c++ - 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，而是返回一个可以在两个方向上迭代计算的数学序列。事实上，我正在迭代整数，使用++ 和 -- 在 int 上。这意味着数据不会存储在不同的结构中，因此当迭代器超出范围时，值也会超出范围。

尽管如此，我希望下一个代码(最小失败示例)示例能够工作，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 分支导致内存违规，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 分支不会导致内存违规，或者至少在我的 valgrind 可以检测到的范围内。但是， vector 中存储的值是“随机的”:

--decrement
*dereference
-16777520

我对发生的事情感到有点惊讶。迭代器应该一直在范围内，但引用似乎失效了。为什么在打印 12 时会出现内存冲突，或者在存储不同于 12 的内容时为什么没有出现内存冲突？

Answer 1

reverse_iterator 不适用于所谓的“隐藏迭代器”，即返回对自身内部事物的引用的迭代器。 reverse_iterator 的operator* 制作包装迭代器的拷贝，递减它，并返回取消引用拷贝的结果。因此，如果取消引用的迭代器返回对自身内部某物的引用，则该引用将变为悬空。

在 C++11 规范中尝试让它工作，但事实证明，如果不为非隐藏迭代器增加大量开销^* 就不可能实现，所以 the specification was reverted to the C++03 version .

---

_{^* 为了支持“隐藏迭代器”，必须添加一个额外的数据成员来存储递减后的当前迭代器，将reverse_iterator 的大小加倍；然后必须使用某种形式的同步，因为 operator * 是 const - 因此必须同时从多个线程调用而不会导致数据竞争 - 但必须修改此附加数据成员。对于这种不常见的用例，向所有 reverse_iterator 添加大量开销。}