Getting the number of members of a union

Question

Is there a way to get the number of members of a union in C++? For example:

union U
{
    int a;
    double b;
    char c;
};

int main()
{
    std::cout << std::union_members_count<U>::value << std::endl;  // prints 3
}

Of course, std::union_members_count<> is fictional.

If there is a way, how do I implement/use it?

There is no such functionality in C++. You will have to wait and hope for reflection proposals to go through. Alternatively use external tools such as clang-query and some preprocessing script. — nwp, Mar 13 '18 at 12:35
i'm working with protocol where each message has fields that are identifiable from a union unique for each message. so the maximum number of fields for a message is dependent on the number of members for the union associated with that message. Unfortunately, the encoder for the message is based on a fixed array whose size is a template parameter. so i have to manually supply the number of members of the union as a template argument for the encoder for each message. If the number of members in the union changes, then i have to change the value for the template argument of the encoder. — hermit.crab, Mar 13 '18 at 13:07
It would be convenient if the size of the encoder array can be automatically deduced based on the number of members of the union, since the protocol has a lot of message types. the encoder, unfortunately, has been in existence for about 10+ years already, and it has no unit tests. so, changing the encoder to use a dynamic array is dangerous. — hermit.crab, Mar 13 '18 at 13:08

score 7 · Accepted Answer · answered Mar 13 '18 at 12:36

7

No, this is not possible in C++.

C++ does not have reflection, a feature for code that describes itself.

answered Mar 13 '18 at 12:36

1 Answers1