为什么基本派生的动态强制转换只允许用于多态类 [英] Why Base-to-Derived Dynamic Casting is Only Allowed for Polymorphic Classes
问题描述
Possible Duplicate:
FAQ: Why does dynamic_cast only work if a class has at least 1 virtual method?
我在C ++中读过，在一组类的层次结构中执行动态转换， cast只允许在类是多态的情况下，例如当基类有一个虚函数等时。这个限制的原因是什么？在基类中有一个纯虚函数代替普通的虚函数更安全吗？
I have read that in C++, performing a dynamic cast down the hierarchy of a set of classes, the cast is allowed only in a situation where the classes are polymorphic, such as when the base class is having a virtual function, etc. What is the reason for this limitation? Is it more 'safe' to have a pure virtual function in place of the normal virtual function in the base class?
谢谢！
推荐答案
此限制的原因是什么？
What is the reason for this limitation?
dynamic_cast 只有在对象是目标类型的实例时才会成功。
非多态类不包含任何类型信息，因此无法判断是否是这种情况;因此，转换不能成功。
dynamic_cast is only supposed to succeed when the object is an instance of the target type.
Non-polymorphic classes don't contain any type information, so there is no way to tell whether this is the case; therefore, the cast cannot succeed.
使用纯虚拟函数代替普通虚拟函数更安全基类？
Is it more 'safe' to have a pure virtual function in place of the normal virtual function in the base class?
就多态性而言，这很好。如果基类有至少一个虚函数，那么它是多态的，因此可以使用 dynamic_cast 。无论是纯还是不影响基类是否可以实例化。
Either is fine, as far as polymorphism is concerned. If the base class has at least one virtual function, then it is polymorphic, and so can be used with dynamic_cast. Whether it's pure or not only affects whether the base class can be instantiated.
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