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This chapter is from the book

Item 5: Use Enumerations for States, Options, and Status Codes

Since Objective-C is based on C, all the features of C are available. One of these is the enumeration type, enum. It is used extensively throughout the system frameworks but is often overlooked by developers. It is an extremely useful way to define named constants that can be used, for example, as error status codes and to define options that can be combined. Thanks to the additions of the C++11 standard, recent versions of the system frameworks include a way to strongly type such enumeration types. Yes, Objective-C has benefitted from the C++11 standard as well!

An enumeration is nothing more than a way of naming constant values. A simple enumeration set might be used to define the states through which an object goes. For example, a socket connection might use the following enumeration:

enum EOCConnectionState {
    EOCConnectionStateDisconnected,
    EOCConnectionStateConnecting,
    EOCConnectionStateConnected,
};

Using an enumeration means that code is readable, since each state can be referred to by an easy-to-read value. The compiler gives a unique value to each member of the enumeration, starting at 0 and increasing by 1 for each member. The type that backs such an enumeration is compiler dependent but must have at least enough bits to represent the enumeration fully. In the case of the preceding enumeration, this would simply need to be a char (1 byte), since the maximum value is 2.

This style of defining an enumeration is not particularly useful, though, and requires the following syntax:

enum EOCConnectionState state = EOCConnectionStateDisconnected;

It would be much easier if you didn’t have to type enum each time but rather use EOCConnectionState on its own. To do this, you add a typedef to the enumeration definition:

enum EOCConnectionState {
    EOCConnectionStateDisconnected,
    EOCConnectionStateConnecting,
    EOCConnectionStateConnected,
};
typedef enum EOCConnectionState EOCConnectionState;

This means that EOCConnectionState can be used instead of the full enum EOCConnectionState:

EOCConnectionState state = EOCConnectionStateDisconnected;

The advent of the C++11 standard brought some changes to enumerations. One such change is the capability to dictate the underlying type used to store variables of the enumerated type. The benefit of doing this is that you can forward declare enumeration types. Without specifying the underlying type, an enumeration type cannot be forward declared, since the compiler cannot know what size the underlying type will end up being. Therefore, when the type is used, the compiler doesn’t know how much space to allocate for the variable.

To specify the type, you use the following syntax:

enum EOCConnectionStateConnectionState : NSInteger { /* ... */ };

This means that the value backing the enumeration will be guaranteed to be an NSInteger. If you so wished, the type could be forward declared like so:

enum EOCConnectionStateConnectionState : NSInteger;

It’s also possible to define the value a certain enumeration member relates to rather than letting the compiler choose for you. The syntax looks like this:

enum EOCConnectionStateConnectionState {
    EOCConnectionStateDisconnected = 1,
    EOCConnectionStateConnecting,
    EOCConnectionStateConnected,
};

This means that EOCConnectionStateDisconnected will use the value 1 rather than 0. The other values follow, incrementing by 1 each time, just as before. Thus, EOCConnectionStateConnected will use the value 3, for example.

Another reason to use enumeration types is to define options, especially when the options can be combined. If the enumeration is defined correctly, the options can be combined using the bitwise OR operator. For example, consider the following enumeration type, found in the iOS UI framework, used to define which dimensions of a view can be resized:

enum UIViewAutoresizing {
    UIViewAutoresizingNone                 = 0,
    UIViewAutoresizingFlexibleLeftMargin   = 1 << 0,
    UIViewAutoresizingFlexibleWidth        = 1 << 1,
    UIViewAutoresizingFlexibleRightMargin  = 1 << 2,
    UIViewAutoresizingFlexibleTopMargin    = 1 << 3,
    UIViewAutoresizingFlexibleHeight       = 1 << 4,
    UIViewAutoresizingFlexibleBottomMargin = 1 << 5,
}

Each option can be either on or off, and using the preceding syntax enables this because each option has just a single bit set in the value represented by it. Multiple options can be bitwise OR’ed together: for example, UIViewAutoResizingFlexibleWidth | UIViewAutoresizingFlexibleHeight. Figure 1.2 shows the bit layout of each enumeration member and the combination of two of the members.

It’s then possible to determine whether one of the options is set by using the bitwise AND operator:

enum UIVewAutoresizing resizing =
    UIViewAutoresizingFlexibleWidth |
    UIViewAutoresizingFlexibleHeight;
if (resizing & UIViewAutoresizingFlexibleWidth) {
    // UIViewAutoresizingFlexibleWidth is set
}
Figure 1.2

Figure 1.2. Binary representation of three options values and two of those values bitwise OR’ed together

This is used extensively throughout the system libraries. Another example from UIKit, the iOS UI framework, uses it as a way of telling the system what device orientations your view supports. It does this with an enumerated type called UIInterfaceOrientationMask, and you implement a method called supportedInterfaceOrientations to indicate the supported orientations:

- (NSUInteger)supportedInterfaceOrientations {
    return UIInterfaceOrientationMaskPortrait |
           UIInterfaceOrientationMaskLandscapeLeft;
}

