关于ARC中readonly @property的问题

问题描述

在我的介面（.h）档案中，我有

  @property（readonly）NSString * foo; 
  

在我的实现（.m）文件中，我有

  @synthesize foo; 
 

在ARC打开的情况下，编译器给出此错误：自动引用计数问题：ARC

如果我添加了一个，那么错误就会消失。 strong ， weak 或 copy 为什么是这样？为什么对于只读属性，这些东西有什么区别，这些区别是什么，为什么程序员要担心它们？为什么编译器不能智能地推导出只读属性的默认设置？

我处于另一个问题： strong ， weak 或 copy 是ARC唯一有意义的东西吗？我不应该再使用 retain 和 assign 吗？

@property 。因此，当编译器看到 @synthesize 时，它会尝试为您合成备用ivar。但你没有指定你想要什么样的ivar。应该是 __ strong 吗？ __ weak ？ __ unsafe_unretained ？最初，属性的默认存储属性是 assign ，这与 __ unsafe_unretained 相同。然而，根据ARC，这几乎总是错误的选择。因此，不是合成不安全的ivar，他们需要你指定你想要什么样的ivar。

In my interface (.h) file, I have

@property(readonly) NSString *foo;

and in my implementation (.m) file, I have

@synthesize foo;

With ARC turned on, the compiler gives me this error: Automatic Reference Counting Issue: ARC forbids synthesizing a property of an Objective-C object with unspecified ownership or storage attribute.

The error goes away if I add a strong, weak, or copy to the property. Why is this? Why would there be any differences between these things for a read-only property, what are those differences, and why does the programmer have to worry about them? Why can’t the compiler intelligently deduce a default setting for a read-only property?

Another question while I’m at it: strong, weak, or copy are the only things that make sense in ARC, right? I shouldn’t be using retain and assign anymore, should I?

解决方案

You've declared a @property that doesn't have a backing ivar. Thus, when the compiler sees @synthesize, it tries to synthesize a backing ivar for you. But you haven't specified what kind of ivar you want. Should it be __strong? __weak? __unsafe_unretained? Originally, the default storage attribute for properties was assign, which is the same as __unsafe_unretained. Under ARC, though, that's almost always the wrong choice. So rather than synthesizing an unsafe ivar, they require you to specify what kind of ivar you want.

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