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