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The Toll-Free Bridge
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Core Foundation and the Objective-C class frameworks are engineered so that select Core Foundation types are interchangeable with equivalent Objective-C classes. The types and classes that form the tollfree bridge are listed in Table 25-2.
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CHAPTER 25 MIXING C AND OBJECTIVE-C
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Table 25-2. Core Foundation Toll-Free Bridge
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Cocoa Class
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NSArray NSAttributedString NSCalendar NSCharacterSet NSData NSDate NSDictionary NSError NSInputStream NSLocale NSMutableArray NSMutableAttributedString NSMutableCharacterSet NSMutableData NSMutableDictionary NSMutableSet NSMutableString NSNumber NSOutputStream NSSet NSString NSTimer NSTimeZone NSURL
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Core Foundation Type
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CFArrayRef CFAttributedStringRef CFCalendarRef CFCharacterSetRef CFDataRef CFDateRef CFDictionaryRef CFErrorRef CFReadStreamRef CFLocaleRef CFMutableArrayRef CFMutableAttributedStringRef CFMutableCharacterSetRef CFMutableDataRef CFMutableDictionaryRef CFMutableSetRef CFMutableStringRef CFNumberRef CFWriteStreamRef CFSetRef CFStringRef CFRunLoopTimerRef CFTimeZoneRef CFURLRef
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Put simply, an object pointer to one of the classes in Table 25-2 is functionally identical to its matching Core Foundation type reference. The single value can be transparently treated as either type. The code in Listing 25-2 demonstrates this with an Objective-C class that incorporates a Core Foundation UUID type. Listing 25-2. Using Core Foundation Types in an Objective-C Class
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CHAPTER 25 MIXING C AND OBJECTIVE-C
#include <CoreFoundation/CoreFoundation.h> @interface Unique : NSObject <NSCoding> { @private __strong CFUUIDRef uuid; } @property (readonly) NSString *uuid; @end @implementation Unique - (id)init { self = [super init]; if (self != nil) { CFUUIDRef newUUID = CFUUIDCreate(kCFAllocatorDefault); uuid = CFMakeCollectable(newUUID); } return self; } - (id)initWithCoder:(NSCoder*)decoder { self = [super init]; if (self != nil) { NSString *uuidString = [decoder decodeObjectForKey:@"UUID"]; CFUUIDRef savedUUID = CFUUIDCreateFromString(kCFAllocatorDefault, (CFStringRef)uuidString); uuid = CFMakeCollectable(savedUUID); } return self; } - (void)encodeWithCoder:(NSCoder*)encoder { [encoder encodeObject:self.uuid forKey:@"UUID"]; } - (NSString*)uuid {
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CHAPTER 25 MIXING C AND OBJECTIVE-C
CFStringRef cfString = CFUUIDCreateString(kCFAllocatorDefault,uuid); NSString *objcString = (NSString*)CFMakeCollectable(cfString); return objcString; } @end The standard Objective-C class frameworks do not include a class that encapsulates Universally Unique Identifiers (UUIDs). The Core Foundation framework, however, has the very handy CFUUIDRef type along with a set of functions for creating and encoding UUIDs. The Unique class in Listing 25-2 stores a CFUUIDRef value, equivalent to an object pointer, to a Core Foundation type as it would any other C pointer. The -init method creates a universally unique ID type and stores a reference to it. The Core Foundation function CFUUIDCreateString( ) creates a textual representation of the UUID from a CFUUIDRef and returns it as a new CFStringRef. Looking at Table 25-2, CFStringRef and NSString are a toll-free bridge. So the reference to the Core Foundation string type can be treated as if it were an NSString object pointer and returned to the caller. Similarly, the -initWithCoder: method begins by decoding an NSString object from the serialized data stream. It passes the NSString object pointer to CFUUIDCreateFromString( ) just as if it were a CFStringRef. The CFUUIDCreateFromString( ) function uses the CFStringRef to reconstruct the UUID opaque type. The toll-free bridge means that you can seamlessly work with many common Core Foundation types exactly as if they were Objective-C objects, and vice versa. The only significant difference between Core Foundation types and their doppelganger Objective-C classes is their memory management. This is discussed in the next section.
Tip There are other Objective-C bridges that you might find interesting. For example, there are bridges from Objective-C to both the Ruby and Python programming languages. Apple once developed a bridge between Objective-C and Java, which has since been deprecated.
The Objective-C classes listed in Table 25-2 are the only ones that have toll-free bridges to Core Foundation types. For example, NSRunLoop is not interchangeable with CFRunLoopRef. To use Core Foundation functions on an NSRunLoop, send the object a -getCFRunLoop message, which will return its underlying CFRunLoopRef.
C Memory Management
C memory management, or lack thereof, was described in 24. Core Foundation memory management either works with, or is layered over, the memory management used by Objective-C. When using Core Foundation types you must either adopt the Core Foundation memory management patterns, or transition the references to the memory management scheme being employed by Objective-C. Core Foundation types use simple reference counting. If you skipped 24, you might want to review it so you understand the basic principles. Here are the key concepts about Core Foundation memory management you need to know:
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