Multiton

Multiton is a draft programming task. It is not yet considered ready to be promoted as a complete task, for reasons that should be found in its talk page.

Description

The multiton pattern is a design pattern which generalizes the singleton pattern. Whereas the singleton allows only one instance of a class to be created, the multiton pattern allows for the controlled creation of multiple instances, which it manages through the use of a map.

Task

Implement a basic Multiton class or other structure and test that it works as intended. If your language does not support the object oriented paradigm, then try to emulate a multiton as best you can with the tools available.

If your language supports multithreading, then you may optionally implement a thread safe variant as well.

Related task

• Singleton

Julia

A registry (just in memory, not on disk) is used below instead of an enum. The registry is protected by a lock on the Multiton constructor, to prevent two threads creating the same object at the same time. <lang julia>struct Multiton{T}

   data::T
   function Multiton(registry, refnum, data)
       if 0 < refnum <= registry.max_instances && registry.instances[refnum] isa Nothing
           lock(registry.spinlock)
           multiton = new{typeof(data)}(data)
           registry.instances[refnum] = multiton
           unlock(registry.spinlock)
           return multiton
       else
           error("Cannot create instance with instance reference number $refnum")
       end
   end
   function Multiton(registry, refnum)
       if 0 < refnum <= registry.max_instances && registry.instances[refnum] isa Multiton
           return registry.instances[refnum]
       else
           error("Cannot find a Multiton in registry with instance reference number $refnum")
       end
   end

end

struct Registry

       spinlock::Threads.SpinLock
       max_instances::Int
       instances::Vector{Union{Nothing, Multiton}}
       Registry(maxnum) = new(Threads.SpinLock(), maxnum, fill(nothing, maxnum))

end

reg = Registry(3) m0 = Multiton(reg, 1, "zero") m1 = Multiton(reg, 2, 1.0) m2 = Multiton(reg, 3, [2]) m3 = Multiton(reg, 1) m4 = Multiton(reg, 2)

for m in [m0, m1, m2, m3, m4]

  println("Multiton is $m")

end

1. produce error
2. m3 = Multiton(reg, 4, "three")

1. produce error

m5 = Multiton(reg, 5)

</lang>

Output:

Multiton is Multiton{String}("zero")
Multiton is Multiton{Float64}(1.0)
Multiton is Multiton{Vector{Int64}}([2])
Multiton is Multiton{String}("zero")
Multiton is Multiton{Float64}(1.0)  
ERROR: LoadError: Cannot find a Multiton in registry with instance reference number 5

Phix

Library: Phix/Class

No attempt is made for thread safety, since multiple threads accessing these would need their own locking anyway, not that it is difficult to invoke enter_cs() and leave_cs().
I thought about adding a get_multiton() function to avoid calling new() all the time, but it would (probably/almost certainly) just invoke new() itself anyway.
Put this (up to "end class") in a separate file, to keep allowed and instances private. Classes are not [yet] supported under pwa/p2js.
Technically I suppose this should really use new_dict()/getd()/setd(), but I'm quite sure you'll cope somehow..

without javascript_semantics
sequence allowed = {"zero","one","two"},
         instances = {NULL,NULL,NULL}

public class Multiton
    public string id
    function Multiton(string id)
        integer k = find(id,allowed)
        if k=0 then crash("not allowed") end if
        if instances[k] = NULL then
            this.id = id&sprintf("_%d",rand(999))
            instances[k] = this
        end if
        return instances[k]
    end function
end class

Multiton a = new({"zero"}),
         b = new({"one"}),
         c = new({"two"}),
         d = new({"zero"}),
--       e = new(),         -- crashes
--       f = new({"three"}) -- crashes
         e = new({"one"}),
         f = new({"two"})

?a.id
?b.id
?c.id
?d.id
?e.id
?f.id

Output:

"zero_651"
"one_111"
"two_544"
"zero_651"
"one_111"
"two_544"

Wren

Library: Wren-dynamic

This more or less follows the lines of the C# example in the linked Wikipedia article.

Although all Wren code runs within the context of a fiber (of which there can be thousands) only one fiber can run at a time and so the language is effectively single threaded. Thread-safety is therefore never an issue. <lang ecmascript>import "/dynamic" for Enum

var MultitonType = Enum.create("MultitonType", ["zero", "one", "two"])

class Multiton {

   // private constructor
   construct new_(type) {
       _type = type
   }

   static getInstance(type) {
       if (!(0...MultitonType.members.count).contains(type)) {
           Fiber.abort("Invalid MultitonType member.")
       }
       if (!__instances) __instances = {}
       if (!__instances.containsKey(type)) __instances[type] = new_(type)
       return __instances[type]
   }

   type { _type }

   toString { MultitonType.members[_type] }

}

var m0 = Multiton.getInstance(MultitonType.zero) var m1 = Multiton.getInstance(MultitonType.one) var m2 = Multiton.getInstance(MultitonType.two)

System.print(m0) System.print(m1) System.print(m2)

var m3 = Multiton.getInstance(3) // produces an error</lang>

Output:

zero
one
two
Invalid MultitonType member.
[./multiton line 13] in getInstance(_)
[./multiton line 33] in (script)