Reference invalidation after applying reverse_iterator on a custom made iterator

Question

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 #include #include class my_iterator : public std::iterator { int d_val = 12; public: my_iterator operator--(int) { std::cout << "decrement-- "; return my_iterator(); } my_iterator &operator--() { std::cout << "--decrement "; return *this; } my_iterator operator++(int) { std::cout << "increment++ "; return my_iterator(); } my_iterator &operator++() { std::cout << "++…

Accepted Answer

`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_iterator`s for such an uncommon use case.

Reference invalidation after applying reverse_iterator on a custom made iterator

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