C++ 求两日期间相隔天数

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#include <iostream>#include <algorithm>#include <cmath>using namespace std;struct Date{    int year;    int month;    int day;    Date(int y = 0, int m = 0, int d = 0):        year(y), month(m), day(d) {}    Date & readIn() {        cin >> year >> month >> day;        return *this;    }    void swap(Date & rhs) {        Date t(*this);        *this = rhs;        rhs = t;    }};bool is_leap(int year) {    return (year % 400)|| (year % 4 && year % 100);}bool operator < (const Date & lhs, const Date & rhs) {    return (lhs.year < rhs.year) ||            (lhs.year == rhs.year) && (lhs.month < rhs.month) ||            (lhs.year == rhs.year) && (lhs.month == rhs.month && lhs.day < rhs.day);}int days_of_month(int year,int month) {    switch(month) {    case 1: case 3: case 5:case 7:    case 8: case 10: case 12:        return 31;    case 2:        return is_leap(year) ? 29: 28;    case 4: case 6: case 9: case 11:        return 30;    default:        return -1;    }}int day_diff(Date lhs, Date rhs) {<span style="white-space:pre"></span>//通过不断用其中一个向另一个逼近来求相差天数    int flag = 1;    if(rhs < lhs) {        swap(lhs, rhs);        flag = -1;    }    int day_shf = 0;    if(lhs.day < rhs.day) {        day_shf += rhs.day - lhs.day;        rhs.day = lhs.day;    }    if(lhs.day > rhs.day) {        day_shf -= lhs.day - rhs.day;        lhs.day = rhs.day;    }    while(lhs.month < rhs.month) {        day_shf += days_of_month(lhs.year ,lhs.month);        ++lhs.month;    }    while(lhs.month > rhs.month) {        day_shf -= days_of_month(rhs.year ,rhs.month);        ++rhs.month;    }<span style="white-space:pre"></span>//两个日期始终以相互为界,故日期大小关系不可能改变    while(lhs.year < rhs.year) {        day_shf += is_leap(lhs.year) ? 366: 365;        ++lhs.year;    }    return day_shf*flag;}int main(){    Date d1, d2;    while(true) {        d1.readIn(); d2.readIn();        cout << day_diff(d1, d2) << endl;    }    return 0;}
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