Toll-Free Bridging

There are a number of data types in the Core Foundation framework and the Foundation framework that can be used interchangeably. This capability, called toll-free bridging, means that you can use the same data type as the parameter to a Core Foundation function call or as the receiver of an Objective-C message. For example, NSLocale (see NSLocale Class Reference) is interchangeable with its Core Foundation counterpart, CFLocale (see CFLocale Reference). Therefore, in a method where you see an NSLocale * parameter, you can pass a CFLocaleRef, and in a function where you see a CFLocaleRef parameter, you can pass an NSLocale instance. You cast one type to the other to suppress compiler warnings, as illustrated in the following example.

NSLocale *gbNSLocale = [[NSLocale alloc] initWithLocaleIdentifier:@"en_GB"];

CFLocaleRef gbCFLocale = (CFLocaleRef) gbNSLocale;

CFStringRef cfIdentifier = CFLocaleGetIdentifier (gbCFLocale);

NSLog(@"cfIdentifier: %@", (NSString *)cfIdentifier);

// logs: "cfIdentifier: en_GB"

CFRelease((CFLocaleRef) gbNSLocale);

CFLocaleRef myCFLocale = CFLocaleCopyCurrent();

NSLocale * myNSLocale = (NSLocale *) myCFLocale;

[myNSLocale autorelease];

NSString *nsIdentifier = [myNSLocale localeIdentifier];

CFShow((CFStringRef) [@"nsIdentifier: " stringByAppendingString:nsIdentifier]);

// logs identifier for current locale

Note from the example that the memory management functions and methods are also interchangeable—you can use CFRelease with a Cocoa object and release and autorelease with a Core Foundation object.

Toll-free bridging has been available since OS X v10.0. Table 13-1 provides a list of the data types that are interchangeable between Core Foundation and Foundation. For each pair, the table also lists the version of OS X in which toll-free bridging between them became available.