如何在C ++中实现对私有基类的转换 [英] how to implement casting to a private base class in C++

问题描述

如何在C ++中实现对私有基类的转换？我不想使用黑客，例如添加朋友等。定义公共类型转换运算符不起作用。

How to implement casting to a private base class in C++? I don't want to use hacks such as adding a friend etc. Defining public casting operator does not work.

编辑：

例如，我有：

class A {
//base class
}

class AX : private A {
//a child
}

class AY : private A {
//another specialized child
}

class B {
//base class
void do (A a) {//do
    }
}

class BX : private B {
//a child
void do (AX a) {
     B::do( static_cast <A> (a) );
    }
}

class BY : private B {
//another specialized child
void do (AY a) {
    B::do( static_cast <A> (a) );
    }
}

EDIT2

为什么要这样做？

假设我必须定义一些重量级的属性，可以是多个类似的属性类型（例如VelocityX VelocityY等）。然后，我希望能够拥有可以具有这些属性的任何集合的类。如果要处理这些属性，很明显，我宁愿将它们强制转换为它们的基本类型，而不是为每个变体添加实现。我不使用公共继承，因为与隐式可见私有接口相比，最好在需要的地方进行显式转换。这不是一个真正的问题，但是我想有一个解决方案：）

Suppose I have to define some property which is quite heavyweight and can be of several similar types (like VelocityX VelocityY etc). Then I want to be able to have classes which can have any set of these properties. If I want to process these properties, it is obvious, that I'd rather cast them to their base type than to add an implementation for each variation. I do not use public inheritance because it's better to explicitly cast where needed than to have the private interface implicitly visible. Not a real problem but I'd like to have a solution :)

推荐答案

如果定义公共转换运算符不起作用，您可以尝试使用常规功能：

If defining a public casting operator does not work, you can try with a regular function:

class D: private B {
    public:
        B& asB() { return static_cast<B&>(*this); }
};
...
D d;
d.asB().methodInB();
...

无论如何，这是什么意思？如果 D 是从 B 私下获得的，则您不应该使用 D 作为外部的 B 。

Anyway, what is the point? If D derives privately from B, then you are not supposed to use a D as a B from the outside.

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