ios - @property definitions with ARC : strong is the default value, 但基本类型的默认值是多少？

Question

我们知道，在ARC规则下，默认值为strong，所以可以使用

@property NSString *downloadPath

但为什么是

@property (readonly) CGFloat progress
@property (readonly) BOOL isSSL 

好吗？由于它们是基本类型，因此应使用 assign。我可以这样理解ARC规则吗:

当属性为基本类型时，默认值为assign，当属性为对象类型时，默认值为strong?

Answer 1

来自clang documentation关于所有权推断:

If an object is declared with retainable object owner type, but without an explicit ownership qualifier, its type is implicitly adjusted to have __strong qualification.

原因可以在 same page 上找到。 :

Using strong by default is safe and consistent with the generic ARC rule about inferring ownership. It is, unfortunately, inconsistent with the non-ARC rule which states that such properties are implicitly assign. However, that rule is clearly untenable in ARC, since it leads to default-unsafe code.

如文档所述，默认的所有权限定符是 assign，但是对于可保留类型(又名对象)，默认的限定符更改为 strong，这有助于编写安全代码的开发人员。