title: Why doesn't unordered_set provide an array access operator
body: <p>I'm curious as to why the STL container <code>unordered_set</code>, which has constant time complexity for random access on average, does not provide a method for accessing elements by some distance from the first element in the container. For example:</p>
  
  <pre><code>T&amp; unordered_set::operator[](size_t index)
  {
      return *(begin() + index);
  }
  </code></pre>