error C2668: 'equal' : ambiguous call to overloaded function
模糊调用重载函数

#include<iostream>
using namespace std;
class DayOfYear
{
private:
    int year,month,day;
public:
    DayOfYear(){}
    DayOfYear(int x,int y){month=x;day=y;}
    int get_month(){return month;}
    int get_day(){return day;}
    void input()
    {
        cin>>month>>day;
    }
    void output()
    {
        cout<<month<<day;
    }
    friend bool equal(DayOfYear c1,DayOfYear c2);
};
bool equal(DayOfYear c1,DayOfYear c2)
{
    if((c1.get_month==c2.get_month)&&(c1.get_day==c2.get_day))
        return true;
    else
        return false;
}
int main()
{
    DayOfYear today,your_birthday(3,21);
    cout<<"Enter today's date:"<<endl;
    today.input();
    cout<<"Today's date is";
    today.output();
    cout<<"Your birthday is";
    your_birthday.output();
    if(equal(today,your_birthday))
        cout<<"Happy Birthday to You!"<<endl;
    else
        cout<<"Happy Unbirthday to You!"<<endl;
    return 0;
}

设计一个DayOfYear类，包含年、月、日等数据成员，并声明一个友元函数equal，在主函数中定义两个DayOfYear类的对象，一个用来存放生日，一个用来存放某一具体的日子，通过友元函数equal来比较这两个对象是否相等，从而来判断某一天是否为生日