下面先看一段代码:
- #include <iostream>
- using namespace std;
- class Point{
- public:
- Point(float x=0.0,float y=0.0):_x(x),_y(y){}
- Point& operator=(const Point& rhs);
- void printData(){
- cout<<"_x="<<_x<<endl;
- cout<<"_y="<<_y<<endl;
- }
- protected:
- float _x,_y;
- };
- inline Point& Point::operator=(const Point& p){
- cout<<"Point::operator="<<endl;
- _x=p._x;
- _y=p._y;
- return *this;
- }
-
- class Point3d:public virtual Point{
- public:
- Point3d(float x=0.0,float y=0.0,float z=0.0):Point(x,y),_z(z){}
- Point3d& operator=(const Point3d& p){
- if(this==&p)
- return *this;
- cout<<"Point3d::operator="<<endl;
- _z=p._z;
- return *this;
- }
- void printData(){
- Point::printData();
- cout<<"_z="<<_z<<endl;
- }
- protected:
- float _z;
- };
- int main(){
- Point3d a(1,2,3),b(4,5,6);
- a=b;
- a.printData();
- return 0;
- }
程序输出结果如下:
分析:派生类定义了赋值操作符,但是并不会隐式的调用基类的赋值操作符,这和构造函数和拷贝构造函数不同。
在看看下面这段代码:
- #include <iostream>
- using namespace std;
- class Point{
- public:
- Point(float x=0.0,float y=0.0):_x(x),_y(y){}
- Point& operator=(const Point& rhs);
- void printData(){
- cout<<"_x="<<_x<<endl;
- cout<<"_y="<<_y<<endl;
- }
- protected:
- float _x,_y;
- };
- inline Point& Point::operator=(const Point& p){
- cout<<"Point::operator="<<endl;
- _x=p._x;
- _y=p._y;
- return *this;
- }
-
- class Point3d:public virtual Point{
- public:
- Point3d(float x=0.0,float y=0.0,float z=0.0):Point(x,y),_z(z){}
- void printData(){
- Point::printData();
- cout<<"_z="<<_z<<endl;
- }
- protected:
- float _z;
- };
- int main(){
- Point3d a(1,2,3),b(4,5,6);
- a=b;
- a.printData();
- return 0;
- }
程序输出结果如下:
分析:派生类没有定义自己的赋值操作符,编译器为派生类合成一个赋值操作符函数(合成的赋值操作符函数,会插入调用基类赋值操作符函数的代码),合成的赋值操作符函数会隐式的调用基类的赋值操作符函数。
再看最后一段代码:
- #include <iostream>
- using namespace std;
- class Point{
- public:
- Point(float x=0.0,float y=0.0):_x(x),_y(y){}
- Point& operator=(const Point& rhs);
- void printData(){
- cout<<"_x="<<_x<<endl;
- cout<<"_y="<<_y<<endl;
- }
- protected:
- float _x,_y;
- };
- inline Point& Point::operator=(const Point& p){
- cout<<"Point::operator="<<endl;
- _x=p._x;
- _y=p._y;
- return *this;
- }
-
- class Point3d:public virtual Point{
- public:
- Point3d(float x=0.0,float y=0.0,float z=0.0):Point(x,y),_z(z){}
- Point3d& operator=(const Point3d& p){
- if(this==&p)
- return *this;
- Point::operator=(p);
- cout<<"Point3d::operator="<<endl;
- _z=p._z;
- return *this;
- }
- void printData(){
- Point::printData();
- cout<<"_z="<<_z<<endl;
- }
- protected:
- float _z;
- };
- int main(){
- Point3d a(1,2,3),b(4,5,6);
- a=b;
- a.printData();
- return 0;
- }
程序输出结果如下:
分析:派生类定义的赋值操作符函数显示调用基类定义的赋值操作符函数,先对基类对象部分赋值,再对派生类对象赋值。