c++ - 如何在 C++11 中为 OuterClass 制作复制粘贴友好的 typedef？

Question

我注意到在 OuterClass 上手动指定 typedef 代价太大，有时会导致令人尴尬的错误。所以我决定在 OuterClass 上制作一个复制粘贴友好的 typedef。这是我得到的:

#include <type_traits>

struct A{
  typedef A NextOuterClass;
  typedef A SelfClass;
  struct B{
    typedef NextOuterClass OuterClass;
    typedef B NextOuterClass;
    typedef B SelfClass;
    struct C{
      typedef NextOuterClass OuterClass;
      typedef C NextOuterClass;
      typedef C SelfClass;
    };
  };
};

#define CHECK(OWNER,TYPE)\
  static_assert(\
  std::is_same<OWNER::TYPE::OuterClass,OWNER>::value,\
  #OWNER"::"#TYPE" - not ok"\
);
CHECK(A,B);
CHECK(A::B,C);
#undef CHECK

int main(){return 0;}

它工作得很好，但并不总是:

#include <type_traits>

struct I{
  typedef I NextOuterClass;
  typedef I SelfClass;
};

struct D{
  typedef D NextOuterClass;
  typedef D SelfClass;
  struct E:public I{
    typedef NextOuterClass OuterClass; // NextOuterClass == I::NextOuterClass
    typedef E NextOuterClass;
    typedef E SelfClass;
    typedef I ParentClass;
  };
};

#define CHECK(OWNER,TYPE)\
  static_assert(\
  std::is_same<OWNER::TYPE::OuterClass,OWNER>::value,\
  #OWNER"::"#TYPE" - not ok"\
);
CHECK(D,E); // D::E - not ok
#undef CHECK

int main(){return 0;}

如果我删除“typedef I NextOuterClass;”，那么它会起作用，但这是一个错误的决定，因为类“I”也可能有子类:

#include <type_traits>

struct I{
  typedef I NextOuterClass;
  typedef I SelfClass;
  struct G{
    typedef NextOuterClass OuterClass;
    typedef G NextOuterClass;
    typedef G SelfClass;
  };
};

struct D{
  typedef D NextOuterClass;
  typedef D SelfClass;
  struct E:public I{
    typedef NextOuterClass OuterClass; // NextOuterClass == I::NextOuterClass
    typedef E NextOuterClass;
    typedef E SelfClass;
    typedef I ParentClass;
  };
};

#define CHECK(OWNER,TYPE)\
  static_assert(\
  std::is_same<OWNER::TYPE::OuterClass,OWNER>::value,\
  #OWNER"::"#TYPE" - not ok"\
);
CHECK(I,G);
CHECK(D,E); // D::E - not ok
#undef CHECK

int main(){return 0;}

我已经尝试利用“private”和“template”的特性，但仍然没有达到预期的行为。

在 C++11 或 C++14 中有什么可靠的方法可以找到 OuterClass？

如果这里有这样的东西就好了:

std::get_outer_class<T>::type
std::is_nested_class<T>::value

Answer 1

您的示例格式不正确:

3.3.7/1 The following rules describe the scope of names declared in classes.

1. The potential scope of a name declared in a class consists not only of the declarative region following the name’s point of declaration, but also of all function bodies, brace-or-equal-initializers of non-static data members, and default arguments in that class (including such things in nested classes).
2. A name N used in a class S shall refer to the same declaration in its context and when re-evaluated in the completed scope of S. No diagnostic is required for a violation of this rule.
3. If reordering member declarations in a class yields an alternate valid program under (1) and (2), the program is ill-formed, no diagnostic is required.

您的方法依赖于名称 NextOuterClass 在嵌套类定义的不同点引用不同的东西。这正是 (2) 所禁止的。