cpp迭代器继承

Question

My requirements are same to the question asked Using Iterators to hide internal container and achieve generic operation over a base container [ 1 ] at stackoverflow. I have a generic pure virtual base container class, which needs to provide an iterator which should be STL complaint so I can use them with cpp algorithm's #include <algorithm> . My implementation uses only an single class instead of two classes as in [ 1 ] solution.

Base pure virtual class

class BaseItr
{
  public:
    class iterator : public std::iterator<std::input_iterator_tag, int>
    {
      public:
        iterator() : _in(NULL) {}
        inline iterator(const iterator& org) : _in(org._in) {}
        inline iterator& operator=(const iterator& other) { _in = other._in; return *this; }
        virtual inline int operator * () { return _in->operator*(); }
        virtual inline iterator& operator++() { (*_in)++; return *this; }
        virtual inline iterator& operator++(int unused) { (*_in)++; return *this; }
        virtual inline bool operator==(const iterator& other) 
        {
          return *(*_in) == *(*(other._in));
        }
        virtual inline bool operator!=(const iterator& other)
        {
          return *(*_in) != *(*(other._in));
        }
        // would use shared pointer insted of this
        //~iterator() { if(_in) { delete _in; } }
        static inline iterator New(iterator *in) { return iterator(in); }
      private:
        iterator(iterator *in) : _in(in) {}
        iterator *_in;
    };

    virtual iterator begin() = 0;
    virtual iterator end() = 0;
};

Implementation

class Itr : public BaseItr
{
  private:
    class iterator : public BaseItr::iterator
    {
      public:
        iterator(int val) : _val(val), BaseItr::iterator() {}
        int operator * () { return _val; }
        inline iterator& operator++() { ++_val; return *this; }
        inline iterator& operator++(int unused) { _val++; return *this; }
      private:
        int _val;
    };
    BaseItr::iterator _begin;
    BaseItr::iterator _end;
  public:
    inline Itr(int start, int end)
    {
      _begin = BaseItr::iterator::New(new iterator(start));
      _end = BaseItr::iterator::New(new iterator(end));
    }

    BaseItr::iterator begin() { return _begin; }
    BaseItr::iterator end() { return _end; }
};

My implementation works was need, I want to know are there any drawbacks with this implementation, Please help me decide with my design to use the appropriate implementation. I have add my full working example code in github:gist https://gist.github.com/3847688

Ref:

• Iterator for custom container with derived classes
• Using Iterators to hide internal container and achieve generic operation over a base container
• Fast and flexible iterator for abstract class
• C++ : Using different iterator types in subclasses without breaking the inheritance mechanism

Answer 1

The most glaring issue: your iterator does not have value semantics.

The STL algorithms are free to copy an iterator if they wish. For example suppose:

template <typename It>
It find(It b, It e, typename std::iterator_traits<It>::const_reference t) {
    for (; b != e; ++b) {
        if (*b == t) { return b; }
    }
    return e;
}

The problem is that if you invoke this algorithm with BaseItr& , then the result is of type BaseItr , and you are thus exposed to Object Slicing , which is undefined behavior .

In order to give value semantics to you iterator, you need to create a wrapper class around an abstract implementation and have the wrapper correctly manage the copy through a virtual clone method. If your iterator ends up with virtual methods, you are doing it wrong.