如何实现STL式迭代器和避免常见的陷阱？ [英] How to implement an STL-style iterator and avoid common pitfalls?

问题描述

我制作了一个集合，我想为其提供一个STL风格的随机访问迭代器。我正在搜索一个迭代器的示例实现，但我没有找到任何。我知道需要const重载 [] 和 * 运算符。迭代器的要求是STL风格还是其他一些要避免的陷阱（如果有）？

I made a collection for which I want to provide an STL-style, random-access iterator. I was searching around for an example implementation of an iterator but I didn't find any. I know about the need for const overloads of [] and * operators. What are the requirements for an iterator to be "STL-style" and what are some other pitfalls to avoid (if any)?

其他上下文：这是一个库并且我不想引入任何依赖，除非我真的需要。我写了我自己的集合，以便能够提供C ++ 03和C ++ 11与相同的编译器之间的二进制兼容性（所以没有STL可能会破坏）。

Additional context: This is for a library and I don't want to introduce any dependency on it unless I really need to. I write my own collection to be able to provide binary compatibility between C++03 and C++11 with the same compiler (so no STL which would probably break).

推荐答案

http://www.cplusplus.com/reference/std/iterator/ 有一个方便的图表，详细说明了C ++ 11标准第24.2.2节的规范。基本上，迭代器具有描述有效操作的标签，并且标签具有层次结构。下面是纯粹的符号，这些类实际上不存在。

http://www.cplusplus.com/reference/std/iterator/ has a handy chart that details the specs of § 24.2.2 of the C++11 standard. Basically, the iterators have tags that describe the valid operations, and the tags have a hierarchy. Below is purely symbolic, these classes don't actually exist as such.

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;
};

您可以专门 std :: iterator_traits< youriterator> ，或者将相同的typedef放在迭代器本身，或者继承 std :: iterator （它有这些typedefs）。我喜欢第二个选项，以避免改变 std 命名空间中的内容，并且为了可读性，但大多数人继承 std :: iterator 。

You can either specialize std::iterator_traits<youriterator>, or put the same typedefs in the iterator itself, or inherit from std::iterator (which has these typedefs). I prefer the second option, to avoid changing things in the std namespace, and for readability, but most people inherit from std::iterator.

struct std::iterator_traits<youriterator> {        
    typedef ???? difference_type; //almost always ptrdif_t
    typedef ???? value_type; //almost always T
    typedef ???? reference; //almost always T& or const T&
    typedef ???? pointer; //almost always T* or const T*
    typedef ???? iterator_category;  //usually std::forward_iterator_tag or similar
};

请注意，iterator_category应该是 std :: input_iterator_tag ， std :: output_iterator_tag ， std :: forward_iterator_tag ， std :: bidirectional_iterator_tag 或 std :: random_access_iterator_tag ，具体取决于迭代器满足的要求。根据您的迭代器，您可以选择专门化 std :: next ， std :: prev ， std :: advance 和 std :: distance ，但这是很少需要的。在非常罕见的情况下，您可能希望专门化 std :: begin 和 std :: end 。

Note the iterator_category should be one of std::input_iterator_tag, std::output_iterator_tag, std::forward_iterator_tag, std::bidirectional_iterator_tag, or std::random_access_iterator_tag, depending on which requirements your iterator satisfies. Depending on your iterator, you may choose to specialize std::next, std::prev, std::advance, and std::distance as well, but this is rarely needed. In extremely rare cases you may wish to specialize std::begin and std::end.

你的容器应该还有一个 const_iterator ，这是一个它类似于你的迭代器，除了它应该可以从迭代器隐式构造，用户应该无法修改数据。它的内部指针通常是指向非常量数据的指针，并且 iterator 从 const_iterator 继承以尽量减少代码重复。

Your container should probably also have a const_iterator, which is a (possibly mutable) iterator to constant data that is similar to your iterator except it should be implicitly constructable from a iterator and users should be unable to modify the data. It is common for its internal pointer to be a pointer to non-constant data, and have iterator inherit from const_iterator so as to minimize code duplication.

我的帖子在编写自己的STL容器具有更完整的容器/迭代器原型。

My post at Writing your own STL Container has a more complete container/iterator prototype.

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