#include <fmt/ostream.h>

​

struct date {

  int year, month, day;

​

  friend std::ostream& operator<<(std::ostream& os, const date& d) {

    return os << d.year << '-' << d.month << '-' << d.day;

  }

};

​

template <> struct fmt::formatter<date> : ostream_formatter {};

​

std::string s = fmt::format("The date is {}", date{2012, 12, 9});

// s == "The date is 2012-12-9"