How to correctly implement custom iterators and const_iterators?

Question

I have a custom container class for which I'd like to write the iterator and const_iterator classes.

I never did this before and I failed to find an appropriate how-to. What are the guidelines regarding iterator creation, and what should I be aware of ?

I'd also like to avoid code duplication (I feel that const_iterator and iterator share many things; should one subclass the other ?).

Foot note: I'm pretty sure Boost has something to ease this but I can't use it here, for many stupid reasons.

Answer 1

• Choose type of iterator which fits your container: input, output, forward etc.
• Use base iterator classes from standard library. For example, std::iterator with random_access_iterator_tag.These base classes define all type definitions required by STL and do other work.

• To avoid code duplication iterator class should be a template class and be parametrized by "value type", "pointer type", "reference type" or all of them (depends on implementation). For example:

  // iterator class is parametrized by pointer type
  template <typename PointerType> class MyIterator {
      // iterator class definition goes here
  };
  
  typedef MyIterator<int*> iterator_type;
  typedef MyIterator<const int*> const_iterator_type;
  

  Notice iterator_type and const_iterator_type type definitions: they are types for your non-const and const iterators.

See Also: standard library reference

EDIT: std::iterator is deprecated since C++17. See a relating discussion here.

Answer 2

I'm going to show you how you can easily define iterators for your custom containers, but just in case I have created a c++11 library that allows you to easily create custom iterators with custom behavior for any type of container, contiguous or non-contiguous.

You can find it on Github

Here are the simple steps to creating and using custom iterators:

1. Create your "custom iterator" class.
2. Define typedefs in your "custom container" class.
   • e.g. typedef blRawIterator< Type > iterator;
   • e.g. typedef blRawIterator< const Type > const_iterator;
3. Define "begin" and "end" functions
   • e.g. iterator begin(){return iterator(&m_data[0]);};
   • e.g. const_iterator cbegin()const{return const_iterator(&m_data[0]);};
4. We're Done!!!

Finally, onto defining our custom iterator classes:

NOTE: When defining custom iterators, we derive from the standard iterator categories to let STL algorithms know the type of iterator we've made.

In this example, I define a random access iterator and a reverse random access iterator:

1. //-------------------------------------------------------------------
   // Raw iterator with random access
   //-------------------------------------------------------------------
   template<typename blDataType>
   class blRawIterator
   {
   public:
   
       using iterator_category = std::random_access_iterator_tag;
       using value_type = blDataType;
       using difference_type = std::ptrdiff_t;
       using pointer = blDataType*;
       using reference = blDataType&;
   
   public:
   
       blRawIterator(blDataType* ptr = nullptr){m_ptr = ptr;}
       blRawIterator(const blRawIterator<blDataType>& rawIterator) = default;
       ~blRawIterator(){}
   
       blRawIterator<blDataType>&                  operator=(const blRawIterator<blDataType>& rawIterator) = default;
       blRawIterator<blDataType>&                  operator=(blDataType* ptr){m_ptr = ptr;return (*this);}
   
       operator                                    bool()const
       {
           if(m_ptr)
               return true;
           else
               return false;
       }
   
       bool                                        operator==(const blRawIterator<blDataType>& rawIterator)const{return (m_ptr == rawIterator.getConstPtr());}
       bool                                        operator!=(const blRawIterator<blDataType>& rawIterator)const{return (m_ptr != rawIterator.getConstPtr());}
   
       blRawIterator<blDataType>&                  operator+=(const difference_type& movement){m_ptr += movement;return (*this);}
       blRawIterator<blDataType>&                  operator-=(const difference_type& movement){m_ptr -= movement;return (*this);}
       blRawIterator<blDataType>&                  operator++(){++m_ptr;return (*this);}
       blRawIterator<blDataType>&                  operator--(){--m_ptr;return (*this);}
       blRawIterator<blDataType>                   operator++(int){auto temp(*this);++m_ptr;return temp;}
       blRawIterator<blDataType>                   operator--(int){auto temp(*this);--m_ptr;return temp;}
       blRawIterator<blDataType>                   operator+(const difference_type& movement){auto oldPtr = m_ptr;m_ptr+=movement;auto temp(*this);m_ptr = oldPtr;return temp;}
       blRawIterator<blDataType>                   operator-(const difference_type& movement){auto oldPtr = m_ptr;m_ptr-=movement;auto temp(*this);m_ptr = oldPtr;return temp;}
   
       difference_type                             operator-(const blRawIterator<blDataType>& rawIterator){return std::distance(rawIterator.getPtr(),this->getPtr());}
   
       blDataType&                                 operator*(){return *m_ptr;}
       const blDataType&                           operator*()const{return *m_ptr;}
       blDataType*                                 operator->(){return m_ptr;}
   
       blDataType*                                 getPtr()const{return m_ptr;}
       const blDataType*                           getConstPtr()const{return m_ptr;}
   
   protected:
   
       blDataType*                                 m_ptr;
   };
   //-------------------------------------------------------------------
   

