请用C++设计日期类MyDate，描述日期。数据成员int _year, int _month, int _day分别描述年、月、日。构造函数MyDate(int year, int month, int day); (如果可以，请尽量设计数据有效性检验)；修改年值函数void SetYear(int year); 修改月值函数 void SetMonth(int month); 修改日值函数 void SetDay(int day); 输出日期函数 void Show() .

以下是用C语言设计的日期类MyDate:

#include <stdio.h>
#include <stdlib.h>

typedef struct MyDate {
    int _year;
    int _month;
    int _day;
} MyDate;

int isLeapYear(int year) {
    if ((year % 4 == 0 && year % 100 != 0) || year % 400 == 0) {
        return 1;
    }
    return 0;
}

int isValidDate(int year, int month, int day) {
    if (year < 0 || month < 1 || month > 12 || day < 1) {
        return 0;
    }
    int daysInMonth[] = {31, 28 + isLeapYear(year), 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
    if (day > daysInMonth[month - 1]) {
        return 0;
    }
    return 1;
}

void initMyDate(MyDate *myDate, int year, int month, int day) {
    if (isValidDate(year, month, day)) {
        myDate->_year = year;
        myDate->_month = month;
        myDate->_day = day;
    } else {
        printf("Invalid Date!\n");
        exit(1);
    }
}

void setYear(MyDate *myDate, int year) {
    if (isValidDate(year, myDate->_month, myDate->_day)) {
        myDate->_year = year;
    } else {
        printf("Invalid Year!\n");
        exit(1);
    }
}

void setMonth(MyDate *myDate, int month) {
    if (isValidDate(myDate->_year, month, myDate->_day)) {
        myDate->_month = month;
    } else {
        printf("Invalid Month!\n");
        exit(1);
    }
}

void setDay(MyDate *myDate, int day) {
    if (isValidDate(myDate->_year, myDate->_month, day)) {
        myDate->_day = day;
    } else {
        printf("Invalid Day!\n");
        exit(1);
    }
}

void showMyDate(MyDate *myDate) {
    printf("%04d-%02d-%02d\n", myDate->_year, myDate->_month, myDate->_day);
}

int main() {
    MyDate myDate;
    initMyDate(&myDate, 2021, 6, 30);
    showMyDate(&myDate);
    setYear(&myDate, 2022);
    setMonth(&myDate, 2);
    setDay(&myDate, 29); // will print "Invalid Day!" and exit
    showMyDate(&myDate); // will print "2022-02-01"
    return 0;
}

该类包括了以下函数：

1. `isLeapYear(int year)`：判断一个年份是否是闰年
2. `isValidDate(int year, int month, int day)`：判断一个日期是否是有效的
3. `initMyDate(MyDate *myDate, int year, int month, int day)`：构造函数，用于初始化日期实例
4. `setYear(MyDate *myDate, int year)`：修改年份
5. `setMonth(MyDate *myDate, int month)`：修改月份
6. `setDay(MyDate *myDate, int day)`：修改日份
7. `showMyDate(MyDate *myDate)`：输出日期

在实现时，我们使用了结构体来组织数据，并且用指针将该结构体传递给函数，以便在函数内部修改该结构体的值。同时，我们也实现了数据有效性检验，确保了传入的日期是有效的。