C++数据切片问题，Plus多态

问题引入：

在C++开发中，数据切片是指当子类对象赋值给父类对象时，父类对象只能接收到子类对象中与父类对象数据成员对应的部分。而子类对象中新增加或修改的数据成员则被丢失，这就是数据切片问题。

1.复用的代码示例 #include <iostream> class Base { public: Base():BaseNum(1){} virtual void say(){std::cout << "Base say " << BaseNum << std::endl;} virtual ~Base(){} private: int BaseNum; }; class Derived :public Base { public: Derived():DerivedNum(2){} virtual ~Derived(){} virtual void say(){std::cout << "Derived say " << DerivedNum << std::endl;} void Special() {std::cout << "Derived Special" << std::endl;} private: int DerivedNum; }; 2.数据切片发生场景示例

结果看用子类初始化父类，能力并没有发生变化

父类不能调用子类重写虚函数

父类不能调用子类普通的虚函数

int main() { //栈对象，基类仅仅拷贝了派生类的基类 Derived d2; Base b2 = d2; b2.say(); return 0; } 输出 Base say 1 3.基类想使用子类重写的虚函数，可以用引用

引用情况下：

父类能调用子类重写虚函数

父类不能调用子类普通的虚函数

int main() { //引用对象，基类也可以实现派生类的多态行为 Derived d3; Base &b3 = d3; b3.say(); return 0; } 输出 Derived say 2 4.对象创建在堆，基类指针可以调用派生类重写的虚函数 int main() { //指针访问对象可以获得多态行为 Derived* d1 = new Derived(); Base* b1 = new Derived(); b1->say(); delete d1; delete b1; return 0; } 输出 Derived say 2 5.完整代码 int main() { //指针访问对象可以获得多态行为 Derived* d1 = new Derived(); Base* b1 = new Derived(); b1->say(); delete d1; delete b1; std::cout << "end.........................................1\n" << std::endl; //栈对象，基类仅仅拷贝了派生类的基类 Derived d2; Base b2 = d2; b2.say(); std::cout << "end.........................................2\n" << std::endl; //引用对象，基类也可以实现派生类的多态行为 Derived d3; Base &b3 = d3; b3.say(); std::cout << "end.........................................3" << std::endl; return 0; } 输出 Derived say 2 end.........................................1 Base say 1 end.........................................2 Derived say 2 end.........................................3