最近碰到了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。

根据苹果官方的文档（https://developer.apple.com/library/ios/#releasenotes/ObjectiveC/RN-TransitioningToARC/Introduction/Introduction.html）：

• __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.

意思就是:

__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。