Function value_or

ext/include/boost/optional/optional.hpp:1341–1347

#ifndef BOOST_NO_CXX11_REF_QUALIFIERS
    template <class U>
    value_type value_or ( U&& v ) const&
      {
        if (this->is_initialized())
          return get();
        else
          return boost::forward<U>(v);
      }

    template <class U>
    value_type value_or ( U&& v ) &&