title: What does std::vector's .at() function really do?
body: <p>What is a difference about inserting a value to the vector between these two methods:</p>
  
  <pre><code>vector&lt;int&gt; vectorlist (12);
  vectorlist[12]=23;
  </code></pre>
  
  <p>Versus:</p>
  
  <pre><code>vector&lt;int&gt; vectorlist (12);
  vectorlist.at(12)=23;
  </code></pre>