How to create an instance of a struct in C++?

Question

The issue is an "Access violation writing location 0x00000000" error message right after I initialize the bk variable to NULL. My guess is I should reserve memory space in advance to assign NULL ( something like Book bk = new Book(); ) but I have not been able to figure out how to do it in C++ till now.

Book.h

#ifndef Book_H
#define Book_H

struct _book;
typedef _book* Book;

Book CreateBook(unsigned int pageNum);

Book.cpp

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

#ifndef Book_CPP
#define Book_CPP

using namespace std;

struct _book
{
    int pageNum;
};

Book CreateBook( unsigned int pageNum){

    Book bk = NULL; 

    bk->pageNum = pageNum;

    return bk;
};

Answer 1

You're assigning bk to NULL and then trying to access a member of it. That's the same as a null pointer in Java, and what you're doing would typically raise a NullPointerException (thanks from the comments). If you want to create a pointer to your struct, you need to use operator new:

bk = new _book;
// ...
return bk;

and then make sure you call delete on the pointer when you're done with it.

I would advise against using pointers here, though. Unlike other languages, C++ lets you create objects by value. It also allows for references and pointers, but only use pointers when you absolutely must. If you simply want to create a book object with a given pageNum, you should create a constructor while you're at it:

struct _book {
    int pageNum;
    _book(int n) : pageNum(n) // Create an object of type _book.
    {
    }
};

and then you can invoke it like

_book myBook(5); // myBook.pageNum == 5

If you're new to C++, please get yourself a good book on it. It's not just a low-level language, and it's also not just an OOP language. It's a multi-paradigm swiss army knife language.

Answer 2

This is what you need:

Book CreateBook( unsigned int pageNum){

    Book bk = new _book();

    bk->pageNum = pageNum;

    return bk;
}

your bk was null and you cannot access pageNum when the pointer is null.

And don't forget to call delete on bk when you are done using it.