2. //-------------------------------------------------------------------
   // Raw reverse iterator with random access
   //-------------------------------------------------------------------
   template<typename blDataType>
   class blRawReverseIterator : public blRawIterator<blDataType>
   {
   public:
   
       blRawReverseIterator(blDataType* ptr = nullptr):blRawIterator<blDataType>(ptr){}
       blRawReverseIterator(const blRawIterator<blDataType>& rawIterator){this->m_ptr = rawIterator.getPtr();}
       blRawReverseIterator(const blRawReverseIterator<blDataType>& rawReverseIterator) = default;
       ~blRawReverseIterator(){}
   
       blRawReverseIterator<blDataType>&           operator=(const blRawReverseIterator<blDataType>& rawReverseIterator) = default;
       blRawReverseIterator<blDataType>&           operator=(const blRawIterator<blDataType>& rawIterator){this->m_ptr = rawIterator.getPtr();return (*this);}
       blRawReverseIterator<blDataType>&           operator=(blDataType* ptr){this->setPtr(ptr);return (*this);}
   
       blRawReverseIterator<blDataType>&           operator+=(const difference_type& movement){this->m_ptr -= movement;return (*this);}
       blRawReverseIterator<blDataType>&           operator-=(const difference_type& movement){this->m_ptr += movement;return (*this);}
       blRawReverseIterator<blDataType>&           operator++(){--this->m_ptr;return (*this);}
       blRawReverseIterator<blDataType>&           operator--(){++this->m_ptr;return (*this);}
       blRawReverseIterator<blDataType>            operator++(int){auto temp(*this);--this->m_ptr;return temp;}
       blRawReverseIterator<blDataType>            operator--(int){auto temp(*this);++this->m_ptr;return temp;}
       blRawReverseIterator<blDataType>            operator+(const int& movement){auto oldPtr = this->m_ptr;this->m_ptr-=movement;auto temp(*this);this->m_ptr = oldPtr;return temp;}
       blRawReverseIterator<blDataType>            operator-(const int& movement){auto oldPtr = this->m_ptr;this->m_ptr+=movement;auto temp(*this);this->m_ptr = oldPtr;return temp;}
   
       difference_type                             operator-(const blRawReverseIterator<blDataType>& rawReverseIterator){return std::distance(this->getPtr(),rawReverseIterator.getPtr());}
   
       blRawIterator<blDataType>                   base(){blRawIterator<blDataType> forwardIterator(this->m_ptr); ++forwardIterator; return forwardIterator;}
   };
   //-------------------------------------------------------------------
   

Now somewhere in your custom container class:

template<typename blDataType>
class blCustomContainer
{
public: // The typedefs

    typedef blRawIterator<blDataType>              iterator;
    typedef blRawIterator<const blDataType>        const_iterator;

    typedef blRawReverseIterator<blDataType>       reverse_iterator;
    typedef blRawReverseIterator<const blDataType> const_reverse_iterator;

                            .
                            .
                            .

public:  // The begin/end functions

    iterator                                       begin(){return iterator(&m_data[0]);}
    iterator                                       end(){return iterator(&m_data[m_size]);}

    const_iterator                                 cbegin(){return const_iterator(&m_data[0]);}
    const_iterator                                 cend(){return const_iterator(&m_data[m_size]);}

    reverse_iterator                               rbegin(){return reverse_iterator(&m_data[m_size - 1]);}
    reverse_iterator                               rend(){return reverse_iterator(&m_data[-1]);}

    const_reverse_iterator                         crbegin(){return const_reverse_iterator(&m_data[m_size - 1]);}
    const_reverse_iterator                         crend(){return const_reverse_iterator(&m_data[-1]);}

                            .
                            .
                            .
    // This is the pointer to the
    // beginning of the data
    // This allows the container
    // to either "view" data owned
    // by other containers or to
    // own its own data
    // You would implement a "create"
    // method for owning the data
    // and a "wrap" method for viewing
    // data owned by other containers

    blDataType*                                    m_data;
};

Answer 3

They often forget that iterator must convert to const_iterator but not the other way around. Here is a way to do that:

template<class T, class Tag = void>
class IntrusiveSlistIterator
   : public std::iterator<std::forward_iterator_tag, T>
{
    typedef SlistNode<Tag> Node;
    Node* node_;

public:
    IntrusiveSlistIterator(Node* node);

    T& operator*() const;
    T* operator->() const;

    IntrusiveSlistIterator& operator++();
    IntrusiveSlistIterator operator++(int);

    friend bool operator==(IntrusiveSlistIterator a, IntrusiveSlistIterator b);
    friend bool operator!=(IntrusiveSlistIterator a, IntrusiveSlistIterator b);

    // one way conversion: iterator -> const_iterator
    operator IntrusiveSlistIterator<T const, Tag>() const;
};

In the above notice how IntrusiveSlistIterator<T> converts to IntrusiveSlistIterator<T const>. If T is already const this conversion never gets used.

