c++ - 嵌套类继承错误

Question

class A {};
class B : private A {};
class C : private B
{
public:
    class D : private A {}; // Error here
};

此代码给出以下错误(在 VS 2013 中):

nested.cpp(8) : error C2247: 'A' not accessible because 'B' uses 'private' to inherit from 'A'

如果我像这样更改 D 的定义，它会得到修复:

class D : private ::A {};

这是正确的行为吗？如果是，为什么？

一开始我以为是因为 C 私自继承自 B 会隐藏基类。但是，如果我消除“中间人”类 B 并使用它:

class A {};
class C : private A
{
public:
    class D : private A {};
};

错误消失了。

Answer 1

引自 cppreference :

A name that is private according to unqualified name lookup, may be accessible through qualified name lookup

考虑到这一点，让我们看看第一个示例的非限定名称查找如何工作:

class A {};

class B : private A {};

class C : private B
{
public:
    class D : private A {}; // Error here
};

1. A 在C 范围内查找。如果在那里定义，就不会有问题。
2. 它发现 A 是由它的基(私有(private))类 B 私有(private)继承的，因此会引发编译器错误。 Clang 说:

   note: constrained by private inheritance here:
   class B : private A {}; 

Again, as per the quote it should work, if you use fully qualified name, like you have shown

class D : private ::A {};

至于你的最后一个例子:

class A {};

class C : private A
{
public:
    class D : private A {};
};

之所以有效，是因为名称查找适用于属于同一类的所有名称。再次引用 cppreference:

All members of a class (bodies of member functions, initializers of member objects, and the entire nested class definitions) have access to all the names to which a class can access.