我如何以优雅和高效的方式实现各种迭代器类别? [英] How can I implement various iterator categories in an elegant and efficient way?
问题描述
我正在实现STL容器,例如vector。我困惑我是迭代器的实现。
I'm implementing STL containers, for example, vector. What confused me is the implementation of iterators.
如果我想实现所有迭代器类别:input_iterator,output_iterator,forward_iterator,bidirectional_iterator和random_access_iterator。
If I want to implement all iterator categories: input_iterator, output_iterator, forward_iterator, bidirectional_iterator and random_access_iterator.
如何管理他们的继承关系?
我阅读了如何实现STL式迭代器,避免常见的陷阱? -
How do I manage their inheritance relationships? I've read How to implement an STL-style iterator and avoid common pitfalls?-Mooing Duck's Answer
这是他的例子符号:
iterator {
iterator(const iterator&);
~iterator();
iterator& operator=(const iterator&);
iterator& operator++(); //prefix increment
reference operator*() const;
friend void swap(iterator& lhs, iterator& rhs); //C++11 I think
};
input_iterator : public virtual iterator {
iterator operator++(int); //postfix increment
value_type operator*() const;
pointer operator->() const;
friend bool operator==(const iterator&, const iterator&);
friend bool operator!=(const iterator&, const iterator&);
};
//once an input iterator has been dereferenced, it is
//undefined to dereference one before that.
output_iterator : public virtual iterator {
reference operator*() const;
iterator operator++(int); //postfix increment
};
//dereferences may only be on the left side of an assignment
//once an input iterator has been dereferenced, it is
//undefined to dereference one before that.
forward_iterator : input_iterator, output_iterator {
forward_iterator();
};
//multiple passes allowed
bidirectional_iterator : forward_iterator {
iterator& operator--(); //prefix increment
iterator operator--(int); //postfix decrement
};
random_access_iterator : bidirectional_iterator {
friend bool operator<(const iterator&, const iterator&);
friend bool operator>(const iterator&, const iterator&);
friend bool operator<=(const iterator&, const iterator&);
friend bool operator>=(const iterator&, const iterator&);
iterator& operator+=(size_type);
friend iterator operator+(const iterator&, size_type);
friend iterator operator+(size_type, const iterator&);
iterator& operator-=(size_type);
friend iterator operator-(const iterator&, size_type);
friend difference_type operator-(iterator, iterator);
reference operator[](size_type) const;
};
< 从类
,我使用代码 random_access_iterator
中的 random_access_iterator b = a + 1
。这将导致编译错误。因为 a + 1
的类是基础迭代器
,而不是 random_access_iterator
。
But I found a problem:
If I have an instance a
from class random_access_iterator
, I use the code random_access_iterator b = a + 1
. This will cause compile error. Because a + 1
's class is base iterator
, not random_access_iterator
.
所以我不认为这是一个合理的解决方案。
So I don't think this is a reasonable solution.
我误解了吗?
感谢
推荐答案
我认为您应该使用CRTP( https://en.wikipedia.org/wiki/Curiously_recurring_template_pattern )。
像这样:
I think you should use CRTP (https://en.wikipedia.org/wiki/Curiously_recurring_template_pattern). Like this:
template <typename T, typename ItT>
struct iiterator_t {
typedef T value_type;
typedef T& reference;
typedef T* pointer;
typedef ItT iterator_type;
virtual iterator_type& operator=(const iterator_type&) = 0;
virtual iterator_type& operator++() = 0;
virtual reference operator*() const = 0;
};
template <typename T, typename ItT>
struct iterator_impl_t : virtual public iiterator_t<T, ItT>{
typedef T value_type;
typedef T& reference;
typedef T* pointer;
typedef ItT iterator_type;
iterator_type& operator=(const iterator_type &rhs)
{
p = static_cast<const iterator_impl_t&>(rhs).p;
return dynamic_cast<iterator_type&>(*this);
}
iterator_type& operator++()
{
++p;
return dynamic_cast<iterator_type&>(*this);
}
reference operator*() const
{
return *p;
}
private:
pointer p;
};
template <typename T, typename ItT>
struct iinput_iterator_t : public virtual iiterator_t<T, ItT> {
typedef T value_type;
typedef T& reference;
typedef T* pointer;
typedef ItT iterator_type;
virtual iterator_type operator++(int) = 0;
};
template <typename T, typename ItT>
struct input_iterator_impl_t :
public virtual iinput_iterator_t<T, ItT>,
public virtual iterator_impl_t<T, ItT>
{
typedef T value_type;
typedef T& reference;
typedef T* pointer;
typedef ItT iterator_type;
iterator_type operator++(int)
{
iterator_type result(dynamic_cast<const iterator_type &>(*this));
++dynamic_cast<iterator_impl_t<T, ItT> &>(*this);
return result;
}
};
template <typename T>
struct iterator :
public virtual iterator_impl_t<T, iterator<T> >
{
};
template <typename T>
struct input_iterator :
public virtual input_iterator_impl_t<T, input_iterator<T>>
{
};
int main(int , char** )
{
iterator<int> i;
iterator<int> i2 = ++i;
input_iterator<int> inpi;
input_iterator<int> inpi2 = inpi++;
return 0;
}
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