Day10（下）.面向对象编程强化

理解面向对象编程的思想，C++的类的概念是把所有的人事物都抽象成一个类，类中定义了人事物的属性和方法，在C++中成为成员变量和成员函数，通过给成员变量赋值，由成员函数进行对成员变量的处理操作返回结果。看下面的代码，深刻理解指针在类中的使用。

#include "iostream"using namespace std;class circle{private:int m_r;double m_s;public:int setR(int r){return m_r = r;}double getS(){return m_s = 3.14*m_r*m_r;}};int main(int argc, char const *argv[]){int r;circle cir;cout << "请输入半径：" << endl;cin >> r;cir.setR(r);cout << "圆的半径是" << cir.getS() << endl;circle *pcir = ○cout << "请输入半径：" << endl;cin >> r;pcir->setR(r);cout << "圆的半径是" << pcir->getS() << endl;system("pause");return 0;}

在类的内部定义声明时，一般有public private protected三种，具体这里就不赘述了，可以百度搜索或看书理解，只给出结论。

结论：

1.private修饰的成员变量和成员函数，只能在类的内部使用

2.publix修饰的成员变量和成员函数，能在类的内部和外部使用

3.protected修饰的成员变量和成员函数，在类的外部不能使用，在类的内部能使用（继承中需要用到）

使用时需要注意的是，如果一个成员变量前面不告诉编译器是什么类型， 那么编译器会默认为private，而private的关键字修饰的意思是把类的成员变量隐藏起来。。

看一个例子，求出立方体的面积和体积，show the codes

#include "iostream"using namespace std;//目标：抽象立方体 求立方体的s 和 vclass  Cube{public:void setA(int a){m_a = a;}int getA(){return m_a;}void setB(int a){m_a = a;}int getB(){return m_b;}void setC(int a){m_a = a;}int getC(){return m_c;}void setABC(int a=0, int b=0, int c=0){m_a = a;m_b = b;m_c = c;}public:int getS(){m_s = 2*(m_a*m_b + m_a*m_c + m_b*m_c);return m_s;}int getV(){m_v = m_a*m_b*m_c;return m_v;}public:char * cubeEqueal(Cube &c1, Cube &c2){if (c1.getA()== c2.getA() &&c1.getB()== c2.getB() &&c1.getC()== c2.getC()){return "相等";}return "不相等";}char * cubeEqueal(Cube &c2){if (m_a== c2.getA() &&m_b== c2.getB() &&m_c== c2.getC()){return "相等";}return "不相等";}private:int m_a;int m_b;int m_c;int m_v;int m_s;};void main(){Cube cube1, cube2, cube3;cube1.setABC(1, 2, 3);cube2.setABC(2, 3, 4);cube3.setABC(1, 2, 3);//cout<<cube1.cubeEqueal(cube1, cube2);cout<<cube1.cubeEqueal(cube2);cout<<cube1.cubeEqueal(cube1);//cout<<cubeEqueal(cube1, cube2)<<endl;//cout<<cubeEqueal(cube1, cube3)<<endl;system("pause");}void main31(){Cube cube1;cube1.setABC(1, 2, 3);cube1.getV();cube1.getS();cout<<"体积为："<<cube1.getV()<<"面积为"<<cube1.getS();system("pause");}//全局函数char * cubeEqueal(Cube &c1, Cube &c2){if (c1.getA()== c2.getA() &&c1.getB()== c2.getB() &&c1.getC()== c2.getC()){return "相等";}return "不相等";}void main33(){Cube cube1, cube2, cube3;cube1.setABC(1, 2, 3);cube2.setABC(2, 3, 4);cube3.setABC(1, 2, 3);cout<<cubeEqueal(cube1, cube2)<<endl;cout<<cubeEqueal(cube1, cube3)<<endl;system("pause");}

