Nested class member access on C++11

问题

In C++11, I am trying to access a member variable of an enclosing class from a nested class in the following way:

struct Enclosing {
    int a;
    struct Nested {
        int f() {
            return a;
        }
    };
};

Even this doesn't compile using g++4.7.2 with -std=c++11, producing error messages of this form:

error: invalid use of non-static data member 'Enclosing::a'

As far as I understand, C++11 treats a nested class as a member of the class, so that supposedly a nested class can access every other member of the enclosing class. Did I do something wrong? Thanks in advance.

Update:

While my question seems to have an answer below, I am not convinced this shall be flagged as duplicate.

I am aware of discussions on the relationship between nested classes and enclosing classes before the C++11 standard, after a lot of searching before posting a question.

Previous relevant discussions like this cite some "updates" in C++11, e.g. C++ nested classes accessibility

But it was not very clear, at least from answers I've read, the full extent that C++11 is "different" from older versions on this matter.

Technically the solution to my question exists in older threads such as Nested class' access to enclosing class' private data members, a fact that had to be pointed out, however inane it makes me seem. But I did not come by any such answer that puts C++11 into context; at least, I don't think my question can be fairly deemed a "duplicate" of a question asked before the C++11 standard.

回答1:

Here is the change within C++11 from cppreference;

Declarations in a nested class can use only type names, static members, and enumerators from the enclosing class (until C++11)

Declarations in a nested class can use any members of the enclosing class, following the usual usage rules for the non-static members. (since C++11)

int x,y; // globals
class enclose { // enclosing class
    int x; // note: private members
    static int s;
 public:
    struct inner { // nested class
        void f(int i) {
            x = i; // Error: can't write to non-static enclose::x without instance
            int a = sizeof x; // Error until C++11,
                              // OK in C++11: operand of sizeof is unevaluated,
                              // this use of the non-static enclose::x is allowed.
            s = i;   // OK: can assign to the static enclose::s
            ::x = i; // OK: can assign to global x
            y = i;   // OK: can assign to global y
        }
        void g(enclose* p, int i) {
            p->x = i; // OK: assign to enclose::x
        }
    };
};

In brief, within C++11, nested class can refer to types and static members of its enclosing class. In addition, it can refer to non-static members only when object of the enclosing class is given to the nested class. A nested class has access to members of its enclosing class including private members.

回答2:

To close this question I'll take this as an answer:

"No, it's not treated as a member of the class, it's just scoped inside of it like anything else. You'll need an instance of Enclosing to access it's members."

this and several other comments addresses the problem in my code. Basically this is something that remains true for C++11.

来源：https://stackoverflow.com/questions/28003447/nested-class-member-access-on-c11

标签

c++

c++11

g++-4.7