C++指针的小知识

指针为什么在使用时，需要强制类型转换。

using namespace std;

class Base {
public:
    int a;
    Base(int b):a(b){}
    void print(){
        cout<< "Base a:" << a << endl;
    }
};

class Derived :public Base{
public:
    int b = 900;
    Derived(int a,int b):Base(a){
        this->b = b;
    }
    void print(){
        cout<< "Derived a:" << a << " b:"<< b << endl;
    }
    void hello(){
        cout<< "Derived : Hello" << b << endl;
    }
};


int main()
{
    Base *pb = new Base(30); // pb指向Base类型的内容
    Derived *pd = new Derived(40,10); // pd指向Derived类型的内容
    pb->print(); // Base a:30
    pd->print(); // Derived a:40 b:10

    Base *pdd = new Derived(30,90);
    pdd->print();// Base a:30，为什么不是Derived a:30 b:90，因为当前指针是指向Base内容的，所以调用的print()是来自基类Base，而不是来自Derived类
    ((Derived *)pdd)->print();// 进行类型强制转换后，print()才是来自Derived类，所以结果是：Derived a:30 b:90

   ((Derived *)pb)->print(); // Derived a:30 b:0，pb是指向Base的，现在指向Derived，但Derived的b没有初始化，所以只能是0

    ((Derived *)pb)->hello();//Derived : Hello0

    return 0;
}

输出的结果：

Base a:30
Derived a:40 b:10
Base a:30
Derived a:30 b:90
Derived a:30 b:0
Derived :