Type erasure

Buffer.swift

import Foundation

// Base on
// https://github.com/bignerdranch/type-erasure-playgrounds/blob/master/playgrounds/Type_Erasure_Slides.playground/Contents.swift

protocol Buffer {
    associatedtype T
    func insert(_ value: T)
}

class ArrayBuffer<T>: Buffer {
    var container: [T]
    init() {
        container = []
    }
    func insert(_ value: T) {
        container.append(value)
    }
}

class AnyBufferBase<T>: Buffer {
    init() {
    }

    func insert(_ value: T) {
    }
} 

final class AnyBufferBox<Base: Buffer>: AnyBufferBase<Base.T> {
    var base: Base
    init(_ base: Base) {
        self.base = base
    }
    override func insert(_ value: T) {
        self.base.insert(value)
    }
}

final class AnyBuffer<T>: Buffer {
    let box: AnyBufferBase<T>

    init<Base: Buffer>(_ base: Base) where Base.T == T {
        box = AnyBufferBox(base)
    }

    func insert(_ value: T) {
        self.box.insert(value)
    }
}

let b1 = AnyBuffer(ArrayBuffer<String>())
b1.insert("!")
let b2 = AnyBuffer(ArrayBuffer<Int>())
b2.insert(2)


// Heterogenous collection could only be referred as Any :-(
let buffers: [String: Any] = ["string": b1, "int": b2]

let b1b = buffers["string"] as! AnyBuffer<String>
b1b.insert("Hello")
b1b.insert("World")