知識點： 1.為了讓member function 在運動時動態進行，需要在它宣告前加關鍵字virtual（虛擬函式） 2. 基類的解構函式前加virtual是因為 在實現多型時，當用基類操作派生類，在析構時防止只析構基類而不析構派生類的狀況發生。 3. 編譯器中沒有顯示解構函式的列印內容是因為解構函式是在main函式結束後進行的，當用pause暫停時，解構函式還沒執行。 4. 派生類可直接用基類的資料成員，在初始化值時，直接呼叫Book(title, author)即可，對應_title(title) , _author(author) 5. 關鍵字protected，被宣告為protected的所有成員都可以被派生類直接訪問。

#include<iostream>
#include<string>
using namespace std;

class LibMat{
public:
	LibMat(){ cout << "LibMat::LibMat() default constructor!\n"; }
	//動態繫結，用虛擬函式，在函式申明前加關鍵字virtual
	virtual ~LibMat(){ cout << "LibMat::~LibMat() destructor!\n"; }
	virtual void print() const
	{
		cout <<
 
 "LibMat::print() --I am a LibMat object!\n";
	}
};

//現在我們定義一個非成員函式print()，它接受一個引數，其形式為const LibMat reference
void print(const LibMat &mat)
{
	cout << "in global print(): about to print mat.print()\n";
	mat.print();
}

//實現派生類Book
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";
	}
	virtual void print()const
	{
		cout << "Book::print()--I am a Book object!\n"
			<< "My title is: " << _title << '\n'
			<< "My author is: " << _author << endl;
	}
	// title()和author()是兩個訪問函式，都是non-virtual inline 函式
	const string &title() const { return _title; }
	const string &author() const { return _author; }

protected:  //被宣告為protected的所有成員都可以被派生類直接訪問
	string _title;
	string _author;
};

//實現Book的派生類
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";
	}

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

	const string &narrator() const { return _narrator; }

protected:
	string _narrator;
};

int main()
{
	
	cout << 1 << endl;
	LibMat libmat;
	print(libmat);

	cout << 2 << endl;
	Book b("The Castle", "Franz Kafka");
	print(b);

	cout << 3 << endl;
	AudioBook ab("Man Without Qualities", "Robert Musil", "Kenneth Meter");
	print(ab);

	system("pause");
}