dynamic_cast<type>()转型与typeid()的使用

按照c++之父的经典之作 c++程序设计语言的说法

dynamic_cast<type>()可以把基类的指针转化为子类的指针，代码如下：

#include<iostream>
#include<string>
using namespace std;
class Person{
public:
 Person(string name,int age){
  this->name=name;
  this->age=age;
 }
 virtual void show(){
  cout<<name<<" "<<age<<endl;
 }
  virtual ~Person(){}
private:
 string name;
 int age;
};
class Student: public Person{
public:
 Student(string name1,int age1,string num1):Person(name1,age1),num(num1){}
    void show(){
     Person::show();
     cout<<num<<endl;
    }
    ~Student(){}
private:
  string num;
};
int main(){
 
 Person *p;
 Person ss("zhang",22);
 p=&ss;
 
  Student *s=dynamic_cast<Student*>(p);
  if(s==0){
   cout<<"NULL POINTER"<<endl;
  }else{
   cout<<"IT IS NOT NULL POINTER"<<endl;
  }
 //s->show();
 
 system("pause");
 return 0;
}

以上的代码理论上是可以运行的，但是输出结果为 NULL POINTER,为什么呢？哪位仁兄可以指点一二，哪里出问题了，小弟感激不尽。

typeid()则可以用来判断类型是否一致，typeid().name()可以返回类型，代码如下

#include<iostream>
#include<string>
using namespace std;
class Person{
public:
 Person(string name,int age){
  this->name=name;
  this->age=age;
 }
 virtual void show(){
  cout<<name<<" "<<age<<endl;
 }
 virtual ~Person(){}
private:
 string name;
 int age;
};
class Student: public Person{
public:
 Student(string name1,int age1,string num1):Person(name1,age1),num(num1){}
 void show(){
  Person::show();
  cout<<num<<endl;
 }
 ~Student(){}
private:
 string num;
};
int main()
{
 Person p("zhang",22);
 Person pp("li",21);
 Student ss("wangwu",25,"091150");
 if(typeid(p)==typeid(pp)){
  cout<<"p,pp类型相同"<<endl;
 }else{
  cout<<"p,pp类型不相同"<<endl;
 }

 if(typeid(p)==typeid(ss)){
  cout<<"p,ss类型相同"<<endl;
 }else{
  cout<<"p,ss类型不相同"<<endl;
 }

 int a=100;
 cout<<"a的类型为"<<typeid(a).name()<<endl;
 cout<<"p的类型为"<<typeid(p).name()<<endl;
 system("pause");
 return 0;
}

欢迎回帖，第一个代码问题出在哪里，为什么s为空，朋友你知道吗？

问题得到了解决：

主函数改成

int main(){

Person *p = new Student("zhang",22,"091150");
Student *s=dynamic_cast<Student*>(p);
if(s==0){
cout<<"NULL POINTER"<<endl;
}else{
cout<<"IT IS NOT NULL POINTER"<<endl;
s->show();
}
system("pause");
return 0;

}