案例分析：圖書館

圖書館

模擬一個生活中小型圖書館的使用程序，功能如下：
1. 向圖書館添加書籍
2. 向圖書館借出書籍
3. 向圖書館歸還書籍
4. 顯示圖書館當前狀態
5. 退出程序

簡單分析如下：
我們需要知道書籍，作者以及讀者之間的聯繫。一本書有作者，有書籍名，如果直接把書歸爲一類，作者和書籍名都是其類內成員，然後用容器裝上書類，再進行相關操作，就會顯得較爲複雜。根據分而治之的思想，我們把其分爲兩類單獨表示，一類爲作者，一類爲書籍名，再將兩類設爲各自的友元類，這樣就能夠爲功能的實現減輕負擔。再根據現實生活中的模擬，每個圖書館都會有借書單，用來顯示書籍的借出情況，這裏，我們也需要對其進行抽象歸類，這樣案例的實現需要定義四類：Author，Book，Patron和CheckedOutBook。
功能的實現則可以抽象爲函數進行操作。所以最後剩下的問題就是容器的選用。這裏選用STL庫裏面的list庫，這麼做的目的是爲了讓功能實現時更加高效方便。

案例分析標準的參考程序如下：

#include "pch.h"
#include <iostream>
#include <string>
#include <list>
#include <algorithm>

using namespace std;
class Patron; //前向聲明，方便使用Patron類

class Book {
public:
    Book() { 
        patron = 0; 
    }
    bool operator== (const Book& bk) const {
        return strcmp(title,bk.title) == 0; 
    }
private:
    char *title; //書籍名
    Patron *patron;  //用來判斷書籍是否被讀者借走，方便後續書籍狀態顯示
    ostream& printBook(ostream&) const;
    friend ostream& operator<< (ostream& out, const Book& bk) {
        return bk.printBook(out);
    }
    friend class CheckedOutBook; //改變書籍狀態
    friend Patron;
    friend void includeBook();
    friend void checkOutBook();
    friend void returnBook();
};

class Author {
public:
    Author() {
    }
    bool operator== (const Author& ar) const {
        return strcmp(name,ar.name) == 0; 
    }
private:
    char *name;
    list<Book> books; //作者的書籍清單
    ostream& printAuthor(ostream&) const;
    friend ostream& operator<< (ostream& out,const Author& ar) {
        return ar.printAuthor(out);
    }
    friend void includeBook();
    friend void checkOutBook();
    friend void returnBook();
    friend class CheckedOutBook;
    friend Patron;
};

class CheckedOutBook {
public:
    CheckedOutBook(list<Author>::iterator ar = 0, 
                   list<Book>::iterator bk = 0) {
        author = ar; 
        book = bk;
    }
    bool operator== (const CheckedOutBook& bk) const {
        return strcmp(author->name,bk.author->name) == 0 &&
               strcmp(book->title,bk.book->title) == 0;
    }
private:
    list<Author>::iterator author; //作者列表迭代器
    list<Book>::iterator book; //書籍列表迭代器
    friend void checkOutBook();
    friend void returnBook();
    friend Patron;
};

class Patron {
public:
    Patron() {
    }
    bool operator== (const Patron& pn) const {
        return strcmp(name,pn.name) == 0; 
    }
private:
    char *name;
    list<CheckedOutBook> books; //借書清單
 ostream& printPatron(ostream&) const;
    friend ostream& operator<< (ostream& out, const Patron& pn) {
        return pn.printPatron(out);
    }
    friend void checkOutBook();
    friend void returnBook();
 friend Book;
};
list<Author> catalog['Z'+1]; 
list<Patron> people['Z'+1];

//輸出重載
ostream& Author::printAuthor(ostream& out) const {
    out << name << endl;
    list<Book>::const_iterator ref = books.begin();
    for ( ; ref != books.end(); ref++)
        out << *ref; // overloaded <<
 return out;
}
ostream& Book::printBook(ostream& out) const {
    out << "    * " << title;
    if (patron != 0)
        out << " - checked out to " << patron->name; // overloaded <<
    out << endl;
    return out;
}
ostream& Patron::printPatron(ostream& out) const {
    out << name;
    if (!books.empty()) {
        out << " has the following books:\n";
        list<CheckedOutBook>::const_iterator bk = books.begin();
        for ( ; bk != books.end(); bk++)
            out << "    * " << bk->author->name << ", " 
                << bk->book->title << endl;
    }
    else out << " has no books\n";
    return out;
}
template<class T>
ostream& operator<< (ostream& out, const list<T>& lst) {
    for (list<T>::const_iterator ref = lst.begin(); ref != lst.end(); ref++)
        out << *ref; // overloaded <<
    return out;
}

