objective-c - NSString 到 CFStringRef 和 CFStringRef 到 ARC 中的 NSString？

Question

我想了解NSString从CFStringRefARC 中获取 a 的正确方法？反方向也一样，CFStringRef在NSStringARC 中？

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？

Answer 1

通常

NSString *yourFriendlyNSString = (__bridge NSString *)yourFriendlyCFString;

和

CFStringRef yourFriendlyCFString = (__bridge CFStringRef)yourFriendlyNSString;

现在，如果您想知道__bridge关键字为何存在，可以参考Apple 文档。在那里你会发现：

__bridge在没有所有权转移的情况下在 Objective-C 和 Core Foundation 之间转移一个指针。

__bridge_retained或CFBridgingRetain将 Objective-C 指针转换为 Core Foundation 指针，并将所有权转移给您。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 指针移动到 Objective-C 并将所有权转移给 ARC。ARC 负责放弃对象的所有权。

这意味着在上述情况下，您正在转换对象而不更改所有权。这意味着在这两种情况下，您都将负责处理字符串的内存。

也可能存在出于某种原因您想要转让所有权的情况。

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 a。

- (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已经转移，所以现在 ARC 将为您启动并释放内存。

另一方面，您可以使用强制转换将 a强制NSString *转换为a ，这样您将拥有该对象，并且您必须通过 using 显式释放它。CFString__bridge_retainedCFRelease

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把它包起来，你可以有

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;