派生类的构造函数和析构函数

5.2派生类的构造函数与析构函数引例：派生类的定义#include<iostream.h>classbase{ intx;protected: inty;public: voidsetXY(intm,intn){x=m;y=n;} voidshowXY(){cout<<"x="<<x<<endl; cout<<"y="<<y<<endl;}};classderived:publicbase{ intz;public:voidsetXYZ(intm,intn,intk){setXY(m,n);z=k;}voidshowXYZ(){showXY();cout<<"z="<<z<<endl;}};basederivedbase中的setXY改用构造函数后，derived中该如何初始化基类成员？5.2.1定义格式1.派生类构造函数的定义格式：设类Y由类X派生而来，则类Y的构造函数一般形式为：Y::Y(参数表0):X(参数表1){//派生类数据成员初始化}2.派生类的析构函数由于析构函数不带参数，在派生类中是否要定义析构函数与基类无关。基类的析构函数不会因为派生类没有析构函数而得不到执行，它们各自是独立的。5.2.1定义格式改写引例#include<iostream.h>classbase{ intx;protected: inty;public:

base(intm,intn){x=m;y=n;} voidshowXY(){cout<<"x="<<x<<endl; cout<<"y="<<y<<endl;}};classderived:publicbase{ intz;public:derived(intm,intn,intk){setXY(m,n);z=k;}voidshowXYZ(){showXY();cout<<"z="<<z<<endl;}};:base(m,n){z=k;}classA{inta;public:A(intx){a=x;}};classB:publicA{intb;public:B(intx,inty):A(x),b(y){}};A(x){b=y;}例5-2-1：构造函数定义示例当基类构造函数不带参数时，派生类可以不定义构造函数。classA{inta;public:

A(){a=10;}};classB:publicA{public:intb;……};5.2.1定义格式说明而当基类构造函数带参数时，它所有的派生类都必须定义构造函数，且参数不能为空。classA{inta;public:A(intx){a=x;}};classB:publicA{intb;public:B(intx,inty):A(x),b(y){}};classC:publicA{public:C(intx):A(x){}};5.2.1定义格式说明续前页5.2.1定义格式说明续前页若基类构造函数没有参数，则派生类的构造函数中可以略去“：基类名（参数表）”。classA{inta;public:

A(){a=10;}};classB:publicA{intb;public:B(intx){b=x;}};如果派生类的基类也是一个派生类，每个派生类只需负责其直接基类的构造。classA{inta;public:A(intx){a=x;}};classB:publicA{intb;public:B(intx,inty):A(x),b(y){}};classC:publicB{intc;public:C(intx,inty,intz):B(x,y),c(z){}};5.2.1定义格式

说明续前页5.2.2调用顺序当说明一个派生类对象时，先调用基类的构造函数，然后调用派生类的构造函数，如果基类仍然时一个派生类，则这个过程递归进行。析构函数调用顺序与构造函数的相反。派生类还包含对象成员时，先调用基类构造函数，再调用对象成员的构造函数。例5-2-2：调用顺序示例#include<iostream.h>classbase{inta;public:base(intx){a=x;cout<<"constructingbase”<<endl;}~base(){cout<<"destroyingbase“<<endl;}};classderived:publicbase{intd;public:derived(intx,inty):base(x){d=y;cout<<"constructingderived“<<endl;}~derived(){cout<<"destroyingderived"<<endl;}};main(){derivedd(5,8);}例5-2-2：调用顺序示例5.2.2程序运行结果constructingbaseconstructingderiveddestroyingderiveddestroyingbase改写derived5.2.2classderived:publicbase{intd;basemember;public:derived(intx,inty,intz):base(x),member(y){d=z;cout<<”constructingderived”<<endl;}~derived(){cout<<”destroyingderived”<<endl;}main(){derivedd(5,8,10);}程序运行结果：5.2.2constructingbaseconstructingbaseconstructingderiveddestroyingderiveddestroyingbasedestroyingbase例5-2-3：正多边形的继承5.2.2分析：所有正多边形都有中心点和颜色两个属性，有移动、改变颜色、打印和求面积四个行为。shapecirclerectangletriangleshape:center、colormove()changeColor()print()area()圆有半径，它有自己的求面积和打印方法。矩形有宽和高，它有自己的求面积和打印方法。三角形有底边（宽）和高，它有自己的求面积和打印方法。classpoint{ intx,y;public: point(inta=0,intb=0){x=a;y=b;}pointgetPoint(){return*this;} voidsetPoint(pointp){x=p.x;y=p.y;} voidshow(){cout<<"("<<x<<","<<y<<")"<<endl;}};classshape{protected:pointcenter;charcolor[10];public:shape(intx,inty,char*c):center(x,y){strcpy(color,c);}voidmove(pointp){center.setPoint(p);}voidprint(){cout<<"Thisisa"<<color<<"shape,centerof";center.show();}voidchangeColor(char*c){strcpy(color,c);}voidarea(){cout<<"Theareaofshapeis0.0“<<endl;}};例5-2-3续classcircle:publicshape{ intradius;public:circle(intx,inty,char*c,intr):shape(x,y,c){radius=r;}voidprint(){cout<<"Thisisa“<<color<<"circle,centerof"; center.show();}voidarea(){cout<<"Theareaofcircleis“<<3.14*radius*radius<<endl;}};classshape{protected:pointcenter;charcolor[10];public:

