1、    定义日期类Date包含年、月、日3个数据成员，编写一个友元函数，求两个日期之间相差的天数。
#include<iostream.h>
#include<stdio.h>
class Date
{
    int year;
    int month;
    int day;
    public:
        Date(int y,int m,int d)
        {
            year=y;month=m;day=d;
        }
        void disp()
        {
           cout<<year<<month<<day<<endl;
        }
        friend int count_day(Date &d,int);
        friend int l(int year);
        friend int h(Date &d1,Date &d2);
};
int count_day(Date &d,int k)
{
    static int day_tab[2][12]={{31,28,31,30,31,30,31,31,30,31,30,31},
                               {31,29,31,30,31,30,31,31,30,31,30,31}};
               // 使用二维数组存放各月天数，第一行对应非闰年，第二行对应闰年
    int j,i,s;
    if(l(d.year))
        j=1;
    else j=0;
    if(k)
    {
        s=d.day;
        for(i=1;i<d.month;i++)
            s+=day_tab[j][i-1];
    }
    else
    {
        s=day_tab[j][d.month]-d.day;
        for(i=d.month+1; i<=12; i++)
            s+=day_tab[j][i-1];
    }
    return s;
}
int l(int year)
{
    if(year%4==0&&year%100!=0||year%400==0)   // 是闰年
        return 1;
    else   // 不是闰年
        return 0;
}
int h(Date &d1,Date &d2)
{
    int days,day1,day2,y;
    if(d1.year<d2.year)
    {
        days=count_day(d1,0);
        for(y=d1.year+1;y<d2.year;y++)
            if(l(y))
                days+=366L;
            else
                days+=365L;
         days+=count_day(d2,1);
    }
    else if(d1.year==d2.year)
    {
        day1=count_day(d1,1);
        day2=count_day(d2,1);
        days=day2-day1;
    }
    else
        days=-1;
    return days;
}
void main()
{
    Date d1(2000,1,1),d2(2002,10,1);
    int ds=h(d1,d2);
    {
    cout<<"输出结果:"<<endl;
    }
    if(ds>=0)
    {
       d1.disp(); printf("与");
       d2.disp(); printf("之间有%d天\n\n",ds);
    }
    else
        cout<<"时间错误!"<<endl;
}

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