在子类中将非纯虚函数转换为纯虚函数 [英] Turning a non-pure virtual function into pure in a subclass

问题描述

所以，我有这个多态层次结构:

ClassA不是抽象的，没有纯虚函数，而是几个虚函数B类:公共A类为某种类型的子类定义扩展接口；是抽象的，带有纯虚函数C类:公共B类可用的类，不再需要子类化

这是交易，我将把 ClassA 和 ClassC 的对象一起扔到容器中并进行迭代.为了执行这个迭代，一个非纯虚函数存在于 ClassA 中，但只有 {} 是空的；也就是说，它是空的，只有在迭代遇到 ClassC 时才可用，在这种情况下它被调用，否则它什么都不做.我不能让它是纯的，否则我不能拥有 ClassA 的对象.

但是为了确保ClassC确实实现了那个函数，迫使那个类的用户这样做，我在ClassB中把这个函数设为纯虚函数.>

这可以接受吗?如果我采用非纯虚函数，将其设为纯，然后在 ClassC 中再次设为非纯，则什么都不会破坏"?

解决方案

如果按照您在解释中提出的方式实施，您就可以了.无需深入了解抽象基类和虚函数的整个部分，标准规定:

<块引用>

C++ § 10.4p2

抽象类是只能作为其他类的基类使用的类；不能创建抽象类的对象，除非作为从它派生的类的子对象.如果一个类至少有一个纯虚函数，那么它就是抽象的.[注意:这样的函数可能会被继承:见下文.— end note ] 通过在类定义的函数声明中使用纯说明符 (9.2) 来指定虚函数是纯的.纯虚函数只需要在使用限定 ID 语法 (5.1) 或如同使用 (12.4) 调用时才需要定义

上面引用的下面"导致此注释:

<块引用>

C++11 § 10.4p5

[注意:抽象类可以从非抽象类派生，纯虚函数可以覆盖非纯虚函数.- 尾注]

So, I have this polymorphic hierarchy:

ClassA
Is not abstract, no pure virtual functions, but a few virtual functions

ClassB:public ClassA
Defines an extended interface for a certain type of subclass; 
 is abstract with pure virtual functions

ClassC:public ClassB
Usable class, no more subclassing

Here's the deal, I will have objects of ClassA and ClassC thrown together into containers and iterated through. To perform this iteration, a non-pure virtual function is present in ClassA but is empty with just {}; that is, it is empty, made available only if the iteration encounters a ClassC in which case it is invoked, otherwise it does nothing. I can't have it be pure otherwise I cannot have objects of ClassA.

But to ensure that ClassC does in fact implement that function, forcing the user of that class to do so, I make this function pure virtual in ClassB.

Is this acceptable? Nothing will "break" if I take a non-pure virtual function, make it pure, then make it non-pure again in ClassC ?

解决方案

You're fine if implemented as you present in your explanation. Without going into entire sections on abstract base classes and virtual functions, the standard dictates:

C++ § 10.4p2

An abstract class is a class that can be used only as a base class of some other class; no objects of an abstract class can be created except as subobjects of a class derived from it. A class is abstract if it has at least one pure virtual function. [ Note: Such a function might be inherited: see below. — end note ] A virtual function is specified pure by using a pure-specifier (9.2) in the function declaration in the class definition. A pure virtual function need be defined only if called with, or as if with (12.4), the qualified-id syntax (5.1)

The "below" referenced above leads to this note:

C++11 § 10.4p5

[Note: An abstract class can be derived from a class that is not abstract, and a pure virtual function may override a virtual function which is not pure. - end note]

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