第6章 继承与派生

1。继承的作用是什么？
   它可以允许不同的类以多种不同的方法共享代码。
2。解释多态，什么是多态？如何使用多态。
3。解释动态绑定与静态绑定的区别？描述各自的试能条件。
   动态绑定：即运行的时候的绑定，多态就是这样的。解决了运行的时候方法的间接调用。
   静态绑定：针对方法的直接调用（不通过指针或者引用）仍然是由编译器进行解析的。
4。如何覆盖基类的一个方法？
   当基类中的一个虚函数在派生类中也存在并且函数签名相同时，我们就说派生类中的版本覆盖了基类中的定义。
5。什么是纯虚函数？其主要用途是什么？
6。什么是抽象类？它有什么用途？有什么操作可以针对实体类进行却不能针对抽象类进行？
7。某个基类函数被隐常的含义是什么？什么可以引起此种情况的发生？
8。哪些成员函数无法从基类函数进行继承？解释其原因。
   ①拷贝构造函数②拷贝赋值函数③析构函数
   如果类没有明确定义拷贝构造函数和拷贝赋值函数，编译器会默认为这个类生成。
   编译器也会默认为析构生成一个默认的析构函数。

#include <QTextStream> #include "student.h" Student::Student(QString nm, long id,QString major,int year) :m_Name(nm) ,m_Major(major) ,m_StudentId(id) ,m_Year(year){} QString Student::getClassName() const {  return "Student"; } QString Student::toString() const {  QString retval;  QTextStream os(&retval);    os << "[" << getClassName() << "]"   << " name:" << m_Name   << " id:" << m_StudentId   << " Major:" << m_Major;  return retval; } Undergrad::Undergrad(QString name, long id,QString major,int year) :Student(name, id, major, year){} QString Undergrad::getClassName() const {  return "Undergrad"; } GradStudent::GradStudent(QString name, long id, QString major, int year, GradStudent::Support support) :Student(name,id,major,year), m_Support(support){} QString GradStudent::toString() const {  QString result;  QTextStream os(&result);  os << Student::toString()   << supportStr(m_Support)   << "/n";  return result; } QString GradStudent::getClassName() const {  return "GradStudent"; } QString GradStudent::supportStr(Support support) {  return "test"; }

 

#include <QApplication> #include <QTextStream> #include  "student.h" static QTextStream cout(stdout,QIODevice::WriteOnly); void graduate (Student* student); int main(int argc, char *argv[]) {  QApplication a(argc, argv);    Undergrad us("Frodo",5562,"Ring Theory", 4);  GradStudent gs("Bilbo",3029,"history",6,GradStudent::fellowship);  cout << "Here is the data for the two students:/n";  cout << gs.toString() << endl;  cout << us.toString() << endl;  cout << "/nHere is what happens when they graduate:/n";  graduate(&us);  graduate(&gs);  return a.exec(); } void graduate (Student* student) {  cout << "/nTHE FOLLOWING"   << student->getClassName()   << "has graduated/n"   << student->toString() << "/n"; }

 

 

#include <QString> /* duo tai pai sheng*/ class Student{ public:  Student(QString nm, long id, QString m_Major, int year = 1);  virtual QString getClassName() const;/*it has been uesd in sub class*/  QString toString() const;/*In sub class, it will be spread*/  virtual ~Student(){}   private:  QString m_Name;  QString m_Major;  long m_StudentId; protected:  int m_Year; }; class Undergrad : public Student { public:  Undergrad(QString name, long id, QString major, int year);  QString getClassName() const; }; class GradStudent : public Student { public:  enum Support {ta, ra, fellowship, other};  GradStudent(QString name, long id, QString major, int year, Support support);  QString getClassName() const;  QString toString() const; protected:  static QString supportStr(Support sup); private:  Support m_Support; };

多态派生6-8

#include <QtCore/QCoreApplication> #include <iostream> using namespace std; class A{ public:  virtual void foo(){   cout << "A's foo" << endl;  }  virtual void bar(){   cout << "A's bar" << endl;  } }; class B:public A{ public:  void foo(){   cout << "B's foo" << endl;  }  void bar(){   cout << "B's bar" << endl;  } }; int main(int argc, char *argv[]) {  QCoreApplication a(argc, argv);  B bobj;  A *aptr = &bobj;  aptr->foo();/*B's foo*/  A aobj = *aptr;  aobj.foo();/*A's foo*/  return a.exec(); }

 

多态派生6-9

#include <QtCore/QCoreApplication> #include <iostream> using namespace std; class A{ public:  virtual void foo(){   cout << "A's foo" << endl;  } }; class B:public A{ public:  void foo(){   cout << "B's foo" << endl;  } }; class C:public B{ public:  void foo(){   cout << "C's foo" << endl;  } }; int main(int argc, char *argv[]) {  QCoreApplication a(argc, argv);  C cobj;  B *bptr = &cobj;  bptr->foo();/*C's foo*/  A *aptr = &cobj;  bptr->foo();/*C's foo*/  return a.exec(); }

 

多态派生6-10

base.h

#include <iostream> using namespace std; class Base{ public:  Base();  void a();  virtual void b();  virtual void c(bool condition = true);  virtual ~Base(){}; }; class Derived : public Base { public:  Derived();  virtual void a();  void b();  void c(); };

base.cpp

#include "base.h" Base::Base(){  cout << "Base::Base" << endl;  //a();  //c(); } void Base::c(bool condition){  cout << "Base::c" << endl; } void Base::a(){  cout << "Base::a" << endl;  //b(); } void Base::b(){  cout << "Base::b" << endl; } Derived::Derived(){  cout << "Derived::Derived" << endl; } void Derived::a(){  cout << "Derived::a" << endl;  //c(); } void Derived::b(){  cout << "Derived::b" << endl; } void Derived::c(){  cout << "Derived::c" << endl; } #include <QtCore/QCoreApplication> #include <iostream> #include "base.h" using namespace std; int main (int argc, char** argv){    QCoreApplication a(argc, argv);    Base b;  Derived d;  cout << "Objects Created" << endl;  b.b();/*Derived::b*/  cout << "Calling derived methods" << endl;  d.a();/*Derived::a*/  d.b();/*Derived::b*/  d.c();/*Derived::c*/  cout << ".. via base class pointers..." << endl;  Base* bp = &d;  bp->a();/*base::a*/  bp->b();/*Derived::b*/  bp->c();/*base::c*/  return a.exec(); }