9继承和多态 应用和示例

继承和多态应用和示例闫哲[e-mail]2005年6月24日内容†继承的内存布局†赋值相容规则†继承举例†虚函数实现原理†多态性举例†设计模式继承的内存布局†通过继承，派生类中包含基类成员classPoint2d{public:Point2d(){//…}//……protected:floatx,y;};classPoint3d:publicPoint2d{public:Point3d():Point2d(){//…}//……private:floatz;};Point3dobj2;Point2dobj1;floatxfloatyfloatzfloatxfloaty基类成员private:构造函数和析构函数的调用顺序访问权限的理解继承的内存布局†对比：对象成员classPoint2d{public:Point2d(){//…}//……protected:floatx,y;};classPoint3d:{public:Point3d():point(){//…}//……private:Point2dpoint;floatz;};Point3dobj2;Point2dobj1;floatxfloatyfloatzfloatxfloatypoint对象成员赋值相同规则†通过公有继承，派生类的对象可以当作基类使用classPoint2d{public:voidshow();protected:floatx,y;};classPoint3d:publicPoint2d{public:voidshow();private:floatz;};voidmain(){Point2dobj2d;Point3dobj3d;obj2d=obj3d;//对象赋值obj2d.show();//调用Point2d::show()}Point3dobj3d;Point2dobj2d;floatxfloatyfloatzfloatxfloaty派生类对象赋值给基类对象向上映射、对象切片赋值相同规则†通过公有继承，派生类指针可以赋给基类指针classPoint2d{public:voidshow();protected:floatx,y;};classPoint3d:publicPoint2d{public:voidshow();private:floatz;};voidmain(){Point2dobj2d;Point3dobj3d;Point3dobj3d;Point2dobj2d;p3dp2dfloatxfloatyfloatzfloatxfloatyp2d//指针赋值Point2d*p2d=&obj2d;Point3d*p3d=&obj3d;p2d=p3d;p2d-show();调用Point2d::show()}多重继承的内存布局†通过多重继承，派生类中包含各基类成员classPoint2d{public://…protected:floatx,y;};classPoint3d:publicPoint2d{public://…protected:floatz;};classVertex{public://…protected:Vertex*next;};classVertex3d:publicPoint3d,publicVertex{public://…protected:floatmumble;};Point2dPoint3dVertex3dVertex多重继承的内存布局floatxfloatyfloatzVertex*nextfloatxfloatyfloatzPoint2d子对象Point3d子对象Point2d子对象Vertex子对象Point3dobj1;floatmumbleVertex*nextVertex3dobj3;Vertexobj2;//对象赋值obj1=obj3;obj2=obj3;//指针赋值Point3d*p3d=&obj3;Vertex*pv=&obj3;//涉及指针偏移赋值相容规则：多重继承的内存布局†多重继承中数据成员的二义性intmemberintmemberinttotal来自base1classbase1{protected:intmember;public:base1(){//…}};classbase2{protected:intmember;public:base2(){//…}};classderived:publicbase1,publicbase2{private:inttotal;public:derived(){//…}voidfunc(){member=0;//二义性}};来自base2derived对象布局base1::member=0;base2::member=0;多重继承的内存布局†虚拟基类之前的内存布局intbintb2classbase{protected:intb;};classbase1:publicbase{protected:intb1;};classbase2:publicbase{protected:intb2;};classderived:publicbase1,publicbase2{floatd;public:intfun();};intbintb1base2的内存布局base1的内存布局intbintb1intbintb2floatd二义性derived的内存布局不能把derived对象当base对象使用多重继承的内存布局通过指针指向共享的虚拟基类†虚拟基类的内存布局intb2intbclassbase{protected:intb;};classbase1:virtualpublicbase{protected:intb1;};classbase2:virtualpublicbase{protected:intb2;};classderived:publicbase1,publicbase2{floatd;public:intfun();};intb1intbbase*pbase*pbase1的内存布局base2的内存布局intb1intb2floatdintbbase*pbase*pderived的内存布局可以把derived对象当base对象使用继承的使用†从现实中捕获一般-特殊关系classAutomobile{public:Automobile();voidstart();voidstop();voidspeedup();voidspeeddown();protected:char*brand;floatprice;floatspeed;};classCar:publicAutomobile{public:Car();voidabsstop();private:floatgas_comsume;};classJeep:publicAutomobile{public:Jeep();voidclimb();private:intdrive;};classTruck:publicAutomobile{public:Truck();voidtransport();private:floatload;};AutomobileCarJeepTruck继承的使用†从相似的类中提取共同的特征，形成类层次在银行业务中，有储蓄(Saving)账户和结算(Checking)账户之分，两个类的设计：SavingaccNumberbalanceSaving()Withdraw()Deposit()Display()CheckingaccNumberbalanceremittanceChecking()Withdraw()Deposit()Display()SetRemit()Consume()储蓄账户结算账户继承的使用//类的定义省略#includeiostream.hSaving::Saving(intaccNo,floatbal=0){accNumber=accNo;balance=bal;}voidSaving::Withdraw(floatbal){if(balbalance&&bal0)balance-=bal;}voidSaving::Deposit(floatbal){if(bal0)balance+=bal;}voidSaving::Display(){cout“SavingAccount[”accNo“]:”balanceendl;}Checking::Checking(intaccNo,floatbal=0){accNumber=accNo;balance=bal;}voidChecking::Withdraw(floatbal){floattmp;//…根据remit值计算tmpif(tmpbalance&&tmp0)balance-=tmp;}voidChecking::Deposit(floatbal){if(bal0)balance+=bal;}voidChecking::Display(){cout“SavingAccount[”accNo“]:”balanceendl;}voidChecking::SetRemit(intrem){remittance=rem;}voidChecking::Consume(floatprice,floatadd=0){floattmp=price+add;if(tmpbalance&&tmp0)balance-=tmp;}继承的使用†Checking类继承Saving类SavingaccNumberbalanceSaving()Withdraw()Deposit()Display()CheckingremittanceChecking()Withdraw()SetRemit()Consume()储蓄账户结算账户继承的使用classSaving{protected:intaccNumber;floatbalance;public:Saving(intaccNo,floatbal=0);voidWithdraw(floatbal);voidDeposit(floatbal);voidDisplay();};classChecking:publicSaving{private:intremittance;public:Checking(intaccNo,floatbal=0);voidWithdraw(floatbal);voidsetRemit(intrem);voidConsume(floatprice,floatadd);};#includeiostream.h//Saving的实现…Checking::Checking(intaccNo,floatbal=0):Saving(accNo,bal){}voidChecking::Withdraw(floatbal){floattmp;//根据remit计算if(tmpbalance&&tmp0)balance-=tmp;}voidChecking::setRemit(intrem){remittance=remit;}VoidChecking::Consume(floatprice,floatadd=0){floattmp=price+add;if(tmpbalance&&tmp0)balance-=tmp;}继承的使用†思考继承关系：结算帐户is-a储蓄帐户吗？SavingChecking增加数据成员overdraft表示透支范围在Withdraw()中允许透支则Checking也得到了该数据成员其实，Checking不是一种Saving，而是不同类型的两种帐户AccountaccNumberbalanceAccount()Withdraw()Deposit()Display()SavingoverdraftSaving()Withdraw()Checki