内存泄漏。
为了防止在实际应用中造成这种情况，我们只需要将上述代码中的 ~Base() { cout << "Mammal destructor... \n";前面添上virtual，即通过将基类的析构函数声明为虚析构函数，成功的通过基类指针调用了派生类的析构函数，完成了内存的释放。

更改后的运行截图：

（四）多继承

c++不仅可以单继承，也可以多继承。一个类有多个基类，那么这种继承关系就叫做多继承。
多继承的声明：
class 派生类名:访问控制 基类名1， 访问控制 基类名2， …
{
成员列表
}

使用多继承会带来哪些问题
多继承比单继承复杂，也更容易出现问题。
多继承的两个主要问题是：
①从两个不同的基类，继承同名方法
②从多个基类间接继承同一个类的多个实例

class A
{
public:int a;
};class B1 : public A 
{
public:int b1;
};class B2 : public A
{
public:int b2;
};class C :public B1, public B2
{
public:void fun(){int  i = a; }
};

可以看到若B1、B2不虚继承会报错，因为多继承原因，C同时继承B1，B2，而B1、B2的基类又相同，则程序不知道是a是B1的还是B2的，此时只需要让B1，B2都虚继承A即可解决问题。

（五）多继承实例

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。

CShape.h

#ifndef CSHAPE_H
#define CSHAPE_H
#include<string>
#include<math.h>
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

CShape.cpp

#include "CShape.h"
#include "graphics.h"
#include <iostream>
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;elsereturn 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;
}

Main.cpp

#include<vector>
#include "graphics.h"
#include<iostream>
#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");vector<CShape*>shapes;vector<CShape*>shapestmp;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truemove_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();elseshapes[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truecopy_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;
}

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