C++迭代器和继承

Question

快速询问下面实现迭代器的最佳方法是什么:

假设我有一个模板化的基类'List'和两个子类"ListImpl1"和"ListImpl2".基类的基本要求是可迭代的,即我可以这样做:

for(List<T>::iterator it = list->begin(); it != list->end(); it++){
   ...
}

我也想允许迭代器添加,例如:

for(List<T>::iterator it = list->begin()+5; it != list->end(); it++){
   ...
}

所以问题是ListImpl1的迭代器的实现将与ListImpl2的迭代器的实现不同.我通过使用包含指向ListIteratorImpl的指针的包装器ListIterator来解决这个问题,该指针包含子类ListIteratorImpl2和ListIteratorImpl2,但它们都变得非常混乱,特别是当您需要在ListIterator中实现operator +时.

有没有想过更好的设计来解决这些问题？

Answer 1

如果您可以使List<T>::iterator非虚拟化,那么将虚拟化委派给List会使事情变得简单:

template<typename T>
class List
{
    virtual void add_assign(iterator& left, int right) = 0;

public:
    class iterator
    {
        const List* list;
        const T* item;
    public:
        iterator(const List* list, const T* item) : list(list), item(item) {}

        iterator& operator +=(int right)
        {
            list->add_assign(*this, right);
            return *this;
        }
        static iterator operator +(iterator const& left, int right)
        {
            iterator result = left;
            result += right;
            return result;
        }
    };

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

否则(例如,如果迭代器需要存储明显不同的数据),那么您必须执行常规的,无聊的指针实现以获得虚拟性:

template<typename T>
class List
{
    class ItImpl
    {
        virtual ItImpl* clone() = 0;
        virtual void increment() = 0;
        virtual void add(int right) = 0;
    };
public:
    class iterator
    {
        ItImpl* impl;
    public:
        // Boring memory management stuff.
        iterator() : impl() {}
        iterator(ItImpl* impl) : impl(impl) {}
        iterator(iterator const& right) : impl(right.impl->clone()) {}
        ~iterator() { delete impl; }
        iterator& operator=(iterator const& right)
        {
            delete impl;
            impl = right.impl->clone();
            return *this;
        }

        // forward operators to virtual calls through impl.
        iterator& operator+=(int right)
        {
            impl->add(right);
            return *this;
        }
        iterator& operator++()
        {
            impl->increment();
            return *this;
        }
    };
};

template<typename T>
static List<T>::iterator operator+(List<T>::iterator const& left, int right)
{
    List<T>::iterator result = left;
    result += right;
    return result;
}

template<typename T>
class MagicList : public List<T>
{
    class MagicItImpl : public ItImpl
    {
        const MagicList* list;
        const magic* the_magic;
        // implement ...
    };
public:
    iterator begin() const { return iterator(new MagicItImpl(this, begin_magic)); }
    iterator end() const { return iterator(new MagicItImpl(this, end_magic)); }
};