C++ custom collection reverse_iterator with similar behaviour to std::vector implementation

问题

I have a template based custom collection (as we cannot use std::vector on the interface). I would like to implement a reverse_iterator specific to this collection. The reverse iterator struct below is a structure nested within the collection class. An iterator (basically a pointer to element type of the collection) is already implemented. This is my first attempt at a reverse iterator.

    template <typename T>
    struct reverse_iterator
    {
        typedef T::iterator iterator;
        typedef T& reference;

        inline reverse_iterator(const iterator & it):_it(it){}
        inline reverse_iterator() : _it(0x0)                {}

        inline iterator base() const                        {iterator it = _it; return --it;}

        inline reverse_iterator operator ++ ()              {return reverse_iterator(--_it);}
        inline reverse_iterator operator -- ()              {return reverse_iterator(++_it);}
        inline reverse_iterator operator ++ (int val)       {_it -= val; return reverse_iterator(_it);}
        inline reverse_iterator operator -- (int val)       {_it += val; return reverse_iterator(_it);}
        inline reverse_iterator operator += (int val)       {_it -= val; return reverse_iterator(_it);}
        inline reverse_iterator operator -= (int val)       {_it += val; return reverse_iterator(_it);}

        inline reverse_iterator operator + (int val) const  {iterator it = _it - val; return reverse_iterator(it);}
        inline reverse_iterator operator - (int val) const  {iterator it = _it + val; return reverse_iterator(it);}

        bool operator == (const iterator & other) const     {return other == base();}
        bool operator != (const iterator & other) const     {return other != base();}

        reference operator*() const {return *base();}
        iterator operator->() const {return base();}

    private:    
        iterator _it;
    };

Is this is workable reverse_iterator or am I missing something ?
Can this be improved?

回答1:

Except for the things mentioned below your implementation is almost the same as the implementation in libstdc++ (v3, but still somewhat accurate). Note that you're currently missing all non-member functions. All in all you should try to match the std::reverse_iterator interface: if you're ever able to use std types you can happily exchange your mylab::reverse_iterator by std::reverse_iterator.

Missing things

You're missing all comparison operators between reverse_iterator, such as operator==, operator!=, operator< and so on.

Strange things

This is basically a list of stuff where your reverse_iterator differs from the standard one.

Usually the pre-increment/-decrement operators return a reference (*this) and not a new object.

The post increment/decrement operators shouldn't take a value:

inline reverse_iterator operator ++ (int)       {
  reverse_iterator tmp = *this;
  ++*this; // implement post-increment in terms of pre-increment!
  // or --_it;
  return tmp;
}
inline reverse_iterator operator -- (int)       { ... }

The compound assignment operators also usually return references.
Your const iterator& constructor should be explicit, otherwise one could accidentally mix reverse and normal iterators.

Instead of a container type T you should use the underlying iterator as template parameter:

template <typename Iterator>
struct reverse_iterator
{
    typedef Iterator iterator;
    typedef typename iterator_traits<Iterator>::reference reference;
    ...
}

This enables you to use reverse_iterator on anything that iterator_traits can handle:

template <class Iterator>
struct iterator_traits{
  typedef typename Iterator::reference reference;
  // Add other things
};

template <class T>
struct iterator_traits<T*>{
  typedef T & reference;
};

With this you can even use reverse_iterator<int *> or similar.

operator-> usually returns a pointer to the underlying object, not an intermediary iterator. You might want to add a pointer typedef to both your traits and your original iterator.
It's very uncommon to check equality between different types. Remove the operator==(const iterator&).

来源：https://stackoverflow.com/questions/21275190/c-custom-collection-reverse-iterator-with-similar-behaviour-to-stdvector-imp

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iterator

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