title: Boost cannot find future::then from demo
body: <p>I wanted to try boost future then, I have boost 1.55 installed and included in make file and I wanted to try official demo</p>
  
  <pre><code>#define BOOST_THREAD_PROVIDES_FUTURE
  
  #include &lt;boost/thread/future.hpp&gt;
  
  using namespace boost;
  
  int main()
  {
    future&lt;int&gt; f1 = async([]() { return 123; });
    future&lt;int&gt; f2 = f1.then([](future&lt;int&gt; f) { return f.get();} );// here .get() won't block });
  } 
  </code></pre>
  
  <p>but I always get error during compilation</p>
  
  <pre><code>error: ‘class boost::future&lt;int&gt;’ has no member named ‘then’
  </code></pre>
  
  <p>When I commented line with f2 it compiles.</p>