例子：输入一点，判断是否在圆内或者圆外

#include "iostream"using namespace std;class Point{private:double x0, y0; //点坐标public:void setPoint(double x, double y){x0 = x;y0 = y;}double getX0(){return x0;}double getY0(){return y0;}};class AdvCircle{private:double x1, y1; //圆心坐标double r1;  //圆半径public:void setCir(double x, double y, double r){x1 = x; y1 = y; r1 = r;}public:char * judge(double x, double y){double s = (x1-x)*(x1-x) + (y1-y)*(y1-y) - r1*r1 ;if (s > 0){return "在圆外";}else{return "在圆内";}}char * judge(Point &p){double s = (x1-p.getX0())*(x1-p.getX0()) + (y1-p.getY0())*(y1-p.getY0()) - r1*r1 ;if (s > 0){return "在圆外";}else{return "在圆内";}}};void main(){AdvCircle c1;Point p1;c1.setCir(2, 2, 3);p1.setPoint(1, 1);cout<<c1.judge(p1.getX0(), p1.getY0())<<endl;cout<<c1.judge(p1)<<endl;system("pause");}//面向过程编程void main1(){double x0, y0; //点坐标double x1, y1; //圆心坐标double r;  //圆半径x0 = 1; y0 = 1;x1 = 2; y1 = 2;r = 3;double s = (x1-x0)*(x1-x0) + (y1-y0)*(y1-y0) - r*r ;//if (s  > 0 ){cout<<"在圆外";}else{cout<<"在圆内";}system("pause");}

继续深入，如果我们想把类的定义放在头文件里，而在源文件里进行成员函数的实现，首先分析一下，以点和圆的类说明：

MyCircle.h

#pragma once#include "MyPoint.h"class MyCircle{public:    void setCir(double x, double y, double r);    public:    char * judge(double x, double y);    char * judge(MyPoint &p);private:    double x1, y1; //Ô²ÐÄ×ø±ê    double r1;  //Ô²°ë¾¶};

MyCircle.cpp

#include "MyCircle.h"void MyCircle::setCir(double x, double y, double r){//这里面也是类的内部x1 = x; y1 = y; r1 = r;}char* MyCircle::judge(double x, double y){double s = (x1-x)*(x1-x) + (y1-y)*(y1-y) - r1*r1 ;if (s > 0){return "在圆外";}else{return "在圆内";}}char* MyCircle::judge(MyPoint &p){double s = (x1-p.getX0())*(x1-p.getX0()) + (y1-p.getY0())*(y1-p.getY0()) - r1*r1 ;if (s > 0){return "在圆外";}else{return "在圆内";}}

MyPoint.h

#pragma onceclass MyPoint{private:double x0, y0; //点坐标public:void setPoint(double x, double y);double getX0();double getY0();};

MyPoint..cpp

#include "MyPoint.h"void MyPoint::setPoint(double x, double y){x0 = x;y0 = y;}double MyPoint::getX0(){return x0;}double MyPoint::getY0(){return y0;}

要认真学习面向对象编程的方法和思想。

另外一点需要提及的是空类

class cls{public:private:protected:};int main(int argc, char const *argv[]){cls c;printf("size:%d",sizeof(c));return 0;}

答案为1.

结论：空类的对象大小不为0，空类对象具有地址。

C++构造函数

构造函数有有参构造函数、无参构造函数和赋值构造函数（也叫拷贝构造函数）。

其中有参构造函数的调用方法只有下面的三种，show you the codes

#include "iostream"using namespace std;//隐式 或 显示 构造函数 析构函数//class Test{public:    //有参构造函数    Test(int a)    {        m_a = a;    }    //无参数构造函数    Test()    {        m_a = 0;    }    //四种应用场景    //赋值构造函数 copy构造函数    Test(const Test &obj)    {    }public:    void print()    {        cout<<"m_a："<<m_a<<endl;    }protected:private:    int m_a;};void main(){    //括号法    Test t1(10); //c++编译器自动调用这个类的有参构造函数    t1.print();    //=法    Test t2 = 20; //c++编译器自动调用这个类的有参构造函数    t2.print();    //手工调用初始化    Test t3 = Test(30);//程序员手工的调用构造函数 进行对象初始化    t3.print();    system("pause");}

一定要多练，多练！！！！！！！多练！！！！！！！多练！！！！！！！