C++ 访问继承中的私有成员
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access private members in inheritance
提问by ofer
I have a class A, which have a field val declared as private. I want to declare a class B, that inherit from A and have an access to val. Is there a way to do it on C++?
我有一个 A 类，它有一个字段 val 声明为私有。我想声明一个类 B，它继承自 A 并可以访问 val。有没有办法在 C++ 上做到这一点？
I want to do it because I need to overload some functions of A, without changing A code at all.
我想这样做是因为我需要重载A的一些功能，而根本不改变A代码。
Thanks.
谢谢。
回答by Constantinius
Quick answer: You don't. Thats what the protectedkey-word is for, which you want to use if you want to grant access to subclasses but no-one else.
快速回答：你没有。这就是protected关键字的用途，如果您想授予对子类的访问权而不是其他人，则要使用该关键字。
privatemeans that no-one has access to those variables, not even subclasses.
private意味着没有人可以访问这些变量，甚至不能访问子类。
If you cannot change code in Aat all, maybe there is a public/protectedaccess method for that variable. Otherwise these variables are not meant to be accessed from subclasses and only hacks can help (which I don't encourage!).
如果您根本无法更改代码A，则该变量可能有public/protected访问方法。否则这些变量不能从子类访问，只有 hacks 可以提供帮助（我不鼓励！）。
回答by Schehaider Aymen
Private members of a base classcan only be accessed by base member functions (not derived classes). So you have no rights not even a chance to do so :)
基类的私有成员只能被基成员函数访问（不能被派生类访问）。所以你没有权利甚至没有机会这样做:)
class Base
class Base
- public: can be accessed by anybody
- private: can only be accessed by only base member functions (not derived classes)
- protected: can be accessed by both base member functions and derived classes
- public: 任何人都可以访问
- private：只能被基成员函数访问（不能被派生类访问）
- protected: 可以被基成员函数和派生类访问
回答by dbrank0
Well, if you have access to base class, you can declare class B as friend class. But as others explained it: because you can, it does not mean it's good idea. Use protected members, if you want derived classes to be able to access them.
好吧，如果您有权访问基类，则可以将 B类声明为友元类。但正如其他人所解释的那样：因为你可以，这并不意味着这是个好主意。如果您希望派生类能够访问它们，请使用受保护的成员。
回答by Eoin
It is doable as describe in this Guru of the Week - GotW #76 - Uses and Abuses of Access Rights. But it's should be considered a last resort.
正如本周 Guru - GotW #76 - 访问权限的使用和滥用中所述，这是可行的。但它应该被认为是最后的手段。
回答by Meysam
You need to define it as protected. Protected members are inherited to child classes but are not accessible from the outside world.
您需要将其定义为protected. 受保护的成员继承给子类，但不能从外部访问。
