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ARC中NSString *与CFStringRef的相互转换

ARC中NSString *与CFStringRef的相互转换

作者: 哈哈哈_哈哈哈 | 来源:发表于2017-05-16 06:24 被阅读401次

    首先看一下典型的NSString与CFStringRef的相互转换

    // CFStringRef to NSString *
    NSString *yourFriendlyNSString = (__bridge NSString *)yourFriendlyCFString;
    
    // NSString * to CFStringRef
    CFStringRef yourFriendlyCFString = (__bridge CFStringRef)yourFriendlyNSString;
    

    上面出现了一个关键字 __bridge ,这个关键字便是整个转换的关键。Apple官方对于 __bridge 的解释如下:

    **__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.
    

    上面这些话总结起来就是:

    __bridge 用于Objective-C和Core Foundation指针之间的转换,这种转换不会更换对象的所有权(ownership)。
    __bridge_retained 或 CFBridgeRetain 用于从Objective-C到Core Foundation的指针转换,并且会将对象的所有权(ownership)转移,所以你需要在不再使用该对象的时候调用CFRelease方法来解除引用。
    __bridge_transfer 或 CFBridgeRelease 用于将非Objective-C指针转换为Objective-C指针,对象的所有权(ownership)会交给ARC,这时你无须担心对象何时释放,交给ARC去做就好了。
    为什么在使用__bridge_retained进行转换时需要自己调用CFRelease来释放对象,其实原因很简单:Core Foundation的对象在ARC的管辖范围之内。

    下面是示例代码:

    // Don't transfer ownership. You won't have to call `CFRelease`
    CFStringRef str =(__bridge CFStringRef)string;
    // Transfer ownership (i.e. get ARC out of the way). The object is now yours and you must call `CFRelease` when you're done with it
    CFStringRef str =(__bridge_retained CFStringRef)string; // you will have to call `CFRelease`
    
    // Don't transfer ownership. ARC stays out of the way, and you must call `CFRelease` on `str` if appropriate (depending on how the `CFString` was created)
    NSString*string =(__bridge NSString*)str;
    // Transfer ownership to ARC. ARC kicks in and it's now in charge of releasing the string object. You won't have to explicitly call `CFRelease` on `str`
    NSString*string =(__bridge_transfer NSString*)str;
    
    

    原文: http://www.outflush.com/nsstring-pointer-and-cfstringref-conversion-in-arc/

    参考:http://blog.csdn.net/fightingbull/article/details/8098133

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