class Date {
  int year_, month_, day_;
 public:
  Date(int year, int month, int day) : year_(year), month_(month), day_(day) {}

  int year() const { return year_; }
  int month() const { return month_; }
  int day() const { return day_; }

  friend std::wostream &operator<<(std::wostream &os, const Date &d) {
    os << d.year_ << L'-' << d.month_ << L'-' << d.day_;
    return os;
  }
};

std::wstring s = format(L"The date is {0}", Date(2012, 12, 9));
EXPECT_EQ(L"The date is 2012-12-9", s);