扩展可选和非可选类型

Question

I am interacting with a C API. For memory safety I chose to not keep the C types internally and just produce them when another C api needs them, so I can work with plain Swift structs instead. In this example I just use the Foo type as a stand-in, this is obviously not the real type.

Now when I need to call a C API, I have those 2 methods on the type, to produce a C pointer I can work with:

struct Foo {
    let bar: Int
}

extension Optional where Wrapped == Foo {

    func withUnsafePointer<T>(_ block: (Foo?) throws -> T) rethrows -> T {
        guard let self = self else {
            return try block(nil)
        }
        return try block(self)
    }
}

extension Foo {

    func withUnsafePointer<T>(_ block: (Foo) throws -> T) rethrows -> T {
        return try block(self)
    }
}

As you can see I need to extend the optional as well to facilitate this. Otherwise a call to foo?.withUnsafePointer(...) would just not be executed.

I don't find this pattern particularly pretty. Is there a better option instead of implementing the method twice?

The whole point of this is, that the C object should only have a limited lifetime (here inside the block).

Answer 1

You can avoid duplicating the withUnsafePointer method by declaring a protocol, making both Foo and Foo? conform to it, and then extending the protocol. This isn't necessarily as pretty as something like extension Foo, Optional<Foo> would be, but AFAIK there's not currently a way to do that in Swift as of the time of this writing.

struct Foo {
    let myCPointer = UnsafeMutablePointer<UInt8>.allocate(capacity: 1)
}

protocol HasMyCPointer {
    associatedtype MyCPointerType
    var myCPointer: MyCPointerType { get }
}

extension Foo: HasMyCPointer {}

extension Optional: HasMyCPointer where Wrapped == Foo {
    var myCPointer: UnsafeMutablePointer<UInt8>? { self?.myCPointer }
}

extension HasMyCPointer {
    func withUnsafePointer<T>(_ block: (Self.MyCPointerType) throws -> T) rethrows -> T {
        return try block(self.myCPointer)
    }
}

let foo = Foo()
let bar: Foo? = nil
let baz: Foo? = Foo()

foo.withUnsafePointer {
    print("Foo: \($0)")
}

bar.withUnsafePointer {
    print("Bar: \($0 as Any)")
}

baz.withUnsafePointer {
    print("Baz: \($0 as Any)")
}

prints:

Foo: 0x00007fade3c05750
Bar: nil
Baz: Optional(0x00007fade3c00220)