C++ 继承

C++ 继承

如何设计C++的类？子类和基类之间的关系？子类默认构造函数的初始化列表，多态性，虚构函数的应用？

//Object-Oriented
#include<iostream>
#include <string>
using namespace std;

class LibMat
{
public:
	LibMat(){cout << "LibMat::LibMat default constructor!\n";}
	virtual ~LibMat(){cout << "LibMat::~LibMat destructor!\n";}
	virtual void print ()const
	{
		cout << "LibMat::print() -- I am a  LibMat object! \n";
	}
};

class Book : public LibMat
{
public:
	Book(const string &title,const string &author):_title(title),_author(author)
	{
		cout<< "Book::Book (" << _title << "," << _author << ") constructor\n";
	}

	virtual ~Book(){cout<< "Book::~Book destructor\n";}
    void print ()const
	{
		cout << "Book::print() -- I am a  Book object! \n";
		cout << "My title is : " << _title << endl;
		cout << "My author is : "<< _author << endl;
	}

	const string & title(){return _title;}
	const string &author(){return _author;}

	protected:
	string _title;
	string _author;
};

class AudioBook : public Book
{
public:
	AudioBook(const string &title,const string &author,const string &narrator):Book(title,author),_narrator(narrator)
	{
		cout<< "AudioBook::AudioBook (" << _title << "," << _author << "," << _narrator << ") constructor\n";
	}

	~AudioBook(){cout<< "AudioBook::~AudioBook destructor\n";}
	void print ()const
	{
		cout << "AudioBook::print() -- I am a  AudioBook object! \n";
		cout << "My title is : " << _title << endl;
		cout << "My author is : "<< _author << endl;
		cout << "My narrator is : "<< _narrator << endl;
	}

		 const string & narrator() const {return _narrator;}
//这里两个const，为何两个，用一个可以吗?当语句为string & narrator() const {return _narrator;}时，会提示编译错误，为何？return是const 而接收的却不是const，接收和返回的类型不匹配。
//为何要用两个const：简单的说为了防止改变私有成员的值。


private:
	string _narrator;
};


void print(const AudioBook & audiobook)
{
	cout << "in gobal print(): about to print mat.print()\n";
	audiobook.print();
}


int main()
{
    AudioBook ab("Mason and Dixon","Thomas Pynchon","Edwin Leonard");
	print(ab);

	//ab.narrator() = "hewei";
	//cout <<"\n\n";

	//print(ab);
}

上述代码的输出：
LibMat::LibMat default constructor!
Book::Book (Mason and Dixon,Thomas Pynchon) constructor
AudioBook::AudioBook (Mason and Dixon,Thomas Pynchon,Edwin Leonard) constructor

in gobal print(): about to print mat.print()
AudioBook::print() -- I am a  AudioBook object!
My title is : Mason and Dixon
My author is : Thomas Pynchon
My narrator is : Edwin Leonard
AudioBook::~AudioBook destructor
Book::~Book destructor
LibMat::~LibMat destructor!

构造函数的调用顺序：在使用AudioBook的时候，会最先初始化基类的基类LibMat，调用它的构造函数，然后再初始化基类Book，最后在调用AudioBook的构造函数。

析构函数的调用顺序：和构造函数的调用顺序是相反的。

构造函数初始化列表：首先初始化最底层的基类，依次初始化。

多态性：子类为了实现自己独特的功能而屏蔽掉基类的功能，用virtual作为关键词修饰，类的每一个对象都与一个虚函数列表，程序在编译阶段并不会指定要调用哪个函数，只有在运行时通过查找各个虚函数类表，找出对应的函数。
  比如上面的代码，AudioBook的函数print，当程序调用print时，会先查找ab对象的函数列表，发现Book 和 LibMat的print函数均是虚函数，则会调用ab自己的print函数。