std::reverse_iterator strange behaviour (UB?) [duplicate] - c++

Question

This question already has answers here:
Reference invalidation after applying reverse_iterator on a custom made iterator
(2 answers)
Closed 5 years ago.
I created a pseudo-container class (it does not contain any elements) and an iterator class for this container.
The following code always outputs '776' on my system (my compiler is GCC 5.4.0)
#include <iostream>
#include <iterator>
class Container
{
public:
class Iterator;
Container()=default;
Container::Iterator begin();
Container::Iterator end();
};
class Container::Iterator: public std::iterator<std::bidirectional_iterator_tag, size_t>
{
public:
Iterator(size_t n);
size_t& operator*();
Container::Iterator& operator--();
const Container::Iterator operator--(int);
bool operator==(const Container::Iterator& rhs) const;
bool operator!=(const Container::Iterator& rhs) const;
private:
size_t n_;
};
Container::Iterator Container::end()
{
return Iterator(777);
}
Container::Iterator::Iterator(size_t n):
n_(n)
{
}
size_t& Container::Iterator::operator *()
{
return n_;
}
Container::Iterator& Container::Iterator::operator--()
{
n_--;
return *this;
}
const Container::Iterator Container::Iterator::operator--(int)
{
Container::Iterator oldVal = *this;
n_--;
return oldVal;
}
int main()
{
Container cont;
std::reverse_iterator<Container::Iterator>revit(cont.end());
//as cppreference says, "For a reverse iterator r constructed from an iterator i, the relationship &*r == &*(i-1) is always true...", so I expect that the output must be the same as if I used instead next commented line, and it does so on my system
// auto it = cont.end(); it--; std::cout << *it << std::endl;
std::cout << *revit << std::endl;
return 0;
}
But when I use any online compiler (with C++11 support), this code outputs just '0' (except one Clang version, then the output is some 'random' big number)
I can't figure out, where is my error?

Answer 1

std::reverse_iterator::operator* is equivalent to
Iterator tmp = current; return *--tmp;
(where current is the underlying iterator wrapped by this instance of reverse_iterator).
*tmp returns a reference to the member of tmp - which goes out of scope and is destroyed when std::reverse_iterator::operator* returns, taking that member with it. Therefore, *revit returns a dangling reference; the subsequent attempt to use it exhibits undefined behavior.