Objective C - when should "typedef" precede "enum", and when should an enum be named?

In sample code, I have seen this:

typedef enum Ename { Bob, Mary, John} EmployeeName;

and this:

typedef enum {Bob, Mary, John} EmployeeName;

and this:

typedef enum {Bob, Mary, John};

but what compiled successfully for me was this:

enum {Bob, Mary, John};

I put that line in a .h file above the @interface line, and then when I #import that .h file into a different class's .m file, methods there can see the enum.

So, when are the other variants needed?

If I could name the enum something like EmployeeNames, and then, when I type "EmployeeNames" followed by a ".", it would be nice if a list pops up showing wha开发者_C百科t the enum choices are.

In C (and hence Objective C), an enum type has to be prefixed with enum every time you use it.

enum MyEnum enumVar;

By making a typedef:

typedef MyEnum MyEnumT;

You can write the shorter:

MyEnumT enumVar;

The alternative declarations declare the enum itself and the typedef in one declaration.

// gives the enum itself a name, as well as the typedef
typedef enum Ename { Bob, Mary, John} EmployeeName;

// leaves the enum anonymous, only gives a name to the typedef
typedef enum {Bob, Mary, John} EmployeeName;

// leaves both anonymous, so Bob, Mary and John are just names for values of an anonymous type
typedef enum {Bob, Mary, John};

The names inside enum { } define the enumerated values. When you give it a name, you can use it as a type together with the keyword enum, e.g. enum EmployeeName b = Bob;. If you also typedef it, then you can drop the enum when you declare variables of that type, e.g. EmployeeName b = Bob; instead of the previous example.

Your third example is the same as your last example - the typedef there is useless - GCC even gives a warning about that case:

warning: useless storage class specifier in empty declaration

Your first and second example are also partly equivalent, in that they both give the enum type a name EmployeeName. The first example also lets you use enum Ename interchangeably with EmployeeName; in the second example, EmployeeName is the only option. The second example has to be written as you have - you can decompose the first example as follows:

enum Ename { Bob, Mary, John };
typedef enum Ename EmployeeName;

Maybe that helps clear things up?