mpile
          error, otherwise (nullary call operator's prototypes can't be templates
          and thus are instantiated as early as the class template).
        </p>
<p>
          Only LValue arguments are accepted. To overcome this limitation, apply
          <a href="http://www.boost.org/libs/functional/forward" target="_top">Boost.Functional/Forward
          Library</a>.
        </p>
<p>
          The type of the target function is allowed to be const qualified or a reference.
          Const qualif