C++关于Date类简单问题

本人初学编程，正在预习C++类的概念，请问这个Date类的代码哪里出错了，怎么调，求解释。

要实现构造函数

#include <iostream>
using namespace std;
class Date{
    int y,m,d;
    public:
    Date(int yy, int mm, int dd);
    int month(){return m;}
    int day(){return d;}
    int year(){return y;}
};
Date::Date(int yy, int mm, int dd){
    y=yy;
    m=mm;
    d=dd;
}
int main() {
    Date birthday(1970,12,30);
    cout<<birthday.year()<<","<<birthday.month()<<","<<birthday.day();
    return 0;
}