title: Why does std::map not have a const accessor?
body: <p>The declaration for the [] operator on a std::map is this:</p>
  
  <pre><code>T&amp; operator[] ( const key_type&amp; x );
  </code></pre>
  
  <p>Is there a reason it isn't this?</p>
  
  <pre><code>T&amp; operator[] ( const key_type&amp; x );
  const T&amp; operator[] const ( const key_type&amp; x );
  </code></pre>
  
  <p>Because that would be incredibly useful any time you need to access a member map in a const method.</p>