title: position value in vector
body: <p>I want to insert an int value in a vector object at position -1, i.e. :</p>
  
  <pre><code>vector&lt;int&gt; v;
  int p = 12;
  vector&lt;int&gt;::iterator it = -1;
  v.insert(it,1,p);
  </code></pre>
  
  <p>is it possible in c++ ? or it must always be positive ?</p>