//輸入輸出獲取
char* getString(char *msg) {   
    char s[82], i, *destin;
    cout << msg;
    cin.get(s,80);
    while (cin.get(s[81]) && s[81] != '\n');  // discard overflowing
    destin = new char[strlen(s)+1];           // characters;
    for (i = 0; destin[i] = toupper(s[i]); i++);
    return destin;
}

//功能4
void status() {   
    register int i;
    cout << "Library has the following books:\n\n";
    for (i = 'A'; i <= 'Z'; i++)
        if (!catalog[i].empty())
            cout << catalog[i];
    cout << "\nThe following people are using the library:\n\n"; 
    for (i = 'A'; i <= 'Z'; i++)
        if (!people[i].empty())
            cout << people[i];
}
//功能1
void includeBook() {   
    Author newAuthor;
    Book gnewBook;
    newAuthor.name = getString("Enter author's name: ");
    newBook.title  = getString("Enter the title of the book: ");
    list<Author>::iterator oldAuthor = 
  find(catalog[newAuthor.name[0]].begin(),
                     catalog[newAuthor.name[0]].end(),newAuthor);
    if (oldAuthor == catalog[newAuthor.name[0]].end()) {
         newAuthor.books.push_front(newBook);
         catalog[newAuthor.name[0]].push_front(newAuthor);
    }
    else (*oldAuthor).books.push_front(newBook);
}
   //功能2
void checkOutBook() {
    Patron patron;
    Author author;
    Book book;
    list<Author>::iterator authorRef;
    list<Book>::iterator bookRef;
    patron.name = getString("Enter patron's name: ");
    while (true) { 
        author.name = getString("Enter author's name: ");
        authorRef = find(catalog[author.name[0]].begin(),
                         catalog[author.name[0]].end(),author);
        if (authorRef == catalog[author.name[0]].end())
             cout << "Misspelled author's name\n";
        else break;
    }
    while (true) { 
        book.title = getString("Enter the title of the book: ");
        bookRef = find((*authorRef).books.begin(),
                       (*authorRef).books.end(),book);
        if (bookRef == (*authorRef).books.end())
             cout << "Misspelled title\n";
        else break;
    }
    list<Patron>::iterator patronRef;
    patronRef = find(people[patron.name[0]].begin(),
                     people[patron.name[0]].end(),patron);
    CheckedOutBook checkedOutBook(authorRef,bookRef);
    if (patronRef == people[patron.name[0]].end()) { // a new patron 
         patron.books.push_front(checkedOutBook);    // in the library;
         people[patron.name[0]].push_front(patron);
         (*bookRef).patron = &*people[patron.name[0]].begin();
    }
    else {
         (*patronRef).books.push_front(checkedOutBook);
         (*bookRef).patron = &*patronRef;
    }
}
   //功能3
void returnBook() {
    Patron patron;
    Book book;
    Author author;
    list<Patron>::iterator patronRef;
    list<Book>::iterator bookRef;
    list<Author>::iterator authorRef;
    while (true) { 
        patron.name = getString("Enter patron's name: ");
        patronRef = find(people[patron.name[0]].begin(),
                         people[patron.name[0]].end(),patron);
        if (patronRef == people[patron.name[0]].end())
             cout << "Patron's name misspelled\n";
        else break;
    }
    while (true) { 
        author.name = getString("Enter author's name: ");
        authorRef = find(catalog[author.name[0]].begin(),
                         catalog[author.name[0]].end(),author);
        if (authorRef == catalog[author.name[0]].end())
             cout << "Misspelled author's name\n";
        else break;
    }
    while (true) {
        book.title = getString("Enter the title of the book: ");
        bookRef = find((*authorRef).books.begin(),
                       (*authorRef).books.end(),book);
        if (bookRef == (*authorRef).books.end())
             cout << "Misspelled title\n";
        else break;
    }
    CheckedOutBook checkedOutBook(authorRef,bookRef);
    (*bookRef).patron = 0;
    (*patronRef).books.remove(checkedOutBook);
}

int menu() {
    int option;
    cout << "\nEnter one of the following options:\n"
         << "1. Include a book in the catalog\n2. Check out a book\n"
         << "3. Return a book\n4. Status\n5. Exit\n"
         << "Your option? ";
    cin >> option;
    cin.get();         // discard '\n';
    return option;
}
int main() {
    while (true)
        switch (menu()) {
            case 1: includeBook();  break;
            case 2: checkOutBook(); break;
            case 3: returnBook();   break;
            case 4: status();       break;
            case 5: return 0;
            default: cout << "Wrong option, try again: ";
        }
    return 0;
}