title: std::map<string, string> to string (first values)
body: <p>I'd like to create <code>std::string</code> that contains first elements of <code>std::map&lt;std::string, std::string&gt;</code> separate by some delimiter (it can be from STL or Boost). 
  Is there any better solution (one line) than loop? Like <code>boost::algorithm::join</code> for <code>std::vector&lt;std::string&gt;</code>. </p>