How to correctly implement custom iterators and const_iterators?

Question

• 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.

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;
};

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.

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.

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;

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 cmp(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.