Generic method to display enum value names

Question

Is there a way to display the name of an enum's value? say we have:

enum fuits{
    APPLE,
    MANGO,
    ORANGE,
};

main(){
enum fruits xFruit = MANGO;
...
printf("%s",_PRINT_ENUM_STRING(xFruit));
...
}

using the preprocessor

#define _PRINT_ENUM_STRING(x) #x

won't work as we need to get the value of the variable 'x' and then convert It to string. Is this at all possible in c/C++?

Answer 1

You could use the preprocessor to do this, I believe this technique is called X-Macros:

/* fruits.def */
X(APPLE)
X(MANGO)
X(ORANGE)

/* file.c */
enum fruits {
#define X(a) a,
#include "fruits.def"
#undef X
};

const char *fruit_name[] = {
#define X(a) #a,
#include "fruits.def"
#undef X
};

Note that the last entry includes a trailing comma, which is allowed in C99 (but not in C89). If that is a problem you can add sentinal values. It is also possible to make the macro more complicated by giving multiple arguments for custom names or enum values, etc:

X(APPLE, Apple, 2)
#define X(a,b,c) a = c,        /* in enum */
#define X(a,b,c) [c] = #b,     /* in name array */

Limitations: You cannot have negative constants and your array is sizeof (char *) * largest_constant. But you could work around both by using an extra lookup table:

int map[] = {
#define X(a,b,c) c,
#include "fruits.def"
#undef X
};

This doesn't work of course. What does work is generating an extra set of enum constants as keys for the names:

enum fruits {
#define X(a,b,c) a ## _KEY,
#include "fruits.def"
#undef X
#define X(a,b,c) a = c,
#include "fruits.def"
#undef X
};

Now you can find the name of X(PINEAPPLE, Pineapple, -40) by using fruit_name[PINEAPPLE_KEY].

People noted that they didn't like the extra include file. You don't need this extra file, you also use a #define. This may be more appropriate for small lists:

#define FRUIT_LIST X(APPLE) X(ORANGE)

And replace #include "fruits.def with FRUIT_LIST in the previous examples.