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

Question

我想了解从 ARC 中的 CFStringRef 获取 NSString 的正确方法？反方向也一样，ARC中的CFStringRef到NSString？

在不造成内存泄漏的情况下，正确的方法是什么？

Answer 1

通常

NSString *yourFriendlyNSString = (__bridge NSString *)yourFriendlyCFString;

和

CFStringRef yourFriendlyCFString = (__bridge CFStringRef)yourFriendlyNSString;

现在，如果你想知道为什么会有 __bridge 关键字，你可以引用 Apple documentation .在那里你会发现:

__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.

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。

也有可能出于某种原因您想要转让所有权。

例如考虑以下代码段

- (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，您可能希望在转换时通过转移所有权来保存 CFRelease。

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

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

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

str 的所有权已经转移，所以现在 ARC 将启动并为您释放内存。

另一方面，您可以使用 __bridge_retained 转换将 NSString * 转换为 CFString，这样您就可以拥有该对象您必须使用 CFRelease 显式释放它。

---

你可以把它包起来

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;