How to correctly implement custom iterators and const_iterators?

I came across this post and was surprised that a simple method is not really mentioned here. Using a pointer to the value like how std::iterator describes is obviously a very generic approach. But you might be able to get away with something much simpler. Of course this is a simplistic approach and might not always be sufficient, but in case it is, I am posting it for the next reader.

Most probably the underlying type in your class is an STL container which already has defined the iterators for you. If that is the case, you can simply use their defined iterators and don't really need to make your own.

Here is an example:

class Foo {

  std::vector<int>::iterator begin() { return data.begin(); }
  std::vector<int>::iterator end() { return data.end(); }

  std::vector<int>::const_iterator begin() const { return data.begin(); }
  std::vector<int>::const_iterator end() const { return data.end(); }


private:
  std::vector<int> data

};

i'm interested to know how correct this is, but seems to work as a roll-your-own iterator to internal data storage

template<typename T>
struct iterator_type
{
    using self_type             = iterator_type;
    using iterator_category     = std::random_access_iterator_tag;
    using difference_type       = std::ptrdiff_t;
    using value_type            = std::remove_cv_t<T>;
    using pointer               = T*;
    using reference             = T&;

    iterator_type( pointer ptr ) noexcept
        : _ptr{ ptr }
    {}

    reference operator*() noexcept { return *_ptr; }
    pointer operator->() noexcept { return _ptr; }

    self_type operator++() noexcept { ++_ptr; return *this; }
    self_type operator++(int) noexcept { self_type tmp = *this; ++_ptr; return tmp; }

    self_type operator--() noexcept { --_ptr; return *this; }
    self_type operator--(int) noexcept { self_type tmp = *this; --_ptr; return tmp; }

    bool operator==( const self_type &other ) const noexcept { return _ptr == other._ptr; }
    bool operator!=( const self_type &other ) const noexcept { return _ptr != other._ptr; }

private:
    pointer _ptr;
};


template<typename T>
using const_iterator_type = iterator_type<std::add_const_t<T>>;

Then i just add these to my class, and seems to work as expected.

template<typename T>
class Container
{
public:
    using iterator               = iterator_type<T>;
    using const_iterator         = const_iterator_type<T>;
    using reverse_iterator       = std::reverse_iterator<iterator>;
    using const_reverse_iterator = std::reverse_iterator<const_iterator>;

...

    iterator begin() { return _begin; }
    iterator end() { return _begin + _size; }

    const_iterator cbegin() const { return _begin; }
    const_iterator cend() const { return _begin + _size; }

    reverse_iterator rbegin() { return reverse_iterator(_begin + _size); }
    reverse_iterator rend() { return reverse_iterator(_begin); }

    const_reverse_iterator crbegin() const { return const_reverse_iterator(_begin + _size); }
    const_reverse_iterator crend() const { return const_reverse_iterator(_begin); }

private:
    T*         _begin;
    size_t     _size;
    size_t     _capacity;
};

the only thing is that to make it with the std::cbegin(), std::rcbegin(), std::cend() and std::rcend() functions I have to extend the std namespace:

namespace std
{
    template<typename T>
    typename Container<T>::const_iterator cbegin( Container<T> &c ) { return c.cbegin(); }

    template<typename T>
    typename Container<T>::const_iterator cend( Container<T> &c ) { return c.cend(); }

    template<typename T>
    typename Container<T>::const_reverse_iterator crbegin( Container<T> &c ) { return c.crbegin(); }

    template<typename T>
    typename Container<T>::const_reverse_iterator crend( Container<T> &c ) { return c.crend(); }
}

Check this below code, it works

#define MAX_BYTE_RANGE 255

template <typename T>
class string
{
public:
    typedef char *pointer;
    typedef const char *const_pointer;
    typedef __gnu_cxx::__normal_iterator<pointer, string> iterator;
    typedef __gnu_cxx::__normal_iterator<const_pointer, string> const_iterator;

    string() : length(0)
    {
    }
    size_t size() const
    {
        return length;
    }
    void operator=(const_pointer value)
    {
        if (value == nullptr)
            throw std::invalid_argument("value cannot be null");
        auto count = strlen(value);
        if (count > 0)
            _M_copy(value, count);
    }
    void operator=(const string &value)
    {
        if (value.length != 0)
            _M_copy(value.buf, value.length);
    }
    iterator begin()
    {
        return iterator(buf);
    }
    iterator end()
    {
        return iterator(buf + length);
    }
    const_iterator begin() const
    {
        return const_iterator(buf);
    }
    const_iterator end() const
    {
        return const_iterator(buf + length);
    }
    const_pointer c_str() const
    {
        return buf;
    }
    ~string()
    {
    }

private:
    unsigned char length;
    T buf[MAX_BYTE_RANGE];

    void _M_copy(const_pointer value, size_t count)
    {
        memcpy(buf, value, count);
        length = count;
    }
};