title: Efficiently erasing elements in tr1::unordered_map
body: <p>I am experimenting with tr1::unordered_map and stumbled upon the problem how to
  efficiently delete elements. The 'erase' method offers to delete either by key or
  by iterator. I would assume the latter to be more efficient, since the former
  presumably involves an implicit find operation. On the other hand my investigations
  on the internet have revealed that iterators may become invalid after calling
  the insert() method.</p>
  
  <p>I am interested in a typical real-world situation, where objects put into a hash table
  have a life span which is long enough such that calls to insert() happen during that
  life span. Thus may I conclude that in such a situation deletion by key is the only
  option left? Are there any alternatives how to delete objects more efficiently? I am
  fully aware that the question only matters in applications, where deletions happen
  often. Whether this will be the case for my current project, remains to be seen, but
  I would rather learn about these issues while designing my project rather than when
  there is already a lot of code present.</p>