请让我理解将派生类对象分配给基类指针-C ++

Question

假设你有一个基类基础和派生类派生。

Base b;
Derived *d;
d=&b; // this line gives error, why? I think it asks for typecasting, why?
//When you assign derived class object address to base class pointer, it works fine. Why not the above case works fine

虽然以下代码有效，

Derived *d=new Base(); //no typecasting required here, why?

上述两种情况有什么区别？

谢谢！

Answer 1

Base is-not-a Derived，所以这两个都没有：

Base b;
Derived *d;
d=&b; 

也不是：

Derived* d = new Base();

有任何意义或作品。两者都会产生类似

的结果

  

错误：从“Base *”到“Derived *”[-fpermissive]

的无效转换

两者都做了完全相同的事情：将Base*分配给Derived*。