About the reverse_iterator

最新推荐文章于 2024-11-06 22:14:03 发布

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最新推荐文章于 2024-11-06 22:14:03 发布

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本文深入探讨了反向迭代器(reverse_iterator)的概念及其在C++中的应用。详细解释了反向迭代器如何通过内部保持原始迭代器的副本，并在操作中反映其行为，如增量和递减。此外，文章还提供了实例代码，展示了如何使用vector的rbegin和rend方法，以及更详细的reverse_iterator定义。

reverse_iterator: 反向迭代器
A copy of the original iterator (the base iterator) is kept internally and used to reflect the operations performed on the reverse_iterator: whenever the reverse_iterator is incremented, its base iterator is decreased, and vice versa. A copy of the base iterator with the current state can be obtained at any time by calling member base.

Notice however that when an iterator is reversed, the reversed version does not point to the same element in the range, but to the one preceding it. This is so, in order to arrange for the past-the-end element of a range: An iterator pointing to a past-the-end element in a range, when reversed, is pointing to the last element (not past it) of the range (this would be the first element of the reversed range). And if an iterator to the first element in a range is reversed, the reversed iterator points to the element before the first element (this would be the past-the-end element of the reversed range).

简单说就是，如果用一个正常iterator去初始化reverse_iterator, 结果就是指向base iterator的前一个。或者你也可以直接指定rbegin() 或者rend().

直接使用：

// vector::rbegin/rend
#include <iostream>
#include <vector>

int main ()
{
  std::vector<int> myvector (5);  // 5 default-constructed ints

  int i=0;

  std::vector<int>::reverse_iterator rit = myvector.rbegin();
  for (; rit!= myvector.rend(); ++rit)
    *rit = ++i;

  std::cout << "myvector contains:";
  for (std::vector<int>::iterator it = myvector.begin(); it != myvector.end(); ++it)
    std::cout << ' ' << *it;
  std::cout << '\n';

  return 0;
}

output是：
myvector contains: 5 4 3 2 1

或者就是更严格的来定义reverse_iterator，for example:

// reverse_iterator example
#include <iostream>     // std::cout
#include <iterator>     // std::reverse_iterator
#include <vector>       // std::vector

int main () {
  std::vector<int> myvector;
  for (int i=0; i<10; i++) myvector.push_back(i);

  typedef std::vector<int>::iterator iter_type;
                                                         // ? 0 1 2 3 4 5 6 7 8 9 ?
  iter_type from (myvector.begin());                     //   ^
                                                         //         ------>
  iter_type until (myvector.end());                      //                       ^
                                                         //
  std::reverse_iterator<iter_type> rev_until (from);     // ^
                                                         //         <------
  std::reverse_iterator<iter_type> rev_from (until);     //                     ^

  std::cout << "myvector:";
  while (rev_from != rev_until)
    std::cout << ' ' << *rev_from++;
  std::cout << '\n';

  return 0;
}

output is:
myvector: 9 8 7 6 5 4 3 2 1 0