> On Mar 31, 2016, at 4:21 PM, Joe Groff via swift-dev <swift-dev@swift.org>
> wrote:
>
>>
>> On Mar 31, 2016, at 3:58 PM, Patrick Pijnappel via swift-dev
>> <swift-dev@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.
If you want to ensure that `foo` remains alive despite this, you can use
`withExtendedLifetime`:
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))")
withExtendedLifetime(foo) {
bar(foo)
}
}
-Joe
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