A couple of helpers defined within the Foundation framework help define enumeration types that also allow you to specify the integral type that will be used to store values that use the enumeration type. These helpers provide backward compatibility such that if you’re targeting a compiler that supports the new standard, that syntax is used, but it falls back to the old syntax if not. The helpers are provided in the form of preprocessor #define macros. One is provided for normal enumeration types, such as the EOCConnectionState example. The other is provided for the case in which the enumeration defines a list of options like the UIViewAutoresizing example. You use them as follows:

typedef NS_ENUM(NSUInteger, EOCConnectionState) {
    EOCConnectionStateDisconnected,
    EOCConnectionStateConnecting,
    EOCConnectionStateConnected,
};
typedef NS_OPTIONS(NSUInteger, EOCPermittedDirection) {
    EOCPermittedDirectionUp    = 1 << 0,
    EOCPermittedDirectionDown  = 1 << 1,
    EOCPermittedDirectionLeft  = 1 << 2,
    EOCPermittedDirectionRight = 1 << 3,
};

This is what the macro definitions look like:

#if (__cplusplus && __cplusplus >= 201103L &&
        (__has_extension(cxx_strong_enums) ||
         __has_feature(objc_fixed_enum))
    ) ||
    (!__cplusplus && __has_feature(objc_fixed_enum))
    #define NS_ENUM(_type, _name)
            enum _name : _type _name; enum _name : _type
    #if (__cplusplus)
        #define NS_OPTIONS(_type, _name)
                _type _name; enum : _type
    #else
        #define NS_OPTIONS(_type, _name)
                enum _name : _type _name; enum _name : _type
    #endif
#else
    #define NS_ENUM(_type, _name) _type _name; enum
    #define NS_OPTIONS(_type, _name) _type _name; enum
#endif

The reason for the various ways of defining the macros is that there are different scenarios. The first case that is checked is whether the compiler supports the new style enumerations at all. This is checked with what looks like some rather complex Boolean logic, but all that it’s checking is that the feature is there. If the feature is not there, it defines the enumeration by using the old style.

If the feature is available, the NS_ENUM type is defined such that it expands out like this:

typedef enum EOCConnectionState : NSUInteger EOCConnectionState;
enum EOCConnectionState : NSUInteger {
    EOCConnectionStateDisconnected,
    EOCConnectionStateConnecting,
    EOCConnectionStateConnected,
};

The NS_OPTIONS macro is defined in different ways if compiling as C++ or not. If it’s not C++, it’s expanded out the same as NS_ENUM. However, if it is C++, it’s expanded out slightly differently. Why? The C++ compiler acts differently when two enumeration values are bitwise OR’ed together. This is something, as shown earlier, that is commonly done with the options type of enumeration. When two values are OR’ed together, C++ considers the resulting value to be of the type the enumeration represents: NSUInteger. It also doesn’t allow the implicit cast to the enumeration type. To illustrate this, consider what would happen if the EOCPermittedDirection enumeration were expanded out as NS_ENUM:

typedef enum EOCPermittedDirection : int EOCPermittedDirection;
enum EOCPermittedDirection : int {
    EOCPermittedDirectionUp    = 1 << 0,
    EOCPermittedDirectionDown  = 1 << 1,
    EOCPermittedDirectionLeft  = 1 << 2,
    EOCPermittedDirectionRight = 1 << 3,
};

Then consider attempting the following:

EOCPermittedDirection permittedDirections =
    EOCPermittedDirectionLeft | EOCPermittedDirectionUp;

If the compiler were in C++ mode (or potentially Objective-C++), this would result in the following error:

error: cannot initialize a variable of type
'EOCPermittedDirection' with an rvalue of type 'int'

You would be required to put in an explicit cast to the result of the ORing, back to EOCPermittedDirection. So the NS_OPTIONS enumeration is defined differently for C++ such that this does not have to be done. For this reason, you should always use NS_OPTIONS if you are going to be ORing together the enumeration values. If not, you should use NS_ENUM.

An enumeration can be used in many scenarios. Options and states have been shown previously; however, many other scenarios exist. Status codes for errors are a good candidate as well. Instead of using preprocessor defines or constants, enumerations provide a means for grouping together logically similar status codes into one enumeration. Another good candidate is styles. For example, if you have a UI element that can be created with different styles, an enumeration type is perfect for that situation.

One final extra point about enumerations has to do with using a switch statement. Sometimes, you will want to do the following:

typedef NS_ENUM(NSUInteger, EOCConnectionState) {
    EOCConnectionStateDisconnected,
    EOCConnectionStateConnecting,
    EOCConnectionStateConnected,
};

switch (_currentState) {
    EOCConnectionStateDisconnected:
        // Handle disconnected state
        break;
    EOCConnectionStateConnecting:
        // Handle connecting state
        break;
    EOCConnectionStateConnected:
        // Handle connected state
        break;
}

It is tempting to have a default entry in the switch statement. However, when used for switching on an enumeration that defines a state machine, it is best not to have a default entry. The reason is that if you add a state later on, the compiler will helpfully warn that the newly added state has not been cared for in the switch statement. A default block handles the new state, so the compiler won’t warn. The same applies to any other type of enumeration defined using the NS_ENUM macro. For example, if used to define styles of a UI element, you would usually want to make sure that switch statements handled all styles.

Things to Remember

  • Use enumerations to give readable names to values used for the states of a state machine, options passed to methods, or error status codes.
  • If an enumeration type defines options to a method in which multiple options can be used at the same time, define its values as powers of 2 so that multiple values can be bitwise OR’ed together.
  • Use the NS_ENUM and NS_OPTIONS macros to define enumeration types with an explicit type. Doing so means that the type is guaranteed to be the one chosen rather than a type chosen by the compiler.
  • Do not implement a default case in switch statements that handle enumerated types. This helps if you add to the enumeration, because the compiler will warn that the switch does not handle all the values.
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