问题
I implemented a bidirectional iterator, however instead of operating on a data structure, it returns a mathematical series which one can iteratively calculate through in both directions. In fact, I'm iterating through the integers, using ++ and -- on an int. This means that the data is not stored in a different structure, and hence when the iterator goes out of scope, so does the value.
Nevertheless, I would expect the next code (minimal failing example) sample to work, as the iterator stays in scope the whole time. But it does not work :(
#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';
}
}
source: http://ideone.com/YJKvpl
The if-branch results in memory violations, as properly detected by valgrind:
--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)
The else-branch doesn't result in memory violations, or at least as far as my valgrind can detect. However, the value stored in the vector is 'random':
--decrement
*dereference
-16777520
I'm a bit surprised by what happens. The iterator should be in scope all the time, yet the reference seems to get invalidated. Why do I get the memory violations, whilst 12 is printed or why don't I get them whilst something different from 12 is stored?
回答1:
reverse_iterator does not work with so-called "stashing iterators", iterators that returns references to things within themselves. The operator* of reverse_iterator makes a copy of the wrapped iterator, decrements it, and returns the result of dereferencing the copy. Hence, if dereferencing iterator returns a reference to something inside itself, the reference will become dangling.
An attempt was made in the C++11 specification to make it work, but it turns out that it's impossible to implement without adding massive overhead* for non-stashing iterators, so the specification was reverted to the C++03 version.
* To support "stashing iterators", an additional data member must be added that stores the decremented current iterator, doubling the size of reverse_iterator; and then some form of synchronization must be used since operator * is const - and so must be simultaneously callable from multiple threads without causing a data race - but must modify this additional data member. That's a lot of overhead to add to all reverse_iterators for such an uncommon use case.
回答2:
As has been mentioned, the C++03 and C++14 standards define reverse_iterator::operator* in this way:
24.5.1.3.4
operator*[reverse.iter.op.star]
reference operator*() const;1 Effects:
Iterator tmp = current;
return *--tmp;
tmp is destroyed after operator* returns, so any references to data stored inside of tmp will become invalid. The C++11 standard changed this and added a note:
24.5.1.3.4
operator*[reverse.iter.op.star]
reference operator*() const;1 Effects:
deref_tmp = current;
--deref_tmp;
return *deref_tmp;
2 [ Note: This operation must use an auxiliary member variable rather than a temporary variable to avoid returning a reference that persists beyond the lifetime of its associated iterator. (See 24.2.) —end note ]
This is actually impossible to implement because of the const qualifier on operator*, so the wording was reverted between C++11 and C++14.
The best solution is probably to just implement your own version of reverse_iterator based on the C++11 wording for whatever version of C++ you are targeting. Fortunately, the specification is very straightforward and easy to follow. As a working example, here's one I wrote for C++14:
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); }
Usage is the same as reverse_iterator:
// 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, ","));
来源：https://stackoverflow.com/questions/28595084/reference-invalidation-after-applying-reverse-iterator-on-a-custom-made-iterator