活跃成员（已标记）的工会

Question

I've got a union I'm running a program verification software over, and would like to say something about which member is active. So I've tried writing it as a tagged union:

struct my_struct {
    enum my_tag {v1, v2};
    struct{
        enum my_tag tag;
        union {
            type1 v1;
            type2 v2;
        }value;
    }my_taggedunion; 
};

and am running a program verification software on it. I would like to add the requirement that, if my_taggedunion.tag == v1 then the active member in the union should be v1 .

I'm trying to write something like "if my_taggedunion.tag == v1 then my_taggedunion.active_member == v1 . What is the correct syntax for the conclusion?

Answer 1

There is no notion of an "active member" as far as C is concerned. Note that according to the ANSI C standard:

If a pointer to a union is cast to the type of a pointer to a member, the result refers to that member.

(see 3.5.2.1 of X3.159-1989 and A8.3 of Kernighan & Ritchie)

You have to keep track of the relevant part of the union yourself, which in your case you are doing through my_tag .