need help for 2d array in class (c++)

Question

In this situation, I want to delete book, but I try declare this code, its not working.

class Library {
private:
    Book **books;
    int counter;
public:
    Library() {
        books = NULL;
        counter = 0;
    }
    void Add(INPUT &tmp) {
        books = new Book*[counter];
        ++counter;
    }
    void Delete() {
        --counter;
        delete[] books[counter];
        books[counter] = NULL;
    }
    int getCounter() {
        return this->counter;
    }
    ~Library() {
        delete[] books;
    }
};

what does not work? If you get compiler errors please include them in the question. If it crashes during runtime please let us know (and note that the code you show here is just a class definition, so there is nothing to run here). — 463035818_is_not_an_ai
– 463035818_is_not_an_ai, Commented Sep 7, 2018 at 13:37
1) Please explain what, exactly, doesn't work for you. 2) Please provide minimal reproducible example. — Algirdas Preidžius
– Algirdas Preidžius, Commented Sep 7, 2018 at 13:37
@MustafayevTural Did you try stepping through your code with a debugger? — Algirdas Preidžius
– Algirdas Preidžius, Commented Sep 7, 2018 at 13:43
@MustafayevTural This doesn't answer my question. Let me expand, on my initial comment, about what does it mean to "step through your code with a debugger": Did you try stepping through your code, line-by-line, while investigating the values of local variables, at each step, while, simultaneously, comparing them with your expectations, to see the point where the behavior of your program, doesn't match your expectations? — Algirdas Preidžius
– Algirdas Preidžius, Commented Sep 7, 2018 at 13:52
@john Well, we don't know what Book is in OP's code, nor how it should be used. They may need a std::vector<std::unique_ptr<Book>> for what we know. — Bob__
– Bob__, Commented Sep 7, 2018 at 15:00

Lightness Races in Orbit · Accepted Answer · 2018-09-07 15:50:51Z

3

Before you get deleting to work, you need to get adding right.

In addition to what Jeffrey said, your Add function probably doesn't work correctly due to an "out by one" error. In the first call you will have books = new Book*[0];. Allocating a zero sized array is legal (see here) but you will not be able to store anything in it.

If you can use a std::vector it will make your code much simpler and less error prone.

class Library {
private:
    std::vector<Book> books;
    // no need for counter, std::vector has size()
public:
    // no need for a constructor, the default constructor
    // will correctly construct 'books'
    void Add(INPUT &tmp) {
        // not sure how you convert 'INPUT' to 'Book'
        books.push_back(tmp);
        // this handles all of the memory management for you
    }
    void Delete() {
        // you need to ensure that books is not empty
        books.pop_back();
    }
    int getCounter() {
        return books.size();
    }
    // no need for a destructor, the default one will
    // do everything
};

If you need two dimensions, then the code is similar but will use a vector of vector.

edited Sep 7, 2018 at 15:50

Lightness Races in Orbit

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answered Sep 7, 2018 at 14:01

Paul Floyd

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4 Comments

Eljay Over a year ago

Perhaps Book book(tmp); books.push_back(book);.

Bob__ Over a year ago

@Eljay I raise with books.emplace_back(tmp);...

Eljay Over a year ago

@Bob • but tmp is not a Book, it is an INPUT.

Bob__ Over a year ago

@Eljay Exactly. In your snippet you suppose there's a constructor of Book accepting an INPUT and what emplace_back does is to construct the element in-place using the provided argument, without a temporary.

Jeffrey · Accepted Answer · 2018-09-07 13:42:55Z

Book **books;

is a pointer to a pointer to a book. It is the old style way of having a list of books (as pointers) or a list of list of books.

Library() {
    books = NULL;
    counter = 0;
}

This will create an empty library. No books.

void Add(INPUT &tmp) {
    books = new Book*[counter];
    ++counter;
}

First thing to notice is that you are not using the tmp book. So you probably won't succeed in storing it anywhere, without using it.

Second thing is that books = new Book*[counter]; allocates a library of books. Space for storing some books. You probably should do that in the constructor. If you do it there, every time you try to add a book, you'll lose all the others, and you'll also leak memory.

There's two possibilities here. You have an old-timer C++ professor, and you'll need to learn about pointers and pointers to pointers, and new, delete. Or you could learn about std::vectors and smart-pointers. This would be a better idea, but I can't tell you how well it will be received in your class.

Also, please state what INPUT is defined as.

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