Basic cache, failing on Ubuntu 14.04 with Swift 3.0

cache.swift

import Foundation

fileprivate class CachedItem<T> {
    let value: T

    fileprivate init(value: T) {
        self.value = value
    }

}

class Cache<K: Hashable, V> {

    private var cache = NSCache<NSNumber, CachedItem<V>>()

    func set(value: V, forKey: K) {
        let key = NSNumber(value: forKey.hashValue)
        let cachedItem = CachedItem(value: value)
        cache.setObject(cachedItem, forKey: key)
    }

    func get(key: K) -> V? {
        let key = NSNumber(value: key.hashValue)
        guard let item = cache.object(forKey: key) else {
             print("Cache MISS")
             return nil
        }
        print("Cache HIT")
        return item.value
    }

}



var cache = Cache<String, String>()

func slowReq() -> String {
    print("processsing...")
    sleep(1)
    return "hello"
}

func fetch() -> String {
    if let result = cache.get(key: "test") {
        return result
    }
    let result = slowReq()
    cache.set(value: result, forKey: "test")
    return result
}

for i in 1...10 {
    let result = fetch()
    print(result)
}