Function value_or_eval

extern/boost/boost/optional/optional.hpp:1388–1394

#if (!defined BOOST_NO_CXX11_REF_QUALIFIERS) && (!defined BOOST_OPTIONAL_DETAIL_NO_RVALUE_REFERENCES)
    template <typename F>
    value_type value_or_eval ( F f ) const&
      {
        if (this->is_initialized())
          return get();
        else
          return f();
      }

    template <typename F>
    value_type value_or_eval ( F f ) &&