属性声明中的__block属性 [英] __block attribute on property declarations

问题描述

我必须修复一些现有的代码，这些代码在LLVM（iOS上）下可以很好地构建，因此它也可以使用llvm-gcc-4.2进行构建。我完成了几乎所有的事情，除了在几个地方发现的这种模式外：

  @property（nonatomic，retain ）__block id myProperty; 
  

我怀疑这里的意图是允许从块中访问属性而不保留自。我该如何删除 __ block 属性，这个gcc在这里不支持，但仍然达到相同效果？

  
 
 @property（nonatomic，retain）__block id myProperty; 
  

没有意义。 __ block 限定符用于本地（堆栈分配）变量，因此它们通过引用传递给块，所以它们可以更新，并且通常[*]存储在堆中而不是堆栈。

因此，限定符 __ block 对与对象实例有关的属性声明没有意义，它们始终是在Obj-C中分配的堆。

只需从属性声明中删除 __ block 对于每一个编译器来说。

如果一个块永远不会被复制到堆中，编译器可能会优化__block变量并且不会将它们移动到堆中。

I have to fix some existing code that builds just fine with LLVM (on iOS) so that it builds with llvm-gcc-4.2 too. I'm done with pretty much everything, except this pattern which is found at a few places:

@property (nonatomic, retain) __block id myProperty;

I suspect the intent here is to allow access to the property from inside a block without retaining self. How can I remove the __block attribute, which gcc doesn't support here, but still achieve the same effect?

解决方案

I'll suggest you've found a compiler bug, the declaration:

@property (nonatomic, retain) __block id myProperty;

is meaningless. The __block qualifier is used on local (stack allocated) variables so they are passed by reference to blocks, so they can be updated, and are usually[*] stored on the heap rather than the stack.

Therefore the qualifier __block has no meaning on a property declaration which is concerned with object instances, which are heap allocated at all times in Obj-C.

Just drop the __block from the property declarations - for every compiler.

[*] If a block is never copied to the heap a compiler may optimize __block variables and not move those to the heap either.

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