派生類的構造函數(shù)和析構函數(shù)

5.2派生類的構造函數(shù)與析構函數(shù)引例：派生類的定義#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改用構造函數(shù)后，derived中該如何初始化基類成員？5.2.1定義格式1.派生類構造函數(shù)的定義格式：設類Y由類X派生而來，則類Y的構造函數(shù)一般形式為：Y::Y(參數(shù)表0):X(參數(shù)表1){//派生類數(shù)據(jù)成員初始化}2.派生類的析構函數(shù)由于析構函數(shù)不帶參數(shù)，在派生類中是否要定義析構函數(shù)與基類無關?；惖奈鰳嫼瘮?shù)不會因為派生類沒有析構函數(shù)而得不到執(zhí)行，它們各自是獨立的。5.2.1定義格式改寫引例#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：構造函數(shù)定義示例當基類構造函數(shù)不帶參數(shù)時，派生類可以不定義構造函數(shù)。classA{inta;public:

A(){a=10;}};classB:publicA{public:intb;……};5.2.1定義格式說明而當基類構造函數(shù)帶參數(shù)時，它所有的派生類都必須定義構造函數(shù)，且參數(shù)不能為空。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定義格式說明續(xù)前頁5.2.1定義格式說明續(xù)前頁若基類構造函數(shù)沒有參數(shù)，則派生類的構造函數(shù)中可以略去“：基類名（參數(shù)表）”。classA{inta;public:

A(){a=10;}};classB:publicA{intb;public:B(intx){b=x;}};如果派生類的基類也是一個派生類，每個派生類只需負責其直接基類的構造。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定義格式

說明續(xù)前頁5.2.2調用順序當說明一個派生類對象時，先調用基類的構造函數(shù)，然后調用派生類的構造函數(shù)，如果基類仍然時一個派生類，則這個過程遞歸進行。析構函數(shù)調用順序與構造函數(shù)的相反。派生類還包含對象成員時，先調用基類構造函數(shù)，再調用對象成員的構造函數(shù)。例5-2-2：調用順序示例#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：調用順序示例5.2.2程序運行結果constructingbaseconstructingderiveddestroyingderiveddestroyingbase改寫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);}程序運行結果：5.2.2constructingbaseconstructingbaseconstructingderiveddestroyingderiveddestroyingbasedestroyingbase例5-2-3：正多邊形的繼承5.2.2分析：所有正多邊形都有中心點和顏色兩個屬性，有移動、改變顏色、打印和求面積四個行為。shapecirclerectangletriangleshape:center、colormove()changeColor()print()area()圓有半徑，它有自己的求面積和打印方法。矩形有寬和高，它有自己的求面積和打印方法。三角形有底邊（寬）和高，它有自己的求面積和打印方法。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續(xù)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續(xù)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續(xù)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續(xù)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續(xù)繼承是重用的基礎面向對象的程序設計中通過繼承實現(xiàn)重用reuse——便于重用，擴充?？梢栽谠械幕A上擴充，不修改原有代碼。即使原來的代碼完全不符合當前的要求，也不必修改原有的代碼。可以只增加新的代碼來實現(xiàn)新的功能。覆蓋與支配規(guī)則函數(shù)覆蓋：在派生類中可以重新定義基類中已定義的同名函數(shù)，我們稱之為函數(shù)覆蓋。支配規(guī)則：派生類的對象引用某函數(shù)時，編譯器先在派生類中找與該函數(shù)完全匹配的函數(shù)，有則調用之；否則到其基類中找，依次再到基類的基類中找，……等等，若都沒有找到則出錯。