在哪些情况下我们需要在ARC下编写__autoreleasing所有权限定符？ [英] In which situations do we need to write the __autoreleasing ownership qualifier under ARC?

问题描述

我正在尝试完成这个难题。

I'm trying to complete the puzzle.

__ strong 是所有Objective-C可保留的默认值对象指针，如NSObject，NSString等。这是一个强大的参考。 ARC在范围的末尾用 -release 进行余额。

__strong is the default for all Objective-C retainable object pointers like NSObject, NSString, etc.. It's a strong reference. ARC balances it with a -release at the end of the scope.

__ unsafe_unretained 等于旧的方式。它用于弱指针而不保留可保留对象。

__unsafe_unretained equals the old way. It's used for a weak pointer without retaining the retainable object.

__弱类似于 __ unsafe_unretained 除了它是一个自动归零的弱引用，这意味着一旦引用的对象被释放，指针就会被设置为nil。这消除了悬空指针和EXC_BAD_ACCESS错误的危险。

__weak is like __unsafe_unretained except that it's an auto-zeroing weak reference meaning that the pointer will be set to nil as soon as the referenced object is deallocated. This eliminates the danger of dangling pointers and EXC_BAD_ACCESS errors.

但是 __ autoreleasing 究竟是什么？当我需要使用这个限定符时，我很难找到实际的例子。我相信它只适用于期望指针指针的函数和方法，例如：

But what exactly is __autoreleasing good for? I'm having a hard time finding practical examples on when I need to use this qualifier. I believe it's only for functions and methods which expect a pointer-pointer such as:

- (BOOL)save:(NSError**);

或

NSError *error = nil;
[database save:&error];

在ARC下必须以这种方式声明：

which under ARC has to be declared this way:

- (BOOL)save:(NSError* __autoreleasing *);

但这太模糊了，我想完全理解为什么 。我找到的代码片段将__autoreleasing放在两颗星之间，这对我来说很奇怪。类型是 NSError ** （指向NSError的指针），所以为什么要在星星之间放置 __ autoreleasing 只需在 NSError ** 前面？

But this is too vague and I'd like to fully understand why. The code snippets I find place the __autoreleasing inbetween the two stars, which looks weird to me. The type is NSError** (a pointer-pointer to NSError), so why place __autoreleasing inbetween the stars and not simply in front of NSError**?

此外，可能还有其他情况我必须依赖 __ autoreleasing 。

Also, there might be other situations in which I must rely on __autoreleasing.

推荐答案

你是对的。正如官方文档所解释的那样：

You're right. As the official documentation explains:

__ autoreleasing表示通过引用传递的参数（id *）并在返回时自动释放。

__autoreleasing to denote arguments that are passed by reference (id *) and are autoreleased on return.

所有这一切都在 ARC过渡指南。

在NSError示例中，声明意味着 __strong ，隐含地：

In your NSError example, the declaration means __strong, implicitly:

NSError * e = nil;

将转换为：

NSError * __strong error = nil;

当您拨打保存方法时：

- ( BOOL )save: ( NSError * __autoreleasing * );

然后编译器必须创建一个临时变量，设置为 __ autoreleasing 。所以：

The compiler will then have to create a temporary variable, set at __autoreleasing. So:

NSError * error = nil;
[ database save: &error ];

将转换为：

NSError * __strong error = nil;
NSError * __autoreleasing tmpError = error;
[ database save: &tmpError ];
error = tmpError;

您可以通过将错误对象声明为 __ autoreleasing ，直接。

You may avoid this by declaring the error object as __autoreleasing, directly.

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