使用ARC能帮我们减轻不少内存管理方面的负担，尤其是对用惯了Java的程序员来说。

最近，在做获取本地通讯录时，遇到CFObject和NSObject转换的问题，由于ARC不能管理Core Foundation Object的生命周期，所以在Core Foundation和ARC之间，我们需要使用到__bridge,__bridge_retained和__bridge_transfer三个转换关键字。

在苹果官方文档，我们找到了：

If you cast between Objective-C and Core Foundation-style objects, you need to tell the compiler about the ownership semantics of the object using either a cast (defined in objc/runtime.h) or a Core Foundation-style macro (defined in NSObject.h):

• __bridge transfers a pointer between Objective-C and Core Foundation with no transfer of ownership.

• __bridge_retained or CFBridgingRetain casts an Objective-C pointer to a Core Foundation pointer and also transfers ownership to you.

  You are responsible for calling CFRelease or a related function to relinquish ownership of the object.

• __bridge_transfer or CFBridgingRelease moves a non-Objective-C pointer to Objective-C and also transfers ownership to ARC.

  ARC is responsible for relinquishing ownership of the object.

__bridge只做类型转换，但是不修改对象（内存）管理权；

__bridge_retained（也可以使用CFBridgingRetain）将Objective-C的对象转换为Core Foundation的对象，同时将对象（内存）的管理权交给我们，后续需要使用CFRelease或者相关方法来释放对象；

__bridge_transfer（也可以使用CFBridgingRelease）将Core Foundation的对象转换为Objective-C的对象，同时将对象（内存）的管理权交给ARC。

例如：

- (void)logFirstNameOfPerson:(ABRecordRef)person {

    NSString *name = (NSString *)ABRecordCopyValue(person, kABPersonFirstNameProperty);

    NSLog(@"Person‘s first name: %@", name);

    [name release];

}

我们可以做这样的修改：

- (void)logFirstNameOfPerson:(ABRecordRef)person {

    NSString *name = (NSString *)CFBridgingRelease(ABRecordCopyValue(person, kABPersonFirstNameProperty));

    NSLog(@"Person‘s first name: %@", name);

}

Objective-C 学习笔记<二>