STL iterator before std::map::begin()

Question

In C++11\'s std::map, is there some valid iterator x such that ++x is guaranteed to equal map::begin()? I would like to detect i

Answer 1

It's very important to realize that Standard Library containers are semi-open ranges [begin, end), i.e. you can iterate one-past-the-end. For bidirectional (and random) iterators you can also do --end() and come back from the brink. Dereferencing one-past-the-end by *end() is undefined behavior, and so is decrementing the begin iterator by --begin() or begin() - 1. There is only one exception to this: std::forward_list which has a non-dereferenceable iterator before_begin() that satisfies ++before_begin() == begin() (but note that for a forward_list you cannot decrement begin() either).

This fundamental asymmetry for bidirectional iterators means that reverse iterators are thin wrappers around regular iterators. In most Standard Library implementations they simply contain a copy base_ of the underyling iterator. Incrementing a std::reverse_iterator calls something like --base_; return *this;, and dereferencing it does auto old = base_; return *--old;. At no point is the underlying iterator decremented to before begin(), and no dereferencing of end() is done that way.

Below are the four ways to iterate over a container supporting bidirectional or random iterators, and the relations between the various iterators (.base() converts a std::reverse_iterator back to its underlying iterator)

#include 
#include 
#include 
#include 

Live Example

If you have data structure that should support bidirectional iteration but there are no member iterators .rbegin() or rend(), you can easily define them yourself by std::reverse_iterator(end()) and std::reverse_iterator(begin()), respectively (this is also the way the Standard Library usually implements them).