c ++：错误C2440：“ <function-style-cast> &#39;：无法从&#39;A转换 <TYPE> &#39;至&#39;B <TYPE> &#39;

Question

I have a base class A and it has a subclass B. A overrides the + operator and B overrides it as well,calls the parent's +operator, and casts the result to B. Then I am getting the error message:

error C2440: '' : cannot convert from 'A' to 'B'

I thought polymorphism worked in a way that should allow this to work?

Answer 1

In polymorphism you cannot convert A to B , you can convert B to A . B is a kind of A, but A is NOT a kind of B.

For example, in the classic Shape classes. If you have a class Shape and a class Rectangle that extends [inherit from] Shape , you cannot convert a Shape instance to Rectangle , but you CAN cast a Rectangle to Shape , because it is a 'kind of' Shape.