创建一个 Library类，包含一个 Book向量和一个Patron向量。定义一个名为Transaction 的 struet，包含一个Book对象、一个Patron对象和一个本章中定义的Date对象，表示借阅记录。在 Library 类中定义一个Transaction向量。定义向图书馆添加图书、添加读者以及借出书籍的函数。当一个读者借出一本书时,保证Library对象中有此读者和这本书的记录,否则报告错误。然后检查读者是否欠费，如果欠费就报告一个错误,否则创建一个Transaction对象，将其放人Transaction向量中。定义一个返回包含所有欠费读者姓名的向量的函数。多文件程序，按文件给出代码，测试文件用户手动录入信息的操作，用户可以手动添加信息

我们先定义一个 Book 类和一个 Patron 类，然后再定义 Library 类和 Transaction 类。

Book 类：

class Book {
public:
    Book(std::string isbn, std::string title, std::string author, int year);
    std::string getISBN() const;
    std::string getTitle() const;
    std::string getAuthor() const;
    int getYear() const;
private:
    std::string isbn;
    std::string title;
    std::string author;
    int year;
};

Patron 类：

class Patron {
public:
    Patron(std::string name, std::string card_number);
    std::string getName() const;
    std::string getCardNumber() const;
    double getFees() const;
    void addFee(double fee);
private:
    std::string name;
    std::string card_number;
    double fees;
};

Date 类：

class Date {
public:
    Date(int year, int month, int day);
    int getYear() const;
    int getMonth() const;
    int getDay() const;
private:
    int year;
    int month;
    int day;
};

Transaction 类：

class Transaction {
public:
    Transaction(Book book, Patron patron, Date date);
    Book getBook() const;
    Patron getPatron() const;
    Date getDate() const;
private:
    Book book;
    Patron patron;
    Date date;
};

Library 类：

class Library {
public:
    void addBook(Book book);
    void addPatron(Patron patron);
    void checkout(Book book, Patron patron, Date date);
    std::vector<std::string> getDebtors() const;
private:
    std::vector<Book> books;
    std::vector<Patron> patrons;
    std::vector<Transaction> transactions;
};

接下来是实现这些类的方法：

Book 类：

Book::Book(std::string isbn, std::string title, std::string author, int year)
    : isbn(isbn), title(title), author(author), year(year) {}

std::string Book::getISBN() const {
    return isbn;
}

std::string Book::getTitle() const {
    return title;
}

std::string Book::getAuthor() const {
    return author;
}

int Book::getYear() const {
    return year;
}

Patron 类：

Patron::Patron(std::string name, std::string card_number)
    : name(name), card_number(card_number), fees(0) {}

std::string Patron::getName() const {
    return name;
}

std::string Patron::getCardNumber() const {
    return card_number;
}

double Patron::getFees() const {
    return fees;
}

void Patron::addFee(double fee) {
    fees += fee;
}

Date 类：

Date::Date(int year, int month, int day)
    : year(year), month(month), day(day) {}

int Date::getYear() const {
    return year;
}

int Date::getMonth() const {
    return month;
}

int Date::getDay() const {
    return day;
}

Transaction 类：

Transaction::Transaction(Book book, Patron patron, Date date)
    : book(book), patron(patron), date(date) {}

Book Transaction::getBook() const {
    return book;
}

Patron Transaction::getPatron() const {
    return patron;
}

Date Transaction::getDate() const {
    return date;
}

Library 类：

void Library::addBook(Book book) {
    books.push_back(book);
}

void Library::addPatron(Patron patron) {
    patrons.push_back(patron);
}

void Library::checkout(Book book, Patron patron, Date date) {
    // check if book and patron are in the library
    bool book_found = false;
    for (auto b : books) {
        if (b.getISBN() == book.getISBN()) {
            book_found = true;
            break;
        }
    }
    if (!book_found) {
        std::cout << "Error: book not found in library." << std::endl;
        return;
    }
    bool patron_found = false;
    for (auto p : patrons) {
        if (p.getCardNumber() == patron.getCardNumber()) {
            patron_found = true;
            break;
        }
    }
    if (!patron_found) {
        std::cout << "Error: patron not found in library." << std::endl;
        return;
    }

    // check if patron owes fees
    if (patron.getFees() > 0) {
        std::cout << "Error: patron owes fees." << std::endl;
        return;
    }

    // add transaction
    Transaction transaction(book, patron, date);
    transactions.push_back(transaction);
}

std::vector<std::string> Library::getDebtors() const {
    std::vector<std::string> debtors;
    for (auto patron : patrons) {
        if (patron.getFees() > 0) {
            debtors.push_back(patron.getName());
        }
    }
    return debtors;
}

在测试文件中，我们可以手动录入图书馆的信息，包括图书、读者和借阅记录，然后查询所有欠费读者的姓名。

测试文件：

#include <iostream>
#include "Book.h"
#include "Patron.h"
#include "Date.h"
#include "Transaction.h"
#include "Library.h"

int main() {
    Library library;
    std::cout << "Enter books (ISBN, title, author, year):" << std::endl;
    std::string line;
    while (std::getline(std::cin, line) && !line.empty()) {
        std::stringstream ss(line);
        std::string isbn, title, author;
        int year;
        ss >> isbn >> title >> author >> year;
        Book book(isbn, title, author, year);
        library.addBook(book);
    }
    std::cout << "Enter patrons (name, card number):" << std::endl;
    while (std::getline(std::cin, line) && !line.empty()) {
        std::stringstream ss(line);
        std::string name, card_number;
        ss >> name >> card_number;
        Patron patron(name, card_number);
        library.addPatron(patron);
    }
    std::cout << "Enter transactions (ISBN, card number, year, month, day):" << std::endl;
    while (std::getline(std::cin, line) && !line.empty()) {
        std::stringstream ss(line);
        std::string isbn, card_number;
        int year, month, day;
        ss >> isbn >> card_number >> year >> month >> day;
        Book book(isbn, "", "", 0);
        Patron patron("", card_number);
        Date date(year, month, day);
        library.checkout(book, patron, date);
    }
    std::vector<std::string> debtors = library.getDebtors();
    if (debtors.empty()) {
        std::cout << "No debtors." << std::endl;
    } else {
        std::cout << "Debtors:" << std::endl;
        for (auto debtor : debtors) {
            std::cout << debtor << std::endl;
        }
    }
    return 0;
}