不懂：求助基类类型指针指向派生类对象？

#include <iostream.h>

class Base
{
public:
Base() { }
virtual ~Base()
{
cout<<" call Base::~Base "<<endl;
}
};

class Derived: public Base
{
public:
Derived() { };
~Derived()
{
cout<<" call Derived::~Derived "<<endl;
}
};

main()
{
Base *pob_b =new Derived;
delete pob_b;

return 0;
}


红色部分是基类类型指针指向派生类对象，我的疑问是：指针不是都有类型限制吗？
Base 型指针怎么能够 “随便” 指向一个其他类的对象（这里就是派生类对象）呢？为什么不出错呢？疑惑ing

请教！