Question

I have a class named book, that contains books.

#ifndef BOOK_H
#include<string>
#include<vector>
#include<iostream>
#define BOOK_H
class book
{
public:
    std::string Author, Title;
    int Year;
    book(){}
    book(std::string author, std::string title, int year);
    ~book(){}
    void add_book();
    std::vector<book*>library;
};  
#endif

book.cpp file

#include "book.h"

book::book(std::string author, std::string title, int year)
:Author(author), Title(title), Year(year){} 

void book::add_book()
{
    int y;
    std::string a, t;
    std::cin>>a;
    std::cin>>t;
    std::cin>>y;
    library.push_back(new book(a, t, y));
}

But when I want to add a new book to the library, I'm getting a segmentation fault during the push_back of the new object in main.cpp file

#include "book.h"

int main()
{
    book* ptr;
    ptr->add_book();
    return 0;
}

Could someone explain to me what the problem is?

I'm new in OOP, and though I've read a lot of posts here I still can't find a solution.

Answer 1

You don't initialise the ptr in main;

int main()
{
    book* ptr = new book(); // initialize it here
    ptr->add_book();
    return 0;
}

This will fix the segfault, however, I'm not sure of the logic associated with the sample code, and possible memory leaks.

I would look to separate the library from the book type for better composition and avoid the heap allocation;

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

class book
{
public:
    std::string Author, Title;
    int Year;
    book(){}
    book(std::string author, std::string title, int year) : Author(author), Title(title), Year(year) {} 
    ~book(){}
};  

struct library {
    std::vector<book> books_;
    void add_book(const book& value) { books_.push_back(value); }
};

int main()
{
    library lib;
    int y;
    std::string a, t;
    std::cin>>a;
    std::cin>>t;
    std::cin>>y;
    lib.add_book(book(a, t, y));
    return 0;
}

Answer 2

Well, here:

book* ptr;
ptr->add_book();

ptr is unassigned, so its use causes seg fault. You should assign it before use:

ptr = new book();

this will cause memory leak so I suggest (if you like dynamic allocation):

auto ptr = std::make_unique<book>();
ptr->add_book();

but, why you need pointer anyway, this:

book bk;
bk.add_book();

will work the same way.

On the other hand, why your bookclass keeps a vector of book instances? You need a library class which will keep a vector of book-s.

Answer 3

You will have to make an object of class Book before using its methods.

Book *ptr = new Book();

std::vector segmentation fault during push_back

3 个答案: