把compare的参数设置成const就无法调用公有函数，如何解决？

Question

把compare的参数设置成const就无法调用公有函数？

int compare(const Date& d){
            if(d.toInt()>toInt()){
                return 1;
            }
            else if(d.toInt()<toInt()){
                return -1;
            }
            return 0;
        }

报错如下
[Error] passing 'const Date' as 'this' argument of 'int Date::toInt()' discards qualifiers [-fpermissive]
不管调哪个函数都不让过编译

#include<iostream>
#include<string>
using namespace std;
class Date{
    private :
        int year,month,day;
    public  :
        Date(int y=1900,int m=1,int d=1):year(y),month(m),day(d){}
        Date(const string&s){
            int a=0;
            for(int i=0;i<4;++i){
                a=a*10+s[i]-'0';
            }
            year=a;
            a=0;
            for(int i=5;i<7;++i){
                a=a*10+s[i]-'0';
            }
            month=a;
            a=0;
            for(int i=8;i<10;++i){
                a=a*10+s[i]-'0';
            }
            day=a;
        }
        void set(int y,int m,int d){
            year=y;
            month=m;
            day=d;
        }
        bool isValid(){
            int d[12]={31,28,31,30,31,30,31,31,30,31,30,31};
            int d2[12]={31,29,31,30,31,30,31,31,30,31,30,31};
            if(isLeapYear()&&month<=12&&month>=1&&day<=d2[month-1]&&day>0){
                return true;
            }
            if(!isLeapYear()&&month<=12&&month>=1&&day<=d[month-1]&&day>0){
                return true;
            }
            return false;
        }
        bool isLeapYear(){
            if(year%100!=0&&year%4==0||year%400==0){
                return true;
            }
            return false;
        }
        int toInt(){
            int x;
            x=year*10000;
            x+=month*100;
            x+=day;
            return x; 
        }
        Date lastDay(){
            Date x;
            int d[12]={31,28,31,30,31,30,31,31,30,31,30,31};
            int d2[12]={31,29,31,30,31,30,31,31,30,31,30,31};
            if(month==1&&day==1){
                x.set(year-1,12,31);
            }
            else if(isLeapYear()&&day==1){
                x.set(year,month-1,d2[month-2]);
            }
            else if(!isLeapYear()&&day==1){
                x.set(year,month-1,d[month-2]);
            }
            else x.set(year,month,day-1);
            return x;
        }
        Date nextDay(){
            Date x;
            int d[12]={31,28,31,30,31,30,31,31,30,31,30,31};
            int d2[12]={31,29,31,30,31,30,31,31,30,31,30,31};
            if(month==12&&day==31){
                x.set(year+1,1,1);
            }
            else if(isLeapYear()&&day==d2[month-1]){
                x.set(year,month+1,1);
            }
            else if(!isLeapYear()&&day==d[month-1]){
                x.set(year,month+1,1);
            }
            else x.set(year,month,day+1);
            return x;
        }
        void print(){
            cout<<year<<'-'<<month<<'-'<<day<<endl;
        }
        int getYear(){
            return year;
        }
        int getMonth(){
            return month;
        }
        int getDay(){
            return day;
        }
        void setYear(int y){
            year=y;
        }
        void setMonth(int m){
            month=m;
        }
        void setDay(int d){
            day=d;
        }
        int compare(const Date& d){
            if(d.toInt()>toInt()){
                return 1;
            }
            else if(d.toInt()<toInt()){
                return -1;
            }
            return 0;
        }
};
int main(){
    Date today(2023,3,15);
    Date yesterday("2023/3/14");
    Date theday;
    theday.set(2222,2,29); 
    cout<<"2222年是闰年吗？"<<endl;
    cout<<theday.isLeapYear()<<endl;
    cout<<"theday是合法的日期吗？"<<endl;
    cout<<theday.isValid()<<endl;
    cout<<"数字化"<<endl;
    cout<<theday.toInt()<<endl;
    cout<<"昨天的前一天是"<<endl; 
    yesterday.lastDay().print();
    cout<<"今天的后一天是"<<endl;
    today.nextDay().print();
    cout<<"今天是几几年？"<<endl;
    cout<<today.getYear()<<endl;
    cout<<"今天是几月？"<<endl;
    cout<<today.getMonth()<<endl;
    cout<<"今天是几号？"<<endl;
    cout<<today.getDay()<<endl;
    cout<<"随便输入一个日期，看看是在今天前面还是后面"<<endl;
    int y,m,d;
    cin>>y>>m>>d;
    Date aday;
    aday.setYear(y);
    aday.setMonth(m);
    aday.setDay(d);
    if(today.compare(aday)>0){
        cout<<"在今天后面"<<endl;
    }
    else if(today.compare(aday)<0){
        cout<<"在今天前面"<<endl;
    }
    else cout<<"是同一个日期啊"<<endl;
} 

Answer 1

参考GPT和自己的思路：

这个问题是因为您在compare函数中使用了toInt函数，但是toInt函数不是const函数，因此无法在const函数中调用。解决方法是将toInt函数也声明为const函数，即在函数声明和定义中都加上const关键字，如下所示：

int toInt() const {
    int x;
    x=year*10000;
    x+=month*100;
    x+=day;
    return x;
}

这样就可以在compare函数中调用toInt函数了。