我应该从一个私人派生类指针转换到它的基类吗？ [英] Should I pointer-cast from a private derived class to its base class?

问题描述

我从C ++常见问题中找到这个问题

一般来说，

从
私人派生类的成员函数或朋友，与基类的
关系是
已知，并且从
PrivatelyDer *向上转换为Base *或
PrivatelyDer& to Base&）是安全的;

但是，PrivatelyDer的用户应该
避免这种不安全的转换，因为
基于

如何理解上述词语？我不认为解释正确或准确。

我有一个这样的代码

  class A {
}; 
 
 B类：private A {
}; 
 
 int main（）{
 
 B * b = new B（）; 
 A * a = new A（）; 
 
 a = b; // wrong 
 a =（A *）b; // right 
 
} 
  

解决方案

从纯粹的机械观点来看，你是对的：对私人基类的转换将工作并产生工作结果。

FAQ的意义是，设计观点一般是错误的。私人继承真的应该是私人的 - 换句话说，即使它可能工作，你不应该知道它会工作，在某些时候它可能会退出工作 - 因为它是正式的实现细节，不是部分的公共接口，他们可以重新实现类而不使用继承。在这一点上，演员不会再工作了（但是因为你使用了演员，编译器可能不会警告你它从你可能不应该做的事情，不能工作的东西）

编辑：是的，演员 必须工作。根据标准的§5.4/ 7：

...以下static_cast和
reinterpret_cast操作
可选地后面跟随一个const_cast
操作）可以使用
转换符号执行显式类型
转换，即使基类
类型不可访问：

- 指向派生的
类类型的对象的指针或派生的
类类型的值可以显式转换
指向
明确的基类类型
的指针或引用;

/ p>

I found this from C++FAQ

Generally, No.

From a member function or friend of a privately derived class, the relationship to the base class is known, and the upward conversion from PrivatelyDer* to Base* (or PrivatelyDer& to Base&) is safe; no cast is needed or recommended.

However users of PrivatelyDer should avoid this unsafe conversion, since it is based on a private decision of PrivatelyDer, and is subject to change without notice.

How to understand the above words? I don't think the explanation is correct or accurate.

I have a code like this

class A{
};

class B: private A{
};

int main(){

    B *b = new B();
    A *a = new A();

    a = b;                    //wrong
    a = (A*)b;            //right

}

解决方案

From a purely mechanical viewpoint, you're right: a cast to a private base class will work and produce working results.

The point of the FAQ is that from a design viewpoint it's generally wrong. Private inheritance is really supposed to mean private -- in other words, even though it may work, you're not supposed to know it'll work, and at some point it may quit working -- since it's officially an implementation detail, not part of the public interface, they could re-implement the class without using inheritance. At that point, the cast wouldn't work any more (but because you've used a cast, the compiler probably won't warn you about it having gone from something you probably shouldn't do to something that can't possibly work at all).

Edit: Yes, the cast does necessarily work. According to §5.4/7 of the standard:

... the following static_cast and reinterpret_cast operations (optionally followed by a const_cast operation) may be performed using the cast notation of explicit type conversion, even if the base class type is not accessible:

— a pointer to an object of derived class type or an lvalue of derived class type may be explicitly converted to a pointer or reference to an unambiguous base class type, respectively;

[emphasis added]

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