c ++：error C2440：'< function-style-cast>' ：不能从'A< TYPE>'到'B< TYPE>
我有一个基类A，它有一个子类B.
覆盖+操作符，B也覆盖它，调用父类的+操作符，并将结果转换为B.然后我得到错误讯息：
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:
错误C2440：''：无法将'A'
转换为'B'
error C2440: '' : cannot convert from 'A' to 'B'
我认为多态性的工作方式应该允许这个工作？
I thought polymorphism worked in a way that should allow this to work?
在多态性中，您不能将 A 转换为 B ，可以转换 B 到 A 。 B是一种A，但A不是B的一种。
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.
例如，在经典的Shape类中。如果你有一个类 Shape 和一个类 Rectangle extends [inherit from] Shape ，您不能将 Shape 实例转换为 Rectangle ，但是您可以转换 Rectangle 到 Shape ，因为它是一种形状。
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.