Usage of union inside a class

Question

I saw some code as follows:

class A 
{
private:
    union {
        B *rep;
        A *next;
    }; // no variables of this anonymous defined!

    void func()
    {
        A *p = new A;

        p->next = NULL; // why p has a member variable of 'next'?
    }
};

I have compiled the above code with VS2010 without any error. Here is the question,

why p has member variable 'next'?

    union {
        B *rep;
        A *next;
    };

As far as I know, this is an anonymous union without even defining a variable. How can we access the member variables inside this union like that?

Answer 1

Because that's pretty much what an anonymous union does, it defines zero-or-more variables in the enclosing namespace (which in a class declaration makes them field names) which occupy overlapping memory. Hence in use it's the same as if you'd declared

class A 
{
private:
    B *rep;
    A *next;

    void func()
    {
        A *p = new A;

        p->next = NULL;
    }
};

...except for rep and next occupying overlapping space (or given that the two pointers will have the same size, the same space), and hence all the dangers and benefits that come with a named union.

Answer 2

Here's the quote from the standard that controls this behavior: section [class.union] (wording from C++0x draft n3242)

A union of the form union { member-specification } ; is called an anonymous union; it defines an unnamed object of unnamed type. The member-specification of an anonymous union shall only define non-static data members. [ Note: Nested types and functions cannot be declared within an anonymous union. — end note ] The names of the members of an anonymous union shall be distinct from the names of any other entity in the scope in which the anonymous union is declared. For the purpose of name lookup, after the anonymous union definition, the members of the anonymous union are considered to have been defined in the scope in which the anonymous union is declared.