[swift-users] @NSCopying semantic does not appear to copy in Swift initializer

Torin Kwok torin at kwok.im
Fri Jan 27 01:30:18 CST 2017

Hello guys,

I wanna ask a question about the behavior of `@NSCopying` semantic in
Swift 3. Well, according to Apple's official documentation:

> In Swift, the Objective-C copy property attribute translates to
> @NSCopying. The type of the property must conform to the NSCopying
> protocol.

However, 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 because 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 making advantage of `@NSCopying` attribute, I 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 found nothing related — 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) 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.

Thank you very much.

Best Regards,
Torin Kwok

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