effective c++ 条款18：让接口容易被正确使用，不易被误用

class Data {
public:
    Date(int month, int day, int year);
};

Date d(30, 3, 1995); //错误，应该是3,30

struct Day {
    explicit Day(int d): val(d) {}
    int val;
};

struct Month {
    explicit Month(int m) : val(m) {}
    int val;
};

struct Year {
    explicit Year(int y) : val(y) {}
    int val;
};

class Date {
public:
    Date(const Month& m, const Day& d, const Year& y);
    ...
};
Date d(30, 3, 1995); //错误，不正确的数据类型
Date d(Day(30), Month(3), Year(1995)); //错误，不正确的数据类型
Date d(Month(3), Day(30), Year(1995)); //正确

class Month {
public:
    static Month Jan() { return Month(1); }
    static Month Feb() { return Month(2); }
    ...
private:
    explicit Month(int m);
};

Date d(Month::Mar(), Day(30), Year(1995));