Implementing custom iterators in C++11

Question

I am reading a book, and I started the Templates chapter. I have already read the chapter about iterators. For practicing, I am trying to implement a doubly-linked class using templates. Below is the header file of my class.

template<typename T>
class list
{
private:
    T* first_node;
    T* last_node;
    struct node_structure
    {
        T* previous;
        T* next;
        T content;
    };
public:
    list();
    void push_back( T& item );
    void push_front( T& item );
    void remove( size_t index );
    size_t size() const;
    T& get_node( size_t index );
    void clear();
    ~list();
};

I could have set two members:

T* begin();
T* end();

...to act like very basic iterators, but they are not iterators actually. So, I have a question:

How do I implement the custom iterator class to be used with all the arithmetic operations and to have begin() , cbegin() , end() , and cend() ?

_{(I am aware that creating a list class will not be as efficient as std::list , but I am doing this just for sake of practicing.)}

Answer 1

If you have a look at an implementation of <iterator> , you'll find something like __normal_iterator , which looks like:

template<typename I>
class iter
{
protected:
    I i;

    using tr = iterator_traits<I>;

public:
    using iterator_type     = I;
    using iterator_category = typename tr::iterator_category;
    using value_type        = typename tr::value_type;
    using difference_type   = typename tr::difference_type;
    using reference         = typename tr::reference;
    using pointer           = typename tr::pointer;

    iter() : i(I()) { }

    explicit iter(const I& i) : i(i) { }

    // Forward iterator requirements
    reference operator*() const { return *i; }

    pointer operator->() const { return i; }

    iter& operator++() { ++i; return *this; }

    iter operator++(int) { return iter(i++); }

    // Bidirectional iterator requirements
    iter& operator--() { --i; return *this; }

    iter operator--(int) { return iter(i--); }

    // Random access iterator requirements
    reference operator[](const difference_type& n) const { return i[n]; }

    iter& operator+=(const difference_type& n) { i += n; return *this; }

    iter operator+(const difference_type& n) const { return iter(i + n); }

    iter& operator-=(const difference_type& n) { i -= n; return *this; }

    iter operator-(const difference_type& n) const { return iter(i - n); }

    const I& base() const { return i; }
};

This is supposed to work on an ordinary pointer or iterator. All you have to do is use this as a template and adjust to what is needed by your custom container. If T is your value_type , then member functions normally return

• begin() -> iter<T*>
• cbegin() -> iter<const T*>
• rbegin() -> std::reverse_iterator<iter<T*> >
• crbegin() -> std::reverse_iterator<iter<const T*> >

However, since you have your node_structure , this is not entirely true and you need to elaborate a bit more.