CopyOnWrite.swift

@propertyWrapper
struct CopyOnWrite<Object> {
    var wrappedValue: Object {
        get { storage.object }
        set { 
            if !isKnownUniquelyReferenced(&storage) {
                storage = storage.copy()
            }
            storage.object = newValue 
        }
    }
    
    public init(wrappedValue: Object) {
        storage = Storage(wrappedValue)
    }
    
    private var storage: Storage
    
    private class Storage {
        var object: Object
        
        init(_ object: Object) {
            self.object = object
        }
        
        func copy() -> Storage {
            print("Copying")
            return Storage(object)
        }
    }
}

struct HTTPRequest {
    @CopyOnWrite var path: String
    @CopyOnWrite var headers: [String:String]
}

var reqA = HTTPRequest(path: "/home", headers: [:])
print("Assigning to B")
var reqB = reqA
print("Mutating B")
reqB.headers["X-RequestId"] = "\(5)"
print(reqA.headers, reqB.headers)