iVars references strong, weak or what?

Question

In Obj-C, properties can be configured to be weak/strong. Instance variables. like following -

@interface MyClass {
NSObject *a;
}

Does the MyClass's object keep weak reference to a or strong, or something else? I think iVar is not released until its object is released. Why don't we specify weak/strong for iVar like properties?

Answer 1

对ivar的默认引用是__strong ，但您可以将其显式设置为__weak或__strong

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Answer 2

You question inspired me and i did a deep search on ObjectiveC memory management. I would like to share something with you that I got from Apple Doc.

Default behavior of instance variable

Instance variable maintains a strong reference to the objects by default

Why don't we specify weak/strong for iVar like properties?

Local variables and non-property instance variables maintain a strong references to objects by default. There's no need to specify the strong attribute explicitly, because it is the default.
A variable maintains a strong reference to an object only as long as that variable is in scope, or until it is reassigned to another object or nil.

If you don't want a variable to maintain a strong reference, you can declare it as __weak, like this:

  NSObject * __weak weakVariable;

Answer 3

@interface MyClass {
__weak NSObject *a;
__strong NSObject *a;
__unsafe_unretained NSObject *obj;
}