CODEWITHSUNDEEP
objective-ciosmemory-management
cateof
cateof

Reputation: 6758

retain release common practice in iOS setter methods

The common practice in IOS setters is the following:

- (void)setMyString:(NSString *)newString {
    if ( newString != myString ) {
        [myString release];
        myString = [newString retain];
    }
}

On the contrary this is not good practice

- (void)setMyString:(NSString *)newString {
    if ( myString != nil ) [myString release];
        myString = [newString retain];
    }
}

What is the reason checking for equality in the first case? What is the problem in the seconds case?

Upvotes: 0

Views: 262

Answers (3)

Metabble
Metabble

Reputation: 11841

I know this is somewhat redundant, but...

If the new and old object are the same then you send release to the old object and it gets deallocated, the pointer to the new object will become a dangling pointer as the object it pointed to will no longer exist (since it pointed to the same object as the old object pointer). Ex. if myString and newString point to the same instance who has a retain count of one, then you subtract one, it'll equal zero. it's too late to add one now, because it'll get deallocated. However, reverse the calls to retain and release and it should be fine. If the retain count is one and you add one, it's now two, and you can safely send release. In general, I'd say before you disown an object, assert ownership of the new one first.

Also, the first type of setter would be what you would use for retain/strong style setter. If it were assign you wouldn't need to retain/release as no ownership is supposed to be asserted. NSStrings often have a copy style setter which copies the argument and uses that, which would create a copy instead of retaining. I would generally use copy for anything with a mutable subclass as you wouldn't want someone passing in a NSMutableString and mutating it behind your back. This page goes into accessors, and you'll notice that they retain the new value before releasing the old one, and explain why.

Upvotes: 0

Euan
Euan

Reputation: 71

You may take this to though the memory management, https://developer.apple.com/documentation/Cocoa/Conceptual/MemoryMgmt/.

Upvotes: 0

BergP
BergP

Reputation: 3545

If you set something like this [object setMyString:[object myString]]; without checking for equality - it will be crash! Because it will be released before you send it message retain. (in case when only object own string). Also in first example we checking for equality to avoid extra operations.

Upvotes: 2

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