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。实际上，我在整数中使用++和 - 迭代整数。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，因为迭代器一直在范围内。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';
}
}
if-branch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可以检测到的情况下。但是，存储在向量中的值是随机的＆＃39;：
--decrement
*dereference
-16777520
我对发生的事情感到有些惊讶。迭代器应该一直在范围内，但引用似乎无效。为什么我会收到内存违规行为，而12是打印出来的，或者为什么我不能存储它们而存储12个不同的内容？
答案 0 :(得分：7)
reverse_iterator不能与所谓的“存储迭代器”一起使用，迭代器返回对它们内部事物的引用。 operator*的{{1}}生成包装迭代器的副本，递减它，并返回解除引用副本的结果。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{{3 }}
* 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reverse_iterator的大小加倍;然后必须使用某种形式的同步，因为reverse_iterator是operator * - 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，添加到所有const的开销很大。
答案 1 :(得分：1)
如前所述，C ++ 03和C ++ 14标准以这种方式定义reverse_iterator::operator*：
24.5.1.3.4
operator*[reverse.iter.op.star]
reference operator*() const;1效果：
Iterator tmp = current;
return *--tmp;
tmp返回后 operator*被销毁，因此对tmp内存储的数据的任何引用都将无效。 C ++ 11标准改变了这一点，并添加了一个注释：
24.5.1.3.4
operator*[reverse.iter.op.star]
reference operator*() const;1效果：
deref_tmp = current;
--deref_tmp;
return *deref_tmp;
2 [注意：此操作必须使用辅助成员变量而不是 一个临时变量，以避免返回超出的引用 其关联迭代器的生命周期。 （见24.2。） - 后注]
由于const上的operator*限定符，实际上无法实现，所以在C ++ 11和C ++ 14之间还原了。
最好的解决方案可能是根据您所针对的C ++版本的C ++ 11措辞实现自己的reverse_iterator版本。幸运的是，该规范非常简单易懂。作为一个工作示例，这是我为C ++ 14写的一个：
template <class Iterator>
class stashing_reverse_iterator :
public std::iterator<
typename std::iterator_traits<Iterator>::iterator_category,
typename std::iterator_traits<Iterator>::value_type,
typename std::iterator_traits<Iterator>::difference_type,
typename std::iterator_traits<Iterator>::pointer,
typename std::iterator_traits<Iterator>::reference
> {
typedef std::iterator_traits<Iterator> traits_type;
public:
typedef Iterator iterator_type;
typedef typename traits_type::difference_type difference_type;
typedef typename traits_type::reference reference;
typedef typename traits_type::pointer pointer;
stashing_reverse_iterator() : current() {}
explicit stashing_reverse_iterator(Iterator x) : current(x) {}
template <class U>
stashing_reverse_iterator(const stashing_reverse_iterator<U>& u) : current(u.current) {}
template <class U>
stashing_reverse_iterator& operator=(const stashing_reverse_iterator<U>& u) {
current = u.base();
return *this;
}
Iterator base() const {
return current;
}
// Differs from reverse_iterator::operator*:
// 1. const qualifier removed
// 2. current iterator is stored in a member field to ensure references are
// always valid after this function returns
reference operator*() {
deref_tmp = current;
--deref_tmp;
return *deref_tmp;
}
pointer operator->() const {
return std::addressof(operator*());
}
stashing_reverse_iterator& operator++() {
--current;
return *this;
}
stashing_reverse_iterator operator++(int) {
stashing_reverse_iterator tmp = *this;
--current;
return tmp;
}
stashing_reverse_iterator& operator--() {
++current;
return *this;
}
stashing_reverse_iterator operator--(int) {
stashing_reverse_iterator tmp = *this;
++current;
return tmp;
}
stashing_reverse_iterator operator+ (difference_type n) const {
return stashing_reverse_iterator(current - n);
}
stashing_reverse_iterator& operator+=(difference_type n) {
current -= n;
return *this;
}
stashing_reverse_iterator operator- (difference_type n) const {
return stashing_reverse_iterator(current + n);
}
stashing_reverse_iterator& operator-=(difference_type n) {
current += n;
return *this;
}
// Differs from reverse_iterator::operator[]:
// 1. const qualifier removed because this function makes use of operator*
reference operator[](difference_type n) {
return *(*this + n);
}
protected:
Iterator current;
private:
Iterator deref_tmp;
};
template <class Iterator1, class Iterator2>
bool operator==(
const stashing_reverse_iterator<Iterator1>& x,
const stashing_reverse_iterator<Iterator2>& y)
{ return x.base() == y.base(); }
template <class Iterator1, class Iterator2>
bool operator<(
const stashing_reverse_iterator<Iterator1>& x,
const stashing_reverse_iterator<Iterator2>& y)
{ return x.base() > y.base(); }
template <class Iterator1, class Iterator2>
bool operator!=(
const stashing_reverse_iterator<Iterator1>& x,
const stashing_reverse_iterator<Iterator2>& y)
{ return !(x.base() == y.base()); }
template <class Iterator1, class Iterator2>
bool operator>(
const stashing_reverse_iterator<Iterator1>& x,
const stashing_reverse_iterator<Iterator2>& y)
{ return x.base() < y.base(); }
template <class Iterator1, class Iterator2>
bool operator>=(
const stashing_reverse_iterator<Iterator1>& x,
const stashing_reverse_iterator<Iterator2>& y)
{ return x.base() <= y.base(); }
template <class Iterator1, class Iterator2>
bool operator<=(
const stashing_reverse_iterator<Iterator1>& x,
const stashing_reverse_iterator<Iterator2>& y)
{ return x.base() >= y.base(); }
template <class Iterator1, class Iterator2>
auto operator-(
const stashing_reverse_iterator<Iterator1>& x,
const stashing_reverse_iterator<Iterator2>& y) -> decltype(y.base() - x.base())
{ return y.base() - x.base(); }
template <class Iterator>
stashing_reverse_iterator<Iterator> operator+(
typename stashing_reverse_iterator<Iterator>::difference_type n,
const stashing_reverse_iterator<Iterator>& x)
{ return stashing_reverse_iterator<Iterator>(x.base() - n); }
template <class Iterator>
stashing_reverse_iterator<Iterator> make_stashing_reverse_iterator(Iterator i)
{ return stashing_reverse_iterator<Iterator>(i); }
用法与reverse_iterator相同：
// prints 5,4,3,2,1, for a sanely implemented number_iterator
std::copy(
make_stashing_reverse_iterator(number_iterator(5)),
make_stashing_reverse_iterator(number_iterator(0)),
std::ostream_iterator<int>(std::cout, ","));