32 Thread support library [thread]

32.9 Futures [futures]

32.9.4 Class future_­error [futures.future.error]

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_;             // exposition only
  };
}