ARC实例变量保留释放 [英] ARC instance variable retain release

问题描述

如果我们有这样的.h文件：

If we have .h file like this:

@interface TestViewController : UIViewController {

    __weak NSObject *object;
}

@end

和.m文件中的方法像这样：

and methods in .m file like this:

- (void)viewDidLoad {

    [super viewDidLoad];
    NSObject *localObject = [[NSObject alloc] init];
    NSLog(@"%ld", CFGetRetainCount((__bridge CFTypeRef)localObject));
    object = localObject;
    NSLog(@"%ld", CFGetRetainCount((__bridge CFTypeRef)object));
    NSLog(@"%ld", CFGetRetainCount((__bridge CFTypeRef)localObject));
}

然后我们得到以下保留计数的输出：

Then we get the following output for retain count:

1

2

1

1
2
1

我的问题是为什么保留计数得到当它被声明为__weak实例变量时，object上增加为2，此外localObject保留计数保持为1.
我认为这是因为ARC插入保留/释放但我不确定。

My question is why does the retain count gets incremented to 2 on "object" when it is declared as "__weak" instance variable, furthermore "localObject" retain count remains 1. I think that's because of how ARC inserts retain/release but I am not sure.

推荐答案

首先，通常的免责声明：保留计数的绝对值并没有告诉你任何有用的信息，有关更多信息，请参阅

First of all the usual disclaimer: the absolute value of the retain count does not tell you anything useful, for more information see

• http://whentouseretaincount.com


• 何时使用-retainCount？

• http://whentouseretaincount.com
• When to use -retainCount?

现在在你的情况下，在行

Now in your case, in the line

NSLog(@"%ld", CFGetRetainCount((__bridge CFTypeRef)object));

你读了一个弱指针的值，而 Clang / ARC文档4.2语义陈述：

you read the value of a weak pointer, and the Clang/ARC documentation 4.2 Semantics states about that:

对于 __ weak 对象，保留当前的指针对象，然后在当前的完整表达式结束时释放
。

For __weak objects, the current pointee is retained and then released at the end of the current full-expression.

因此，当传递给 CFGetRetainCount（）时，对象的保留计数增加1。你也可以在生成的汇编代码中看到：

Therefore the object's retain count is increased by one while passed to CFGetRetainCount(). You can see that also in the generated assembly code:

movq    %rbx, %rdi
callq   _objc_retain
movq    %rax, %rbx
.loc    1 21 0                  
movq    %rbx, %rdi
callq   _CFGetRetainCount
movq    %r15, %rdi
movq    %rax, %rsi
xorb    %al, %al
callq   _NSLog
movq    _objc_release@GOTPCREL(%rip), %r12
movq    %rbx, %rdi
callq   *%r12

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