[swift-dev] isUniquelyReferenced issues

[Joe Groff]

> On Mar 31, 2016, at 3:58 PM, Patrick Pijnappel via swift-dev <swift-dev at swift.org> wrote:
> 
> In trying to implement a COW type, but I'm running into problems with isUniqueReferenced breaking in even fairly simple cases. For example (with -O) the code below prints "bar: false", commenting out the print in test() makes it print "bar: true", and removing the var parameter var foo: Foo and using var foo = foo instead breaks it again. Am I doing something wrong here?
> 
> class FooStorage { var x: Int = 0 }
> 
> struct Foo { var storage = FooStorage() }
> 
> func bar(var foo: Foo) {
> 	print("bar: \(isUniquelyReferencedNonObjC(&foo.storage))")
> }
> 
> func test() {
> 	var foo = Foo()
> 	print("test: \(isUniquelyReferencedNonObjC(&foo.storage))")
> 	bar(foo)
> }
> 
> test()

You're not doing anything wrong, this is just the ARC optimizer at work. `foo` inside `test` is dead after the call to `bar`, so ownership is transferred directly to `bar`'s parameter.

-Joe