Enum in C++ like Enum in Ada?

Question

At one point I had looked at implementing a class/template in C++ that would support an Enum that would behave like it does in Ada. It has been some time since I thought about this problem and I was wondering if anyone has ever solved this problem?

EDIT:

My apologies, I should clarify what functionality I thought were useful in the Ada implementation of the Enum. Given the enumeration

type fruit is (apple, banana, cherry, peach, grape);

We know that fruit is one of the listed fruits: apple, banana, cherry, peach, grape. Nothing really different there from C++.

What is very useful are the following pieces of functionality that you get with every enum in Ada without any additional work:

• printing out an enumerated value generates the string version
• you can increment the enumerated variable
• you can decrement the enumerated variable

I hope this defines the problem a bit more.

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Notes added from comments:

• See: Wikipedia (Judah Himango, 2008-11-19 at 0:09)
• See: Wikibooks

Useful features of Ada enumerations

• The first value in the enumeration is fruit'first which gives apple.
• The last value in the enumeration is fruit'last which gives grape.
• The increment operation is fruit'succ(apple) which gives banana.
• The decrement operation is fruit'pred(cherry) which also gives banana.
• The conversion from enumeration to integer is fruit'pos(cherry) which returns 2 because Ada uses 0-based enumerations.
• The conversion from integer to enumeration is fruit'val(2) which returns cherry.
• The conversion from enumeration to string is fruit'Image(apple) which returns the (upper-case) string "APPLE".
• The conversion from string to enumeration is fruit'Value("apple") which returns the value apple.

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See also related SO questions:

• Which Typesafe Enum in C++ Are You Using?
• Next or previous enum

Answer 1

Okay, let's leave C++ aside for a moment. C++ is just a superset of C (which means everything that can be done in C can be done in C++ as well). So let's concentrate on plain-C (because that's a language I know well). C has enumerations:

enum fruit { apple, banana, cherry, peach, grape };

This is perfectly legal C and the values are contiguous, and apple has the value zero and banana has the value apple + 1. You can create enumerations with holes, but only if you explicitly make holes like this

enum  fruit { apple = 0, banana, cherry = 20, peach, grape };

While apple is 0 and banana is 1, cherry is 20, thus peach is 21 and grape is 22 and everything between 1 and 20 is undefined. Usually you don't want holes. You can do the following:

enum fruit { apple = 0, banana, cherry, peach, grape };
enum fruit myFruit = banana;
myFruit++;
// myFruit is now cherry
printf("My fruit is cherry? %s\n", myFruit == cherry ? "YES" : "NO");

This will print YES. You can also do the following:

enum fruit { apple = 0, banana, cherry = 20, peach, grape };
enum fruit myFruit = banana;
myFruit++;
// myFruit is now cherry
printf("My fruit is cherry? %s\n", myFruit == cherry ? "YES" : "NO");

This will print NO, and the value of myFruit is not the same as any of the enumeration constants.

BTW, to avoid that you must say "enum fruit myFruit", you can avoid the enum with a typedef. Just use "typedef enum fruit fruit;" on an own line. Now you can say "fruit myFruit" without enum in front. It is often done directly when the enum is defined:

typedef enum fruit { apple = 0, banana, cherry, peach, grape } fruit;

fruit myFruit;

Disadvantage is that you don't know anymore that fruit is an enum, it might be an object, a structure or anything else. I usually avoid these type of typedefs, I rather write enum in front if an enum and struct in front if a struct (I will just use them here because it looks nicer).

Getting the string value is not possible. At runtime an enumeration is just a number. That means, it's not possible if you don't know what kind of enumeration that is (as 0 might be apple, but it might also be a different thing of a different enumeration set). However, if you know it is a fruit, then it's easy to write a function that will do it for you. The preprocessor is your friend :-)

typedef enum fruit {
    apple = 0,
    banana,
    cherry,
    peach,
    grape
} fruit;

#define STR_CASE(x) case x: return #x
const char * enum_fruit_to_string(fruit f) {
    switch (f) {
        STR_CASE(apple); STR_CASE(banana); STR_CASE(cherry);
        STR_CASE(peach); STR_CASE(grape);
    }
    return NULL;
}
#undef STR_CASE

static void testCall(fruit f) {
    // I have no idea what fruit will be passed to me, but I know it is
    // a fruit and I want to print the name at runtime
    printf("I got called with fruit %s\n", enum_fruit_to_string(f));
}

int main(int argc, char ** argv) {
    printf("%s\n", enum_fruit_to_string(banana));
    fruit myFruit = cherry;
    myFruit++; // myFruit is now peach
    printf("%s\n", enum_fruit_to_string(myFruit));
    // I can also pass an enumeration to a function
    testCall(grape);
    return 0;
}

Output:

banana
peach
I got called with fruit grape

This is exactly what you wanted or am I totally on the wrong track here?

Answer 2

One of my colleagues has implemented a tool to generate classes that do most (if not all) of what you want:

http://code.google.com/p/enumgen/

The current implementation is in Lisp, but do not hold that against him :-)