C Enumeration, NS
What's the correct way to create an enumeration with a specific type in Objective-C? How does NS_ENUM and NS_OPTIONS work? NS_OPTIONS are used for masks, like NSAutoresizing? Thanks.
Code from NSObjCRuntime.h
#define NS_ENUM(_type, _name) enum _name : _type _name; enum _name : _type
#define NS_OPTIONS(_type, _name) _type _name; enum : _type
example from NSHipster. NS_OPTIONS is used in a similar way, but for enums which would ordinarily be a bit mask
instead of
typedef enum {
UITableViewCellStyleDefault,
UITableViewCellStyleValue1,
UITableViewCellStyleValue2,
UITableViewCellStyleSubtitle
} UITableViewCellStyle;
or
typedef enum {
UITableViewCellStyleDefault,
UITableViewCellStyleValue1,
UITableViewCellStyleValue2,
UITableViewCellStyleSubtitle
};
typedef NSInteger UITableViewCellStyle;
do this:
typedef NS_ENUM(NSInteger, UITableViewCellStyle) {
UITableViewCellStyleDefault,
UITableViewCellStyleValue1,
UITableViewCellStyleValue2,
UITableViewCellStyleSubtitle
};
an example NS_OPTIONS enum:
typedef NS_OPTIONS(NSUInteger, UIViewAutoresizing) {
UIViewAutoresizingNone = 0,
UIViewAutoresizingFlexibleLeftMargin = 1 << 0,
UIViewAutoresizingFlexibleWidth = 1 << 1,
UIViewAutoresizingFlexibleRightMargin = 1 << 2,
UIViewAutoresizingFlexibleTopMargin = 1 << 3,
UIViewAutoresizingFlexibleHeight = 1 << 4,
UIViewAutoresizingFlexibleBottomMargin = 1 << 5
};
There is a difference between the two except that they infer different kind of enumerations.
When compiled in Objective-C++ mode, they generate different code:
this is the original code:
typedef NS_OPTIONS(NSUInteger, MyOptionType) {
MyOptionType1 = 1 << 0,
MyOptionType2 = 1 << 1,
};
typedef NS_ENUM(NSUInteger, MyEnumType) {
MyEnumType1 = 1 << 0,
MyEnumType2 = 1 << 1,
};
this is the code when macros are expanded in Objective-C
compiling:
typedef enum MyOptionType : NSUInteger MyOptionType; enum MyOptionType : NSUInteger {
MyOptionType1 = 1 << 0,
MyOptionType2 = 1 << 1,
};
typedef enum MyEnumType : NSUInteger MyEnumType; enum MyEnumType : NSUInteger {
MyEnumType1 = 1 << 0,
MyEnumType2 = 1 << 1,
};
this is the code when macros are expanded in Objective-C++
compiling:
typedef NSUInteger MyOptionType; enum : NSUInteger {
MyOptionType1 = 1 << 0,
MyOptionType2 = 1 << 1,
};
typedef enum MyEnumType : NSUInteger MyEnumType; enum MyEnumType : NSUInteger {
MyEnumType1 = 1 << 0,
MyEnumType2 = 1 << 1,
};
See the difference of NS_OPTIONS between two modes?
HERE IS THE REASON
:
There is a new feature in C++ 11, you can declare a type for you enumeration, before that, the type holding enumeration is decided by compiler according to the largest value of enumerations.
So in C++ 11, since you can decide the size of your enumeration by yourself, you could forward declare enums without actually define them, like this:
//forward declare MyEnumType
enum MyEnumType: NSInteger
//use myEnumType
enum MyEnumType aVar;
//actually define MyEnumType somewhere else
enum MyEnumType: NSInteger {
MyEnumType1 = 1 << 1,
MyEnumType2 = 1 << 2,
}
This feature is handy, and Objective-C imports this feature , but it brings a problem, when doing bitwise calculation, like this:
enum MyEnumType aVar = MyEnumType1 | MyEnumType2;
This code can't compile in C++/Objective-C++ compiling, since aVar is considered of type NSInteger
but MyEnumType1 | MyEnumType2
MyEnumType1 | MyEnumType2
is of type MyEnumType
, this assignment can't perform without a type cast, C++ forbids implicit type casting .
At this time, we need NS_OPTIONS, NS_OPTIONS fall back to enum before C++ 11, so that there is no MyEnumType
indeed, MyEnumType
is just another name for NSInteger
, so that code like
enum MyEnumType aVar = MyEnumType1 | MyEnumType2;
will compile, since it is assigning NSInteger
to NSInteger
.
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