Описание тега x-macros

X-Macros are a use of the c-preprocessor to use the same tokens in different patterns.
They either use #includes or additional macros for the input to transform:

#define MAPPING X(0, 55 do_a) X(1, 42, do_b) X(23, 7, do_c)

// Sometime later:
    switch(expr) {
        #define X(a, b, c) case a: return b + c(a, b);
        MAPPING
        #undef X
    default:
        abort();
    }

Or maybe:

    switch(expr) {
        #define X(a, b, c) case a: return b + c(a, b);
        #include "mapping.inc"
        #undef X
    default:
        abort();
    }

Why use X?
It's extremely short, and unlikely to already be #define-d.