Method operator<<

example/matcher.cpp:18–21

  matcher(bool result, const std::string& str) : result_{result}, str_{str} {}
  constexpr explicit operator bool() const { return result_; }
  friend auto operator<<(std::ostream& os, const matcher& self)
      -> std::ostream& {
    return (os << self.str_);
  }

 private:
  const bool result_{};