Compliant Solution
In the first compilation module, which is included by the user, the abstract data type is declared as a pointer to a struct. The struct is declared as an incomplete type.
struct string_mx; typedef struct string_mx *string_m;
In the "private" compilation unit struct string_mx is fully defined but non-visible to a user of the API.
union str_union_t {
char *cstr;
wchar_t *wstr;
};
struct string_mx {
size_t size; // This is the size of the string allocation (includes any NULL)
// Note that size is the number of characters, not the number of bytes
// For strtype == STRTYPE_WSTR, you must multiply size by sizeof(wchar_t)
// to compute the number of bytes
size_t maxsize; // maximum size of string
unsigned char strtype;
union str_union_t charset; // set of valid characters
union str_union_t str; // the real string
};