Question

我想了解从ARC NSString获取CFStringRef的正确方法？同样朝着相反的方向，在ARC中CFStringRef到NSString

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

典型地

NSString *yourFriendlyNSString = (__bridge NSString *)yourFriendlyCFString;

和

CFStringRef yourFriendlyCFString = (__bridge CFStringRef)yourFriendlyNSString;

现在，如果您想知道__bridge关键字的原因，可以参考Apple documentation。你会在那里找到：

__bridge 在Objective-C和Core Foundation之间传输指针而不转让所有权。

__bridge_retained 或 CFBridgingRetain 将指向Core Foundation指针的Objective-C指针强制转换为您，并将所有权转让给您。   您有责任调用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。

这意味着在上述情况下，您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你都不会负责处理字符串的内存。

在某些情况下，您可能还希望转让所有权。

例如，请考虑以下代码段

- (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，您可能希望通过在投放时转移所有权来保存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NSString *强制转换投射CFString到__bridge_retained，这样您就拥有了该对象，并且您必须明确释放它使用CFRelease。

要把它包起来，你可以拥有

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;

ARC中的NSString到CFStringRef和CFStringRef到NSString？

1 个答案:

NSString→CFString

CFString→NSString