Stringification on a enumeration member

Question

Hello,

I need to convert an enumeration member (its value, not the identifier) to a string. I have tried the following, which works for a MACRO (TYPE_A), but not for an enum value (typeA). This is a little bit strange in my opinion.

Do you have any idea how to do this?

#define _tostr(a) #a
#define tostr(a) _tostr(a)

typedef enum _SPECIAL_FOLDER_ID {
    typeA = 3,
    typeB = 4,
} SPECIAL_FOLDER_ID;

#define TYPE_A 3

int main() {
    //this is working, but the information is a macro (TYPE_A)
    printf("The string is " tostr(TYPE_A) ".\n");

    //this is not working for typeA (defined in an enumeration)
    printf("The string is " tostr(typeA) ".\n");
    return 0;
}

The output is:

The string is 3.
The string is typeA.

I need to modify the code in some way so that the second line of the output will be "The string is 3."

Thanks!

PS: I do not want to print the value using printf. I need a static string containing that value. I only use printf to test the result...

Answer 1

Whats wrong with the following?

printf("The string is %d.\n", typeA );

You seem to be overcomplicating matters somewhat ...

Answer 2

What's wrong with good old formated output?

printf("The string is %d", typeA)

Answer 3

Using the two nested macros is a trick that forces the preprocessor expand any macro arguments (TYPE_A -> 3).

Enum values, however, are not expanded by the preprocessor, they are expanded by the compiler.

Answer 4

The preprocessor does not know C. It simply knows "text".
When it processes your file, typeA is just 5 letters. Only the compiler will know (after the preprocessor is done) that typeA has a value, and that the value is 3.

Answer 5

There is no automatic way to do this since the compiler converts ENUMs to literal numbers.

The easy way is to have a static array of strings and be careful to keep it in sync.

char names[][8] { "","","","typeA","typeB", etc }
Then just use "names[typeA]"

A safer way is to have a function with a big switch statement.

Answer 6

There's not really a good way to accomplish this. The best I could come up with is

#define typeA_ 3
#define typeB_ 4

enum
{
    typeA = typeA_,
    typeB = typeB_
};

#define tostr__(E) #E
#define tostr_(E) tostr__(E)
#define tostr(E) tostr_(E ## _)

Answer 7

I tend to use the static array approach. It's still a kluge, but it's reasonably clear and it works.

enum {
  typeA = 3,
  typeB = 4,
  NUM_LETTERS = 5
} Letters;

static const char* letterNames[NUM_LETTERS] {
  "", "", "",
  "3",
  "4"
};

printf("The string is " letterNames[(int)typeA] ".\n");

It looks as though Christoph's answer is good, but I have to be honest that I'm not familiar enough with macros to understand it ;-)

Edit; Another way: You mention that you want a 'static string' but I'm not sure that you need it at compile time. Can you use sprintf at the start of runtime? This solution would look something like this...

enum {
  typeA = 3,
  typeB = 4,
  NUM_LETTERS = 5
} Letters;


int main(void){
    char * typeAString = new char[sizeof("This is at least as long as the typeA string")];
    sprintf(typeAString, "This is the %d string", typeA);
    // use your string here
    return 0;
}

The way I used new here is not what I would recommend, but it shows of the idea of using sprintf in your program.