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。

好的，我理解了您的要求。以下是实现您描述的Library类的代码（包括Book、Patron和Date类的实现）：

#include <iostream>
#include <vector>
#include <string>

class Date {
public:
    Date(int year, int month, int day) : year_(year), month_(month), day_(day) {}
    int year() const { return year_; }
    int month() const { return month_; }
    int day() const { return day_; }
private:
    int year_;
    int month_;
    int day_;
};

class Book {
public:
    Book(const std::string& title, const std::string& author, int isbn, int num_copies)
        : title_(title), author_(author), isbn_(isbn), num_copies_(num_copies) {}
    std::string title() const { return title_; }
    std::string author() const { return author_; }
    int isbn() const { return isbn_; }
    int num_copies() const { return num_copies_; }
    void set_num_copies(int num_copies) { num_copies_ = num_copies; }
private:
    std::string title_;
    std::string author_;
    int isbn_;
    int num_copies_;
};

class Patron {
public:
    Patron(const std::string& name, int library_card_num, double fees_owed)
        : name_(name), library_card_num_(library_card_num), fees_owed_(fees_owed) {}
    std::string name() const { return name_; }
    int library_card_num() const { return library_card_num_; }
    double fees_owed() const { return fees_owed_; }
    void set_fees_owed(double fees_owed) { fees_owed_ = fees_owed; }
private:
    std::string name_;
    int library_card_num_;
    double fees_owed_;
};

struct Transaction {
    Book book;
    Patron patron;
    Date date;
};

class Library {
public:
    void add_book(const Book& book) {
        books_.push_back(book);
    }

    void add_patron(const Patron& patron) {
        patrons_.push_back(patron);
    }

    void lend_book(const Book& book, const Patron& patron, const Date& date) {
        bool book_found = false;
        bool patron_found = false;
        for (Book& b : books_) {
            if (b.isbn() == book.isbn()) {
                if (b.num_copies() > 0) {
                    b.set_num_copies(b.num_copies() - 1);
                    book_found = true;
                    break;
                }
            }
        }
        for (Patron& p : patrons_) {
            if (p.library_card_num() == patron.library_card_num()) {
                if (p.fees_owed() == 0) {
                    patron_found = true;
                    break;
                }
            }
        }
        if (!book_found) {
            std::cout << "Error: book not found in library." << std::endl;
        }
        if (!patron_found) {
            std::cout << "Error: patron not found in library or has unpaid fees." << std::endl;
        }
        if (book_found && patron_found) {
            transactions_.push_back({book, patron, date});
        }
    }

    std::vector<std::string> get_overdue_patrons() const {
        std::vector<std::string> overdue_patrons;
        for (const Patron& p : patrons_) {
            if (p.fees_owed() > 0) {
                overdue_patrons.push_back(p.name());
            }
        }
        return overdue_patrons;
    }

private:
    std::vector<Book> books_;
    std::vector<Patron> patrons_;
    std::vector<Transaction> transactions_;
};

您可以使用上述代码来创建一个Library对象，并调用其成员函数来添加图书、添加读者和借阅图书。例如：

int main() {
    Library library;

    // Add books
    Book book1("The Catcher in the Rye", "J.D. Salinger", 12345, 2);
    Book book2("To Kill a Mockingbird", "Harper Lee", 67890, 1);
    library.add_book(book1);
    library.add_book(book2);

    // Add patrons
    Patron patron1("Alice", 123, 0);
    Patron patron2("Bob", 456, 10.5);
    library.add_patron(patron1);
    library.add_patron(patron2);

    // Lend books
    Date today(2022, 11, 1);
    library.lend_book(book1, patron1, today); // should succeed
    library.lend_book(book1, patron2, today); // should fail (book already lent out)
    library.lend_book(book2, patron2, today); // should fail (patron has unpaid fees)

    // Get overdue patrons
    std::vector<std::string> overdue_patrons = library.get_overdue_patrons();
    std::cout << "Overdue patrons: ";
    for (const std::string& name : overdue_patrons) {
        std::cout << name << " ";
    }
    std::cout << std::endl;

    return 0;
}

输出应该为：

Error: patron not found in library or has unpaid fees.
Error: patron not found in library or has unpaid fees.
Overdue patrons: Bob