On Mon, May 29, 2017 at 11:48 PM, Anton Bronnikov <anton.bronnikov@me.com> wrote:
Like I mentioned, it is necessary to use optional, but it can not be nil.
Consider the following example where I have to use an optional in order to
break retention cycle:
protocol Container : class {
var component: Component { get }
}
class ConcreteContainer : Container {
var component: Component! = nil
init() {
component = Component(container: self)
}
}
class Component {
unowned let container: Container
init(container: Container) {
self.container = container
}
}
*Playground execution failed: error: scratchpad.playground:5:7: error:
type 'ConcreteContainer' does not conform to protocol 'Container'*
*class ConcreteContainer : Container {*
* ^*
*scratchpad.playground:2:9: note: protocol requires property 'component'
with type 'Component'; do you want to add a stub?*
* var component: Component { get }*
* ^*
*scratchpad.playground:6:9: note: candidate has non-matching type
'Component!'*
* var component: Component! = nil*
* ^*
Cheers,
Anton
P.S. Declaring protocol as
protocol Container : class {
var component: Component! { get }
}
.. would also work of course, but my question is not about how to move
forward, rather about whether such setup is deliberate.
On 29 May 2017, at 08:20, Zhao Xin <owenzx@gmail.com> wrote:
Why you have to use `unwrapped optional` at the first place? If you have
to use it, it means it could be nil. So it won't conform the protocol,
which requires the `value` never nil.
Zhaoxin
On Mon, May 29, 2017 at 12:37 PM, Anton Bronnikov via swift-users < > swift-users@swift.org> wrote:
Hi All,
If I have a protocol with a property requirement such as:
protocol Foo {
var value: Int { get }
}
.. and I need to conform to it using an implicitly unwrapped optional
like:
struct Bar : Foo {
let value: Int! = nil
}
.. then the compiler fails with an error:
*Playground execution failed: error: scratchpad.playground:5:8: error:
type 'Bar' does not conform to protocol 'Foo'*
*struct Bar : Foo {*
* ^*
*scratchpad.playground:2:9: note: protocol requires property 'value' with
type 'Int'; do you want to add a stub?*
* var value: Int { get }*
* ^*
*scratchpad.playground:6:9: note: candidate has non-matching type 'Int!'*
* let value: Int! = nil*
* ^*
Technically, I understand why the error, and currently work around it
with a cumbersome:
struct Bar : Foo {
var value: Int { return _value! }
let _value: Int? = nil
}
However, I keep wondering - would it not be making sense to accept
implicitly unwrapped optionals as a conformance to be base-type
requirements? It sort of works just like that in all other parts of the
language.
Or in other words, is this by design, or should I probably create an
issue for this?
Thank you.
Cheers,
Anton
P.S. The example is oversimplified for the sake of clarity, in practice
the property just has to be an optional - implicitly unwrapped or not -
because it’s a part of the composite that holds unowning reference to its
container object.
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