为什么PRIVATE成员函数不能是另一个类的朋友函数？ [英] Why can't a PRIVATE member function be a friend function of another class?

问题描述

class x
{
    void xx() {}
};

class y
{
    friend void x::xx();
};

这会导致类似

错误：friend function'xx'是'x'的私人成员

error: friend function 'xx' is a private member of 'x'

推荐答案

[class.friend] / 9：

[class.friend]/9:

由朋友声明提名的名称应在包含好友声明的类的
范围内可访问。

A name nominated by a friend declaration shall be accessible in the scope of the class containing the friend declaration.

原因很简单; private 会员必须遵守一个明确而明确的规则：

The reason is quite simple; private members shall obey a clear and definite rule:

可以

A member of a class can be

• private 即

• private; that is, its name can be used only by members and friends of the class in which it is declared.

允许私有成员在不是朋友的类中的声明中被命名将是不一致的：它允许另一个类访问实现细节，而不被明确允许。和访问可以意味着从调用到声明的任何东西。
可以通过 x 整个 y 的朋友规避：

Allowing private members to be named in declarations inside a class that isn't a friend would be inconsistent: It allows another class to access an implementation detail without being explicitly allowed to. And "access" can mean anything from a call to a declaration.
This can be circumvented by making x a friend of y in its entirety:

class x
{
    void xx() {}
};

class y
{
    friend x;
};

演示 。

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