shape(intx,inty,char*c):center(x,y){strcpy(color,c);}voidmove(pointp){center.setPoint(p);}voidprint(){cout<<"Thisisa"<<color<<"shape,centerof";center.show();}voidchangeColor(char*c){strcpy(color,c);}voidarea(){cout<<"Theareaofshapeis0.0“<<endl;}};例5-2-3续classrectangle:publicshape{protected: intwidth,height;public:

rectangle(intx,inty,char*c,intw,inth):shape(x,y,c){width=w;height=h;}voidprint(){cout<<"Thisisa"<<color<<"rectangle,centerof"; center.show();}voidarea(){cout<<"Theareaofrectangleis"<<width*height<<endl;}};classshape{protected:pointcenter;charcolor[10];public:

shape(intx,inty,char*c):center(x,y){strcpy(color,c);}voidmove(pointp){center.setPoint(p);}voidprint(){cout<<"Thisisa"<<color<<"shape,centerof";center.show();}voidchangeColor(char*c){strcpy(color,c);}voidarea(){cout<<"Theareaofshapeis0.0“<<endl;}};例5-2-3续classtriangle:publicrectangle{public:

triangle(intx,inty,char*c,intw,inth):rectangle(x,y,c,w,h){}voidprint(){cout<<"Thisisa"<<color<<"triangle,centerof"; center.show();}voidarea(){cout<<"Theareaoftriangleis"<<width*height/2<<endl;}};classrectangle:publicshape{protected: intwidth,height;public:

rectangle(intx,inty,char*c,intw,inth):shape(x,y,c){width=w;height=h;}voidprint(){cout<<"Thisisa"<<color<<"rectangle,centerof"; center.show();}voidarea(){cout<<"Theareaofrectangleis"<<width*height<<endl;}};例5-2-3续main(){ circlec(50,50,"red",50); c.print(); c.area(); c.move(point(100,100)); c.changeColor("blue"); c.print(); rectangler(10,10,"yellow",10,30); r.print(); r.area(); r.move(point(20,20));r.changeColor("green");r.print();trianglet(20,20,"white",40,50);t.print();t.area();t.move(point(30,30));t.changeColor("black");t.print(); }例5-2-3续继承是重用的基础面向对象的程序设计中通过继承实现重用reuse——便于重用，扩充。可以在原有的基础上扩充，不修改原有代码。即使原来的代码完全不符合当前的要求，也不必修改原有的代码。可以只增加新的代码来实现新的功能。覆盖与支配规则函数覆盖：在派生类中可以重新定义基类中已定义的同名函数，我们称之为函数覆盖。支配规则：派生类的对象引用某函数时，编译器先在派生类中找与该函数完全匹配的函数，有则调用之；否则到其基类中找，依次再到基类的基类中找，……等等，若都没有找到则出错。