Xcode-beta (7A152u): Protocol "inheritence" needs better access controls

Number:rdar://21850541 Date Originated:16-Jul-2015 12:13 AM
Status:Open Resolved:
Product:Developer Tools Product Version:Xcode-beta (7A152u)
Classification:Enhancement Reproducible:Always
A protocol (this isn’t really limited to protocols though) really needs a way to separate the API for the consumers of the protocol and the implementors of the protocol.

Take this example:

    protocol KeyValueCodable : _KeyValueCodable {
        mutating func setValue<T>(value: T, forKey: String) throws
        func getValue<T>(forKey: String) throws -> T
    protocol _KeyValueCodable {
        static var _codables: [KVCTypeInfo] { get }
        var _kvcstore: Dictionary<String, Any> { get set }
    extension KeyValueCodable {
        mutating func setValue<T>(value: T, forKey: String) {
            // default implementation goes here...
        func getValue<T>(forKey: String) -> T {
            // default implementation goes here...

Everything prefixed with “_” is “private to consumers, public to implementors”. 

As the protocol author, I absolutely do **not** want this to be possible:

    var kvc = SomeKVCType()
    kvc._kvcstore["haha"] = "this shouldn’t happen…"

However, I need those exposed to create default implementations. 

This bug is probably a duplicate of many others describing how the access modifiers we have today really are not sufficient. This is an area that C++ came closer to being correct on.


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