Answer 4

Boost has something to help: the Boost.Iterator library.

More precisely this page: boost::iterator_adaptor.

What's very interesting is the Tutorial Example which shows a complete implementation, from scratch, for a custom type.

template <class Value>
class node_iter
  : public boost::iterator_adaptor<
        node_iter<Value>                // Derived
      , Value*                          // Base
      , boost::use_default              // Value
      , boost::forward_traversal_tag    // CategoryOrTraversal
    >
{
 private:
    struct enabler {};  // a private type avoids misuse

 public:
    node_iter()
      : node_iter::iterator_adaptor_(0) {}

    explicit node_iter(Value* p)
      : node_iter::iterator_adaptor_(p) {}

    // iterator convertible to const_iterator, not vice-versa
    template <class OtherValue>
    node_iter(
        node_iter<OtherValue> const& other
      , typename boost::enable_if<
            boost::is_convertible<OtherValue*,Value*>
          , enabler
        >::type = enabler()
    )
      : node_iter::iterator_adaptor_(other.base()) {}

 private:
    friend class boost::iterator_core_access;
    void increment() { this->base_reference() = this->base()->next(); }
};

The main point, as has been cited already, is to use a single template implementation and typedef it.

Answer 5

I don't know if Boost has anything that would help.

My preferred pattern is simple: take a template argument which is equal to value_type, either const qualified or not. If necessary, also a node type. Then, well, everything kind of falls into place.

Just remember to parameterize (template-ize) everything that needs to be, including the copy constructor and operator==. For the most part, the semantics of const will create correct behavior.

template< class ValueType, class NodeType >
struct my_iterator
 : std::iterator< std::bidirectional_iterator_tag, T > {
    ValueType &operator*() { return cur->payload; }

    template< class VT2, class NT2 >
    friend bool operator==
        ( my_iterator const &lhs, my_iterator< VT2, NT2 > const &rhs );

    // etc.

private:
    NodeType *cur;

    friend class my_container;
    my_iterator( NodeType * ); // private constructor for begin, end
};

typedef my_iterator< T, my_node< T > > iterator;
typedef my_iterator< T const, my_node< T > const > const_iterator;

Answer 6

There are plenty of good answers but I created a template header I use that is quite concise and easy to use.

To add an iterator to your class it is only necessary to write a small class to represent the state of the iterator with 7 small functions, of which 2 are optional:

#include <iostream>
#include <vector>
#include "iterator_tpl.h"

struct myClass {
  std::vector<float> vec;

  // Add some sane typedefs for STL compliance:
  STL_TYPEDEFS(float);

  struct it_state {
    int pos;
    inline void begin(const myClass* ref) { pos = 0; }
    inline void next(const myClass* ref) { ++pos; }
    inline void end(const myClass* ref) { pos = ref->vec.size(); }
    inline float& get(myClass* ref) { return ref->vec[pos]; }
    inline bool equals(const it_state& s) const { return pos == s.pos; }

    // Optional to allow operator--() and reverse iterators:
    inline void prev(const myClass* ref) { --pos; }
    // Optional to allow `const_iterator`:
    inline const float& get(const myClass* ref) const { return ref->vec[pos]; }
  };
  // Declare typedef ... iterator;, begin() and end() functions:
  SETUP_ITERATORS(myClass, float&, it_state);
  // Declare typedef ... reverse_iterator;, rbegin() and rend() functions:
  SETUP_REVERSE_ITERATORS(myClass, float&, it_state);
};

Then you can use it as you would expect from an STL iterator:

int main() {
  myClass c1;
  c1.vec.push_back(1.0);
  c1.vec.push_back(2.0);
  c1.vec.push_back(3.0);

  std::cout << "iterator:" << std::endl;
  for (float& val : c1) {
    std::cout << val << " "; // 1.0 2.0 3.0
  }
  
  std::cout << "reverse iterator:" << std::endl;
  for (auto it = c1.rbegin(); it != c1.rend(); ++it) {
    std::cout << *it << " "; // 3.0 2.0 1.0
  }
}

I hope it helps.

Answer 7

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

};

Answer 8

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(); }
}

Answer 9

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;
    }
};