C preprocessor: declare structure data from a given enum value

Question

I am trying to automatically declare a given data structure from an enum value. A simple example is worth thousand words:

#define X_FIELDS \
    X(A, upd_foo) \
    X(B, upd_bar)

enum MSGS_TYPES {
#define X(msg_type, data_type) msg_type,
    X_FIELDS
#undef X
    MSGS_SIZE
};

#define DECLARE_MSG(name, type)           X(type) name = { type };  

typedef struct {
    enum MSGS_TYPES type;
    int x;
} upd_foo;

typedef struct {
    enum MSGS_TYPES type;
    int y;
} upd_bar;

int main() {
    DECLARE_MSG(msg, B);     
    printf("%d\n", msg.type);
    msg.y = 0;
    return 0;
}

Here, I'd like for DECLARE_MSG macro to declare
upd_bar msg = { B };
Is there any way to achieve that?

Note that I already tried something like:

#define T_A    upd_foo
#define T_B    upd_bar

#define _T(type)  T_type
#define X(type)   _T(type)
#define DECLARE_MSG(name, type)           X(type) name = { type };  

But it obviously isn't working, complaining that ‘T_type’ is an unknown type name.

Thank you very much!

Answer 1

T_type is considered single token, and type argument is not found on macro expansion.

You need to use concatenation operator ##:

#define _T(type)  T_##type