How can you use a reverse iterator with an iterator that is a proxy

Question

Given you have an iterator that is effectively a proxy and contains the data that it returns, how can you make a reverse iterator?

std::reverse_iterator implementation of the dereferencing operator creates a temporary object which it then decrements and dereferences, with code that is something like:

  reference operator*() const {
    iterator tmp = current;
    return *--tmp;
  }

with the result that what it returns to you is a pointer to data that goes out of scope before you get hold of it.

This has rather unfortunate results.

How can you get round this?

Answer 1

It looks like you'll need to write your own custom reverse iterator implementation for this specific case since your iterator type is not compatible with this specific implementation of reverse_iterator.

According to http://en.cppreference.com/w/cpp/iterator/reverse_iterator, some implementations do also store a decremented copy of the iterator but not all. Boost::reverse_iterator does not appear to store an additional copy.

Answer 2

I think the standard library implementation is incorrect. If you look at 24.5.1.3.4 [reverse.iter.op.star] in the C++11 standard you'll find the following:

deref_tmp = current;
--deref_tmp;
return *deref_tmp;

Note: This operation must use an auxillary member variable rather than a temporary variable to avoid returning a reference that persists beyond the lifetime of the associated iterator.

In the standard deref_tmp is a for-exposition-only data member of reverse_iterator.