What is a typedef enum in Objective C?

迈不过友情╰ 2022-05-24 05:52 215阅读 0赞

原帖: [http://stackoverflow.com/questions/707512/what-is-a-typedef-enum-in-objective-c][http_stackoverflow.com_questions_707512_what-is-a-typedef-enum-in-objective-c]

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I don't think I fundamentally understand what a enum is, and when to use it.

For example:

typedef enum {     kCircle,     kRectangle,     kOblateSpheroid } ShapeType;

What is really being declared here?

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Three things are being declared here: an anonymous enumerated type is declared, `ShapeType` is being declared a typedef for that anonymous enumeration, and the three names `kCircle`, `kRectangle`, and`kOblateSpheroid` are being declared as integral constants.

Let's break that down. In the simplest case, an enumeration can be declared as

enum tagname { ... };

This declares an enumeration with the tag `tagname`. In C and Objective-C (but not C++), any references to thismust be preceded with the `enum` keyword. For example:

enum tagname x;  // declare x of type 'enum tagname' tagname x;  // ERROR in C/Objective-C, OK in C++

In order to avoid having to use the `enum` keyword everywhere, a typedef can be created:

enum tagname { ... }; typedef enum tagname tagname;  // declare 'tagname' as a typedef for 'enum tagname'

This can be simplified into one line:

typedef enum tagname { ... } tagname;  // declare both 'enum tagname' and 'tagname'

And finally, if we don't need to be able to use `enum tagname` with the `enum` keyword, we can make the `enum`anonymous and only declare it with the typedef name:

typedef enum { ... } tagname;

Now, in this case, we're declaring `ShapeType` to be a typedef'ed name of an anonymous enumeration.`ShapeType` is really just an integral type, and should only be used to declare variables which hold one of the values listed in the declaration (that is, one of `kCircle`, `kRectangle`, and `kOblateSpheroid`). You can assign a `ShapeType` variable another value by casting, though, so you have to be careful when reading enum values.

Finally, `kCircle`, `kRectangle`, and `kOblateSpheroid` are declared as integral constants in the global namespace. Since no specific values were specified, they get assigned to consecutive integers starting with 0, so `kCircle` is 0, `kRectangle` is 1, and `kOblateSpheroid` is 2.

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A user defined type that has the possible values of  `kCircle` ,  `kRectangle` , or  `kOblateSpheroid` . The values inside the enum (kCircle, etc) are visible outside the enum, though. It's important to keep that in mind ( `int i = kCircle;`  is valid, for example).

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A enum declares a set of ordered values - the typedef just adds a handy name to this. The 1st element is 0 etc.

typedef enum { Monday=1, ... } WORKDAYS; WORKDAYS today = Monday;

The above is just a enumeration of shapeType tags.

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[http_stackoverflow.com_questions_707512_what-is-a-typedef-enum-in-objective-c]: http://stackoverflow.com/questions/707512/what-is-a-typedef-enum-in-objective-c