NSString到CFStringRef和CFStringRef到ARC中的NSString吗？

Question 1

我想了解NSString从CFStringRefARC中获取的正确方法吗？同为去相反的方向，CFStringRef以NSString在ARC？

在不造成内存泄漏的情况下执行此操作的正确方法是什么？

Question 2

通常

NSString *yourFriendlyNSString = (__bridge NSString *)yourFriendlyCFString;

和

CFStringRef yourFriendlyCFString = (__bridge CFStringRef)yourFriendlyNSString;

现在，如果您想知道为什么使用__bridge关键字，可以参考Apple文档。在那里您会发现：

__bridge 在Objective-C和Core Foundation之间转移指针，而没有所有权转移。

__bridge_retained或CFBridgingRetain将Objective-C指针转换为Core Foundation指针，也将所有权转让给您。您负责调用CFRelease或相关函数来放弃对象的所有权。

__bridge_transfer或CFBridgingRelease将非Objective-C指针移至Objective-C，并将所有权转移到ARC。ARC负责放弃对象的所有权。

这意味着在以上情况下，您将在不更改所有权的情况下投射对象。这意味着在任何情况下，您都不负责处理字符串的内存。

在某些情况下，您可能出于某些原因要转移所有权。

例如考虑以下片段

- (void)sayHi {
    CFStringRef str = CFStringCreateWithCString(NULL, "Hello World!", kCFStringEncodingMacRoman);

    NSString * aNSString = (__bridge NSString *)str;

    NSLog(@"%@", aNSString);

    CFRelease(str); //you have to release the string because you created it with a 'Create' CF function
}

在这种情况下，您可能希望CFRelease通过在投射时转移所有权来保存。

- (void)sayHi {
    CFStringRef str = CFStringCreateWithCString(NULL, "Hello World!", kCFStringEncodingMacRoman);

    NSString * aNSString = (__bridge_transfer NSString *)str;
// or alternatively
    NSString * aNSString = (NSString *)CFBridgingRelease(str);

    NSLog(@"%@", aNSString);
}

的所有权str已转让，因此现在ARC将启动并为您释放内存。

在另一种方法中，您可以使用强制类型NSString *转换CFString将a__bridge_retained强制转换为a ，这样您将拥有该对象，并且必须使用显式释放它CFRelease。

总结一下，你可以

NSString→CFString

// 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`

CFString→NSString

// 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;