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 return temp } int main( ) { Date datel(2000,1,1),date2(0,0,0) Date date3(datel) 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