title: Parse std::string with Boost ptree
body: <p>I have this code</p>
  
  <pre><code>std::string ss = "{ \"item1\" : 123, \"item3\" : 456, \"item3\" : 789 }";
  
  // Read json.
  ptree pt2;
  std::istringstream is(ss);
  read_json(is, pt2);
  std::string item1= pt2.get&lt;std::string&gt;("item1");
  std::string item2= pt2.get&lt;std::string&gt;("item2");
  std::string item3= pt2.get&lt;std::string&gt;("item3"); 
  </code></pre>
  
  <p>That I need to parse a JSON string into <code>std::string</code>'s as shown above, I tried to put a catch statement here but the actual error is just <code>&lt;unspecified file&gt;(1):</code></p>
  
  <p>So I assume that the read_json is just reading file, not std::string, in what way that this <code>std::string</code> can be parsed?</p>