Function emplace

ext/include/boost/optional/optional.hpp:1168–1171

    // upon exception *this is always uninitialized
    template<class... Args>
    void emplace ( Args&&... args )
    {
      this->emplace_assign( boost::forward<Args>(args)... );
    }

    template<class... Args>
    explicit optional ( in_place_init_t, Args&&... args )