Convert objective-c typedef to its string equivalent

Question

Assuming that I have a typedef declared in my .h file as such:

typedef enum {
  JSON,
  XML,
  Atom,
  RSS
} FormatType;

I would like to build a function that converts the numeric value of the typedef to a string. For example, if the message [self toString:JSON] was sent; it would return 'JSON'.

The function would look something like this:

-(NSString *) toString:(FormatType)formatType {
  //need help here
  return [];
}

Incidentally, if I try this syntax

[self toString:FormatType.JSON];

to pass the typedef value to the method, I get an error. What am I missing?

Thanks,

Craig Buchanan

Answer 1

You can't do it easily. In C and Objective-C, enums are really just glorified integer constants. You'll have to generate a table of names yourself (or with some preprocessor abuse). For example:

// In a header file
typedef enum {
  JSON,
  XML,
  Atom,
  RSS
} FormatType;

NSString * const FormatType_toString[];

// In a source file
NSString * const FormatType_toString[] = {
  @"JSON",
  @"XML",
  @"Atom",
  @"RSS"
};
...
// To convert enum to string:
NSString *str = FormatType_toString[theEnumValue];

The danger of this approach is that if you ever change the enum, you have to remember to change the array of names. You this problem with some preprocessor abuse, but it's tricky and ugly.

This particular implementation also assumes that the enum values are assigned their defaults, namely starting at 0 and increasing. If you use non-default values, you'll have to modify the name array to have the right names at the right indices.

Answer 2

This is really a C question, not specific to Objective-C (which is a superset of the C language). Enums in C are represented as integers. So you need to write a function that returns a string given an enum value. There are many ways to do this. An of strings such that the enum value can be used as an index into the array or a map structure (e.g. an NSDictionary) that maps an enum value to a string work, but I find that these approaches are not as clear as a function that makes the conversion explicit (and the array approach, although the classic C way is dangerous if your enum values are not continguous from 0). Something like this would work:

- (NSString*)formatTypeToString:(FormatType)formatType {
    NSString *result = nil;

    switch(formatType) {
        case JSON:
            result = @"JSON";
            break;
        case XML:
            result = @"XML";
            break;
        case Atom:
            result = @"Atom";
            break;
        case RSS:
            result = @"RSS";
            break;
        default:
            [NSException raise:NSGenericException format:@"Unexpected FormatType."];
    }

    return result;
}

Your related question about the correct syntax for an enum value is that you use just the value (e.g. JSON), not the FormatType.JSON sytax. FormatType is a type and the enum values (e.g. JSON, XML, etc.) are values that you can assign to that type.

Answer 3

First of all, with regards to FormatType.JSON: JSON is not a member of FormatType, it's a possible value of the type. FormatType isn't even a composite type — it's a scalar.

Second, the only way to do this is to create a mapping table. The more common way to do this in Objective-C is to create a series of constants referring to your "symbols", so you'd have NSString *FormatTypeJSON = @"JSON" and so on.