它无法从其对应的迭代器定义const_iterator

Question

I am extending stl container with a self defined container so as to provide more flexible control on the operations of the elements

class MyContainer;

template <typename T> class myiterator :public iterator<bidirectional_iterator_tag, T>
{
  friend class MyContainer;
  private:
    T *pointer;

    myiterator(T *pt):pointer(pt) {}

  public:
    T& operator*() {return (*pointer);}

    const myiterator<T>& operator++()
    {
      pointer->current_iterator++;
       return *this;
    }

    bool isEnd(void) const
    {
      return pointer->current_iterator == pointer->data.end();
    }
  };

class MyContainer
{
  friend class myiterator<MyContainer>;
  public:
    typedef myiterator<MyContainer> iterator;
    typedef myiterator<MyContainer const> const_iterator;

  private:
    map<int, int> data;
    map<int, int>::const_iterator current_iterator;

  public:
    MyContainer() {current_iterator = data.begin(); }

    void addDataPair(int key, int value) {data[key] = value;}

    int first() const {return (*current_iterator).first;}
    int second() const {return (*current_iterator).second;}

    iterator begin() 
    {
      current_iterator = data.begin();
      return iterator(this);
    }

    const_iterator begin() const
    {
      return const_iterator(this);
    }
  };

This code run ok, if I use iterator as follow

MyContainer h;

h.addDataPair(1, 1);
h.addDataPair(2, 2);
h.addDataPair(3, 3);

for (MyContainer::iterator it=h.begin(); !it.isEnd(); ++it)
{
  cout << (*it).first() << " " << (*it).second() << endl;
}

But it won't compile if I change iterator to const_iterator. I read some article, which mention that to define constant iterator, we simply replace the value_type from X to X const, that's how I did in my code. But I soon find that it might won't work in my case because the reference returned by an iterator is the container itself in my case. I don't know then how to make the const_iterator work without duplicate the coding.

In addition, my iterator is derived from std::iterator but I found that I cannot override the constructor for my iterator. Is that any way that I can pass more than one parameters into my iterator besides the T *pt ? Thanks.

Answer 1

First problem:

If you change this:

for (MyContainer::iterator it=h.begin(); !it.isEnd(); ++it)

to

for (MyContainer::const_iterator it=h.begin(); !it.isEnd(); ++it)

then you get a non-const iterator from begin() and end() and try to initialize a const_iterator from it, but that's a different type and your my_iterator template doesn't have a constructor that allows construction from a different type.

You can fix that by adding:

template<typename> friend class myiterator;

template<typename T2>
  myiterator(myiterator<T2> const& i) : pointer(i.pointer) { }

You should also make operator* const (it doesn't alter the iterator to dereference it.)

But there's still a bigger problem, a const_iterator points to a const MyContainer , but const_iterator::operator++ needs to alter that object, which it can't because it's const. So you can't increment your const_iterator ie can't use it to iterate! You might want to rethink that design.