CODEWITHSUNDEEP
objective-cautomatic-ref-countingobjective-c-blocks
realvadim
realvadim

Reputation: 154

weakSelf and strongSelf in Objective-C

In case when there are many references to weakSelf inside block, it is recommended to create its strong version. The code looks like this:

__weak typeof(self) weakSelf = self;
self.theBlock = ^{
    __strong typeof(self) strongSelf = weakSelf;

    [strongSelf doSomething];
    [strongSelf doSomethingElse];
};

What I am concerned with is this line in the above code:

__strong typeof(self) strongSelf = weakSelf;

Isn't it erroneous when we write typeof(self)? Is referencing self allowed here?

In the tutorials they sometimes write:

__strong typeof(weakSelf) strongSelf = weakSelf;

Both versions are used 50/50. Are the both correct?

Upvotes: 2

Views: 656

Answers (1)

CRD
CRD

Reputation: 53000

Isn't it erroneous when we write typeof(self)? Is referencing self allowed here?

(A) No. (B) yes

typeof is an (Objective-)C language extension which in a declaration (here you are declaring strongSelf) is handled by the compiler as compile time - there is no use of typeof in the resulting compiled code. A primary use of typeof is in #define macros which enables a single macro to be expanded to work on different types; again such macro expansion occurs at compile time.

In your case you are constructing a block in an instance method, in abstract your code will look something like:

@implementation SomeClass {

- (someReturnType) someInstanceMethod {

... ^{ typeof(self) strongSelf = weakself; ... } ...

} }

Here typeof(self) is effectively just a "shorthand" for SomeClass. The use of self is processed at compile time and no reference to runtime object referenced by self is captured.

Both versions are used 50/50. Are the both correct?

The are identical in meaning. The rules of ARC state that if no qualifier is present then __strong is assumed, so one version relies on this and the other makes the qualifier explicit.

HTH

Upvotes: 2

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