[swift-evolution] @NSCopying currently does not affect initializers

Torin Kwok torin at kwok.im
Fri Jan 27 22:34:39 CST 2017

Hello guys,

Note: This issue has been originally presented in swift-users mailling list <https://lists.swift.org/pipermail/swift-users/Week-of-Mon-20170123/004552.html>. And then I post it again here at the suggestion <https://lists.swift.org/pipermail/swift-users/Week-of-Mon-20170123/004561.html> of Jordan Rose:

It might be reasonable to change this behavior, but it probably deserves a bit of discussion on swift-evolution; it's not 100%, for-sure a bug.
--- the original content follows this line ---

I encountered a strange behavior when I declared a property with the @NSCopying attribute:

// `Person` class inherits from `NSObject` class and conforms to `NSCopying` protocol
@NSCopying var employee: Person
and then assigned an external instance of Person class protocol to this property within the designated init methods:

// Designated initializer of `Department` class
init( employee externalEmployee: Person ) {
 self.employee = externalEmployee

 // Assertion would fail since Swift do not actually copy the value assigned to this property         
 // even though `self.employee` has been marked as `@NSCoyping`
 // assert( self.employee !== externalEmployee )
If I indeed require the deep copying behavior during the init process, instead of taking advantage of @NSCopying attribute, I would have to invoke the copy() method manually:

init( employee externalEmployee: Person ) {
 // ...
 self.employee = externalEmployee.copy() as! Person  
 // ...
In fact, what really makes me confusing is that @NSCopying semantic does work properly within the other parts of the class definition such as normal instance methods, or external scope. For instance, if we're assigning an external instance of Person to the self.employee proper of Department directly through setter rather than initializer:

department.employee = johnAppleseed
then self.employee property and johnAppleseed variable will no longer share the same underlying object now. In the other words, @NSCopying attribute makes sense.

After I looked through a great deal of results given by Google, and dicussions on StackOverflow, I finally end up with nothing helpful — the vast majority of articles, documentations as well as issues talking about this similar topics only focus on the basic concepts and effects of @NSCopying itself but do not mentioned this strange behavior at all — besides one radar descriping the same problem (rdar://21383959 <rdar://21383959>) and a final conclusion mentioned in a guy's Gist comment: ... values set during initialization are not cloned ...

That is, @NSCopying semantic has no effect in initializers.

Then, what I want to figure out is the reason why @NSCopying semantic will become effectless implicitly whithin initializers of a class, and the special considerations behind this behavior, if any.

--- END ---


Your observation is correct: @NSCopying currently does not affect initializers. This is because accessing a property in an initializer always does direct access to the storage rather than going through the setter.
I have tested the identical logic in Objective-C and the NSCopying semantic works perfectly within Obj-C's class initializer.

I have no idea whether it's a bug or special consideration. After all, as a special consideration, it seems too strange that this behavior has not been obviously documented.

Best Regards,
Torin Kwok

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