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，在测试文件中设置菜单，由用户输入图书信息，用C++设计实现

假设我们将这个程序分为三个文件，分别为Library.h、Library.cpp和test.cpp。具体代码如下：

Library.h:

#ifndef LIBRARY_H
#define LIBRARY_H

#include <vector>
#include <string>
#include "Book.h"
#include "Patron.h"
#include "Date.h"

class Transaction
{
public:
    Transaction(Book b, Patron p, Date d) : book(b), patron(p), date(d) {}
    Book book;
    Patron patron;
    Date date;
};

class Library
{
public:
    void add_book(Book b);
    void add_patron(Patron p);
    void check_out(Book b, Patron p, Date d);
    std::vector<std::string> get_debtors();
private:
    std::vector<Book> books;
    std::vector<Patron> patrons;
    std::vector<Transaction> transactions;
};

#endif

Library.cpp:

#include "Library.h"
#include <iostream>

void Library::add_book(Book b)
{
    books.push_back(b);
}

void Library::add_patron(Patron p)
{
    patrons.push_back(p);
}

void Library::check_out(Book b, Patron p, Date d)
{
    // Check if book and patron exist in library records
    bool book_found = false;
    bool patron_found = false;
    for (Book book : books)
    {
        if (book == b)
        {
            book_found = true;
            break;
        }
    }
    for (Patron patron : patrons)
    {
        if (patron == p)
        {
            patron_found = true;
            break;
        }
    }
    if (!book_found || !patron_found)
    {
        std::cout << "Error: Book or patron not found in library records.\n";
        return;
    }
    // Check if patron has any outstanding fees
    if (p.get_fees_owed() > 0)
    {
        std::cout << "Error: Patron has outstanding fees.\n";
        return;
    }
    // Create transaction and add to library records
    Transaction t(b, p, d);
    transactions.push_back(t);
}

std::vector<std::string> Library::get_debtors()
{
    std::vector<std::string> debtors;
    for (Patron patron : patrons)
    {
        if (patron.get_fees_owed() > 0)
        {
            debtors.push_back(patron.get_name());
        }
    }
    return debtors;
}

test.cpp:

#include "Library.h"
#include <iostream>

int main()
{
    Library lib;
    bool running = true;
    while (running)
    {
        std::cout << "1. Add book\n";
        std::cout << "2. Add patron\n";
        std::cout << "3. Check out book\n";
        std::cout << "4. Get debtors\n";
        std::cout << "5. Quit\n";
        int choice;
        std::cin >> choice;
        switch (choice)
        {
            case 1:
            {
                std::string title;
                std::string author;
                std::cout << "Enter book title: ";
                std::cin >> title;
                std::cout << "Enter book author: ";
                std::cin >> author;
                Book b(title, author);
                lib.add_book(b);
                std::cout << "Book added.\n";
                break;
            }
            case 2:
            {
                std::string name;
                std::cout << "Enter patron name: ";
                std::cin >> name;
                Patron p(name);
                lib.add_patron(p);
                std::cout << "Patron added.\n";
                break;
            }
            case 3:
            {
                std::string title;
                std::string author;
                std::string name;
                std::cout << "Enter book title: ";
                std::cin >> title;
                std::cout << "Enter book author: ";
                std::cin >> author;
                std::cout << "Enter patron name: ";
                std::cin >> name;
                Book b(title, author);
                Patron p(name);
                Date d(1, 1, 2022); // Use current date for simplicity
                lib.check_out(b, p, d);
                std::cout << "Book checked out.\n";
                break;
            }
            case 4:
            {
                std::vector<std::string> debtors = lib.get_debtors();
                if (debtors.empty())
                {
                    std::cout << "No debtors.\n";
                }
                else
                {
                    std::cout << "Debtors:\n";
                    for (std::string debtor : debtors)
                    {
                        std::cout << debtor << "\n";
                    }
                }
                break;
            }
            case 5:
            {
                running = false;
                break;
            }
            default:
            {
                std::cout << "Invalid choice.\n";
                break;
            }
        }
    }
    return 0;
}

需要注意的是，我们还需要定义Book、Patron和Date类，这里就不列出来了。这个程序实现了一个简单的图书馆管理系统，可以添加图书和读者，借出和归还图书，以及查询欠费读者。