Can't understand class constructor

Question

I have a class.

class Books
{
private:
    int m_books;
public:
    Books(int books=0)
    {
        m_books = books;
    }

    Books(const Books &source)  //Here is what I don't understand.
    {
        m_books = source.m_books;
    }
};

I can't understand why it has to be Books(const Books &source), and not Books(const Books source).

Answer 1

When you have

Books(const Books &source)

the source is passed by reference. When you have

Books(const Books source)

it would have been passed by value. But to pass by value you are the copy constructor. So to avoid an infinite recursion, the copy constructor must accept a reference.