Core Foundation和Foundation中可互换使用

在Core Foundation和Foundation中有许多可互换使用的数据类型。这意味着可以使用相同的数据类型，作为Core Foundation框架方法调用或Objective－C消息的参数或接收者。这种在Core Foundation框架和Foundation框架之间交换使用数据类型的技术就叫 Toll-Free Bridging(无缝桥接)。
#if __has_feature(objc_arc)
// After using a CFBridgingRetain on an NSObject, the caller must take responsibility for calling CFRelease at an appropriate time.
NS_INLINE CF_RETURNS_RETAINED CFTypeRef CFBridgingRetain(id X) {
return (__bridge_retained CFTypeRef)X;
}

 NS_INLINE id CFBridgingRelease(CFTypeRef CF_CONSUMED X) {
return (__bridge_transfer id)X;
}

#else

// This function is intended for use while converting to ARC mode only.
NS_INLINE CF_RETURNS_RETAINED CFTypeRef   
CFBridgingRetain(id X) {
return X ? CFRetain((CFTypeRef)X) : NULL;
}

// This function is intended for use while converting to ARC mode only.
NS_INLINE id CFBridgingRelease(CFTypeRef CF_CONSUMED X) {
return [(id)CFMakeCollectable(X) autorelease];
}

#endif

但并不是所有的CF对象都支持 Toll-Free Bridging！官方介绍：[https://developer.apple.com/library/ios/documentation/General/Conceptual/CocoaEncyclopedia/Toll-FreeBridgin/Toll-FreeBridgin.html]