c++ - 动态分配的对象未初始化(Dynamic allocated object not initializing)

Question

I'm trying to prepare a library managament system for my school homework.

(我正在尝试为我的学校作业准备一个图书馆管理系统。)

I wrote the code but in "bool Library::add_book(const Book&)" function, I can not initialize book to dynamic array.

(我编写了代码，但是在“ bool Library :: add_book（const Book＆）”函数中，我无法将book初始化为动态数组。)

I overloaded = operator to initialize "Book" class and when i declare a "Book" object in main, it works.

(我重载了=运算符以初始化“ Book”类，当我在main中声明“ Book”对象时，它起作用了。)

But for dynamic allocated array, it is not working.

(但是对于动态分配的数组，它不起作用。)

Thanks everyone for help...

(谢谢大家的帮助...)

#include <iostream>

class Author{
    public:
        std::string name;
        Author(){}
        Author(const std::string& name){
            this -> name = name;
        }
};

class Book{
    private:
        std::string _title, _isbn;
        int _edition;
        Author *_author = new Author;
    public:
        Book(){
            *_author = Author("NULL");
            _isbn = "INVALID";
        }
        Book(const std::string& title,
            const std::string& isbn,
            const std::string& author,
            int edition){
                _title = title;
                _isbn = isbn;
                _edition = edition;
                *_author = Author(author);
            }
        bool query_isbn(const std::string&);
        Book& operator=(const Book&);
        friend std::ostream &operator <<(std::ostream &, Book&);
};

class Library{
    private:
        Book *_book = nullptr;
        int *_numberOfBorrows = nullptr;
        int _numberOfBook, _maxNumberBook;
    public:
        Library(int maxNumberBook){
            _maxNumberBook = maxNumberBook;
            _book = new Book[_maxNumberBook];
            _numberOfBorrows = new int[_maxNumberBook];
            for(int i = 0; i < _maxNumberBook; i++){
                _book[i] = Book();
                _numberOfBorrows[i] = 0;
            }
        }
        ~Library(){
            delete []_book;
            _book = NULL;
            delete []_numberOfBorrows;
            _numberOfBorrows = NULL;
        }
        bool add_book(const Book&);
        void operator+= (const Book&);
        const Book& borrow_book(const std::string&);
        friend std::ostream& operator <<(std::ostream&, Library&);
};

bool Book::query_isbn(const std::string& isbn){
    if(_isbn == isbn)
        return true;
    else
        return false;
}

Book& Book::operator=(const Book& book){
    Book ret;
    ret._author -> name = book._author -> name;
    ret._edition = book._edition;
    ret._isbn = book._isbn;
    ret._title = book._title;
    return ret;
}

std::ostream &operator << (std::ostream& output, Book& obj){
    output << obj._author -> name << ", " << obj._title << ", " << obj._isbn << std::endl;
    return output;
}

bool Library::add_book(const Book& book){
    for(int i = 0; i < _maxNumberBook; i++){
        if(_book[i].query_isbn("INVALID")){
            _book[i] = book;
            _numberOfBook++;
            return true;
        }
    }
    return false;
}

void Library::operator+= (const Book& book){
    add_book(book);
}

const Book& Library::borrow_book(const std::string& isbn){
    for(int i = 0; i < _maxNumberBook; i++){
        if(_book[i].query_isbn(isbn)){
            _numberOfBorrows[i] = _numberOfBorrows[i] + 1;
            return _book[i];
        }
    }
    std::cout << "ISBN with " << isbn << " does not exist" << std::endl;
}

std::ostream& operator <<(std::ostream& output, Library& lib){
    for(int i = 0; i < lib._maxNumberBook; i++){
        output << lib._book[i];
        output << "Borrowed: " << lib._numberOfBorrows[i] << std::endl;
        for(int i = 0; i < 38; i++)
            output << '=';
            std::cout << std::endl;
    }
    return output;
}

int main(int argc, char** argv){
    Library lib(3);
    Book obj1("The 8051 Microcontroller & Embedded Systems", "1234-456789123", "Mazidi", 1),
        obj2("Fundamentals of Database Systems", "7899-456456123", "Elmasri", 2),
        obj3("Electric Circuits", "1478-258963258", "Nilsson", 3);
    lib.add_book(obj1);
    lib.add_book(obj2);
    lib.add_book(obj3);
    lib.borrow_book("6584-258963258");
    lib.borrow_book("1234-456789123");
    lib.borrow_book("7899-456456123");
    lib.borrow_book("1478-258963258");
    std::cout << lib;



    return 0;
}

I want it to output book properties, but it outputs same as below...

(我希望它输出书籍属性，但其输出与以下相同...)

ISBN with 6584-258963258 does not exist
ISBN with 1234-456789123 does not exist
ISBN with 7899-456456123 does not exist
ISBN with 1478-258963258 does not exist
NULL, , INVALID
Borrowed: 0
======================================
NULL, , INVALID
Borrowed: 0
======================================
NULL, , INVALID
Borrowed: 0
======================================

  ask by bacimen translate from so

Answer 1

等待大神答复