Rule of reordering member declaration in class

问题:

I'm reading the c++14 N3797 and I've encountered with 3.3.7/1:

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.

There are (1) and (2):

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, default arguments, exception-specifications, and brace-or-equal-initializers of non-static data members 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.

That is if we write the following:

class A
{
    int a;
    int b;
}

then the program is ill-formed. Reorering member declaration yields an alternate valid program:

class A
{
    int b;
    int a;
}

Might I don't understand this rule correctly?

回答1:

The "alternate valid program" is referring to a situation in which each ordering of the elements in the class yields a valid interpretation of the program, but the meaning changes depending on the ordering.

In your case, changing the order of a and b is allowed, but since their relative order can't affect the meaning of the program, the behavior is defined.

For this to happen, you must use a name in the class that has already been defined with some other meaning outside the class. For example:

typedef void *T;

struct whatever {
    T a;
    typedef long T;
};

Here, the relative order of the declaration of a and the typedef of T affects the meaning of the code. As it's written right now, a has type void *, because the global typedef void *T; is in scope when the T a; is parsed.

If, however, we rearranged the two so as:

typedef void *T;

struct whatever {
    typedef long T;
    T a;
};

...the T a; is equivalent to long a;. The meaning of the program is different due to the relative ordering of the two declarations, so the behavior is undefined.