Toll-Free Bridging

刚刚趁有空在翻看苹果开发文档时，在NSDictionary类的介绍中看到

NSDictionary is toll-free bridged with its Core Foundation counterpart,CFDictionaryRef

大概意思是NSDictionary和它的Core Foundation的对象CFDictionaryRef是toll-free bridged的

可toll-free bridged是什么意思？ 免费桥接？ 等价交换？

懵逼

然后好奇点了roll-free bridged,看了下它的官方文档。

大概摘抄如下：

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.

大致意思是：

Core Foundation框架和Foundation框架中有许多数据类型可以互换使用。该功能被称为Toll-Free Bridging，意味着你可以把相同的数据类型作为Core Foundation函数调用的参数或者作为OC消息的receiver。 例如，NSLocale可以和它的Core Foundation对象CFLocale互换。因此，当你看到一个方法需要一个NSLocale *参数，你可以传入一个CFLocaleRef,当一个方法需要CFLocaleRef参数，你可以传入一个NSLocate实例。你可以把一种类型强制转换为另一种类型来消除编译器的警告

苹果爸爸还很贴心的给出了一个🌰

image.png

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

打印：

cfIdentifier: en_GB
nsIdentifier: en_US

从示例中可以看到，内存管理函数和方法也是可以互换的：我们可以将CFRelease和Cocoa对象一起使用，也可以将release``autorelease和Core Foundation对象一起使用。

下面这个表格列出了Core Foundation和Foundation可以互换的数据类型

Core Foundation type	Foundation class
CFArrayRef	NSArray
CFAttributedStringRef	NSAttributedString
CFBooleanRef	NSNumber
CFCalendarRef	NSCalendar
CFCharacterSetRef	NSCharacterSet
CFDataRef	NSData
CFDateRef	NSDate
CFDictionaryRef	NSDictionary
CFErrorRef	NSError
CFLocaleRef	NSLocale
CFMutableArrayRef	NSMutableArray
CFMutableAttributedStringRef	NSMutableAttributedString
CFMutableCharacterSetRef	NSMutableCharacterSet
CFMutableDataRef	NSMutableData
CFMutableDictionaryRef	NSMutableDictionary
CFMutableSetRef	NSMutableSet
CFMutableStringRef	NSMutableString
CFNullRef	NSNull
CFNumberRef	NSNumber
CFReadStreamRef	NSInputStream
CFRunLoopTimerRef	NSTimer
CFSetRef	NSSet
CFStringRef	NSString
CFTimeZoneRef	NSTimeZone
CFURLRef	NSURL
CFWriteStreamRef	NSOutputStream

需要注意的是，并不是所有的数据类型都是toll-free bridged的，尽管它们的名字看起来像是支持toll-free bridged. 比如，NSRunLoop 和 CFRunLoopRef并不是toll-free bridged, NSBundle 和 CFBundleRef不是toll-free bridged, 另外 NSDateFormatter 和 CFDateFormatterRef也不是toll-free bridged.

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