在自定义迭代器上应用reverse_iterator后引用失效

Question

我实现了一个双向迭代器,但它不是对数据结构进行操作,而是返回一个数学系列,可以在两个方向上迭代计算.事实上,我正在迭代整数,使用++和 - 在int上.这意味着数据不存储在不同的结构中,因此当迭代器超出范围时,值也是如此.

尽管如此,我希望下一个代码(最小失败示例)示例能够正常工作,因为迭代器始终保持在范围内.但它不起作用:(

#include <iostream>
#include <iterator>
#include <vector>

class my_iterator : public std::iterator<std::bidirectional_iterator_tag, int> {
  int d_val = 12;
public:
  my_iterator  operator--(int) { std::cout << "decrement--\n"; return my_iterator(); }
  my_iterator &operator--()    { std::cout << "--decrement\n"; return *this; }
  my_iterator  operator++(int) { std::cout << "increment++\n"; return my_iterator(); }
  my_iterator &operator++()    { std::cout << "++increment\n"; return *this; }

  int &operator*() { std::cout << "*dereference\n"; return d_val; }

  bool operator==(my_iterator const  &o) { return false; }
  bool operator!=(my_iterator const  &o) { return true ; }
};


int main() {
  auto it = std::reverse_iterator<my_iterator>();
  int &i = *it;
  if (true)
  {
    std::cout << i << '\n';
  }
  else
  {
    std::vector<int> vec;
    vec.push_back(i);
    std::cout << vec[0] << '\n';
  }
}

来源:http://ideone.com/YJKvpl

if-branch会导致内存违规,正如valgrind正确检测到的那样:

--decrement
*dereference
==7914== Use of uninitialised value of size 8
==7914==    at 0x4EC15C3: ??? (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.20)
==7914==    by 0x4EC16FB: std::ostreambuf_iterator<char, std::char_traits<char> > std::num_put<char, std::ostreambuf_iterator<char, std::char_traits<char> > >::_M_insert_int<long>(std::ostreambuf_iterator<char, std::char_traits<char> >, std::ios_base&, char, long) const (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.20)
==7914==    by 0x4EC1C7C: std::num_put<char, std::ostreambuf_iterator<char, std::char_traits<char> > >::do_put(std::ostreambuf_iterator<char, std::char_traits<char> >, std::ios_base&, char, long) const (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.20)
==7914==    by 0x4ECEFB9: std::ostream& std::ostream::_M_insert<long>(long) (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.20)
==7914==    by 0x40087B: main (divine.cc:25)
==7914== 
==7914== Conditional jump or move depends on uninitialised value(s)
==7914==    at 0x4EC15CF: ??? (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.20)
==7914==    by 0x4EC16FB: std::ostreambuf_iterator<char, std::char_traits<char> > std::num_put<char, std::ostreambuf_iterator<char, std::char_traits<char> > >::_M_insert_int<long>(std::ostreambuf_iterator<char, std::char_traits<char> >, std::ios_base&, char, long) const (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.20)
==7914==    by 0x4EC1C7C: std::num_put<char, std::ostreambuf_iterator<char, std::char_traits<char> > >::do_put(std::ostreambuf_iterator<char, std::char_traits<char> >, std::ios_base&, char, long) const (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.20)
==7914==    by 0x4ECEFB9: std::ostream& std::ostream::_M_insert<long>(long) (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.20)
==7914==    by 0x40087B: main (divine.cc:25)
==7914== 
==7914== Conditional jump or move depends on uninitialised value(s)
==7914==    at 0x4EC1724: std::ostreambuf_iterator<char, std::char_traits<char> > std::num_put<char, std::ostreambuf_iterator<char, std::char_traits<char> > >::_M_insert_int<long>(std::ostreambuf_iterator<char, std::char_traits<char> >, std::ios_base&, char, long) const (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.20)
==7914==    by 0x4EC1C7C: std::num_put<char, std::ostreambuf_iterator<char, std::char_traits<char> > >::do_put(std::ostreambuf_iterator<char, std::char_traits<char> >, std::ios_base&, char, long) const (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.20)
==7914==    by 0x4ECEFB9: std::ostream& std::ostream::_M_insert<long>(long) (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.20)
==7914==    by 0x40087B: main (divine.cc:25)
==7914== 
12
==7914== 
==7914== HEAP SUMMARY:
==7914==     in use at exit: 0 bytes in 0 blocks
==7914==   total heap usage: 0 allocs, 0 frees, 0 bytes allocated
==7914== 
==7914== All heap blocks were freed -- no leaks are possible
==7914== 
==7914== For counts of detected and suppressed errors, rerun with: -v
==7914== Use --track-origins=yes to see where uninitialised values come from
==7914== ERROR SUMMARY: 5 errors from 3 contexts (suppressed: 0 from 0)

else-branch不会导致内存违规,或者至少在我的valgrind可以检测到的情况下.但是,存储在向量中的值是"随机的":

--decrement
*dereference
-16777520

发生了什么让我感到有些惊讶.迭代器应该一直在范围内,但引用似乎无效.为什么我会得到内存违规,而12是打印的,或者为什么我不能获得它们而存储了与12不同的内容？

Answer 1

reverse_iterator不能与所谓的"存储迭代器"一起工作,迭代器返回对它们内部事物的引用.该operator*的reverse_iterator使包裹迭代器的副本,递减,然后返回取消引用复制的结果.因此,如果解除引用迭代器返回对其自身内部内容的引用,则引用将变为悬空.

在C++ 11规范中尝试使其工作,但事实证明,如果不为非存储迭代器添加大量开销^*,则无法实现,因此规范被还原为C++ 03版本.

_{^*为了支持"存储迭代器",必须添加一个额外的数据成员来存储递减的当前迭代器,使其大小加倍reverse_iterator; 然后某种形式的同步,必须因为所用operator *IS const-因此必须是从多个线程同时调用,而不会造成数据争用-但必须修改该附加数据成员.reverse_iterator对于这种不常见的用例,添加到所有s 的开销很大.}