有以下程序: class Date { public: Date(int y,int m,int d) { year = y month = m day = d } Date(int y = 2000) { year = y month = 10 day = 1 } Date(Date &d) { year = d.year month = d.month day = d.day } void print() { cout<<year<<'.'<<month<<'.'<<day<<endl } private: int year,month,day } Date fun(Date d) { Date temp temp = d resurn temp } int main () { Date date1 (2000,1,1),date2 (0,0,0) Date date3 (date1) date2 = fun(date3) return 0 } 程序执行时,Date类的拷贝构造函数被调用的次数是