The enum (abbreviation of enumeration) is used to enumerate a set of values (enumerators). A value is an abstract thing represented by a symbol (a word).
For example, a basic enum could be
enum { xs,s,m,l,xl,xxl,xxxl,xxxxl };
This enum is called anonymous because you do not have a symbol to name it. But it is still perfectly correct. Just use it like this
enum { xs,s,m,l,xl,xxl,xxxl,xxxxl } myGrandMotherDressSize;
Ok. The life is beautiful and everything goes well. But one day you need to reuse this enum to define a new variable to store myGrandFatherPantSize, then you write:
enum { xs,s,m,l,xl,xxl,xxxl,xxxxl } myGrandMotherDressSize;
enum { xs,s,m,l,xl,xxl,xxxl,xxxxl } myGrandFatherPantSize;
But then you have a compiler error "redefinition of enumerator". Actually, the problem is that the compiler is not sure that you first enum and you are second describe the same thing.
Then if you want to reuse the same set of enumerators (here xs...xxxxl) in several places you must tag it with a unique name. The second time you use this set you just have to use the tag. But don't forget that this tag does not replace the enum word but just the set of enumerators. Then take care to use enum as usual. Like this:
// Here the first use of my enum
enum sizes { xs,s,m,l,xl,xxl,xxxl,xxxxl } myGrandMotherDressSize;
// here the second use of my enum. It works now!
enum sizes myGrandFatherPantSize;
you can use it in a parameter definition as well:
// Observe that here, I still use the enum
- (void) buyANewDressToMyGrandMother:(enum sizes)theSize;
You could say that rewriting enum everywhere is not convenient and makes the code looks a bit strange. You are right. A real type would be better.
This is the final step of our great progression to the summit. By just adding a typedef let's transform our enum in a real type. Oh the last thing, typedef is not allowed within your class. Then define your type just above. Do it like this:
// enum definition
enum sizes { xs,s,m,l,xl,xxl,xxxl,xxxxl };
typedef enum sizes size_type
@interface myClass {
...
size_type myGrandMotherDressSize, myGrandFatherPantSize;
...
}
Remember that the tag is optional. Then since here, in that case, we do not tag the enumerators but just to define a new type. Then we don't really need it anymore.
// enum definition
typedef enum { xs,s,m,l,xl,xxl,xxxl,xxxxl } size_type;
@interface myClass : NSObject {
...
size_type myGrandMotherDressSize, myGrandFatherPantSize;
...
}
@end
If you are developing in Objective-C with XCode I let you discover some nice macros prefixed with NS_ENUM. That should help you to define good enums easily and moreover will help the static analyzer to do some interesting checks for you before to compile.
Good Enum!
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