54     struct Envelope  struct
56       typedef T element_type;
57       typedef const T *const_pointer;
58       typedef T *pointer;
61       explicit Envelope ( Args &&... args )  in Envelope()  argument
71       explicit operator bool () const noexcept { return true; }  in operator bool()
73 …st typename std::add_lvalue_reference< element_type >::type operator* () const { return element_; }  in operator *()
74       typename std::add_lvalue_reference< element_type >::type operator* () { return element_;  }  in operator *()
76       const_pointer operator-> () const { return &element_; }  in operator ->()
77       pointer operator-> () { return &element_; }  in operator ->()
80       element_type element_;