title: C++ lambda for sort algorithm
body: <p>I'd like to sort a <code>map&lt;pair&lt;string, int&gt;, int&gt;</code> dbg; by value using lambda :</p>
  <p>For this I have</p>
  <pre><code>void test()
  {
     map&lt;pair&lt;string, int&gt;, int&gt; dbg;
     sort( dbg.begin(), dbg.end(),
           []( pair&lt;pair&lt;string, int&gt;, int&gt;&amp; lht, pair&lt;pair&lt;string, int&gt;, int&gt;&amp; rht) {
        return lht.second &gt; rht.second;
     });
  }
  </code></pre>
  <p>But compilation failed with a lot of errors. What is the right lamda prototype here?</p>