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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.........................................1Base say  1
end.........................................2Derived say  2
end.........................................3