33 Concurrency support library [thread]
namespace std {
  class future_error : public logic_error {
  public:
    explicit future_error(future_errc e);
    const error_code& code() const noexcept;
    const char*       what() const noexcept;
  private:
    error_code ec_;             
  };
}
 explicit future_error(future_errc e);
Effects: Initializes 
ec_ with 
make_error_code(e). const error_code& code() const noexcept;
const char* what() const noexcept;
Returns: An 
ntbs incorporating 
code().message().