Add a variable to a class instance at runtime

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Revision as of 22:48, 10 May 2017 by rosettacode>Dmitry Kandalov (→‎{{header|Kotlin}}: Updated example see for details)
Add a variable to a class instance at runtime
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Demonstrate how to dynamically add variables to an object (a class instance) at runtime.

This is useful when the methods/variables of an instance are based on a data file that isn't available until runtime. Hal Fulton gives an example of creating an OO CSV parser at An Exercise in Metaprogramming with Ruby. This is referred to as "monkeypatching" by Pythonistas and some others.


In ActionScript this can be done using an Object object <lang actionscript>var object:Object = new Object(); = "bar";</lang> Or by creating a dynamic class <lang actionscript>package {

   public dynamic class Foo
       // ...

}</lang> <lang actionscript>var foo:Foo = new Foo(); = "zap";</lang>


Ada is not a dynamically typed language. Yet it supports mix-in inheritance, run-time inheritance and interfaces. These three allow us to achieve the desired effect, however questionably useful it could be. The example declares an interface of the class (Class). Then a concrete type is created (Base). The object E is an instance of Base. Later, at the run time, a new type Monkey_Patch is created such that it refers to E and implements the class interface per delegation to E. Monkey_Patch has a new integer member Foo and EE is an instance of Monkey_Path. For the user EE appears as E with Foo. <lang ada>with Ada.Text_IO; use Ada.Text_IO;

procedure Dynamic is

  package Abstract_Class is
     type Class is limited interface;
     function Boo (X : Class) return String is abstract;
  end Abstract_Class;
  use Abstract_Class;
  package Base_Class is
     type Base is new Class with null record;
     overriding function Boo (X : Base) return String;
  end Base_Class;
  package body Base_Class is
     function Boo (X : Base) return String is
        return "I am Class";
     end Boo;
  end Base_Class;
  use Base_Class;
  E : aliased Base;  -- An instance of Base


  -- Gone run-time
     type Monkey_Patch (Root : access Base) is new Class with record
        Foo : Integer := 1;
     end record;
     overriding function Boo (X : Monkey_Patch) return String;
     function Boo (X : Monkey_Patch) return String is
     begin -- Delegation to the base
        return X.Root.Boo;
     end Boo;
     EE : Monkey_Patch (E'Access); -- Extend E
     Put_Line (EE.Boo & " with" & Integer'Image (EE.Foo));

end Dynamic;</lang> Sample output:

I am Class with 1


Works with: AutoHotkey_L

<lang AutoHotkey>e := {} := 1 </lang>


It's not really intended that you should do this, but if you must you can: <lang bbcbasic> INSTALL @lib$+"CLASSLIB"

     REM Create a base class with no members:
     DIM class{method}
     REM Instantiate the class:
     PROC_new(myobject{}, class{})
     REM Add a member at run-time:
     member$ = "mymember#"
     PROCaddmember(myobject{}, member$, 8)
     REM Test that the member can be accessed:
     PROCassign("myobject." + member$, "PI")
     PRINT EVAL("myobject." + member$)
     DEF PROCaddmember(RETURN obj{}, mem$, size%)
     LOCAL D%, F%, P%
     DIM D% DIM(obj{}) + size% - 1, F% LEN(mem$) + 8
     P% = !^obj{} + 4
     WHILE !P% : P% = !P% : ENDWHILE : !P% = F%
     $$(F%+4) = mem$ : F%!(LEN(mem$) + 5) = DIM(obj{})
     !(^obj{} + 4) = D%
     DEF PROCassign(v$, n$)
     IF EVAL("FNassign(" + v$ + "," + n$ + ")")
     DEF FNassign(RETURN n, v) : n = v : = 0</lang>


This solution saves the original members and methods in a variable, using pattern matching. Then, using macro expansion, a new object is created with an additional member variable and also an additional method. Because the new object is assigned to the same variable as the original object, the original object ceases to exist. <lang bracmat>( ( struktuur

 = (aMember=) (aMethod=.!(its.aMember))

& new$struktuur:?object & out$"Object as originally created:" & lst$object & A value:?(object..aMember) & !object:(=?originalMembersAndMethods) & new

   $ ( 
     ' ( (anotherMember=)
 : ?object

& out

 $ "

Object with additional member and method and with 'aMember' already set to some interesting value:" & lst$object & some other value:?(object..anotherMember) & out$" Call both methods and output their return values." & out$("aMember contains:" (object..aMethod)$) & out$("anotherMember contains:" (object..anotherMethod)$) &);</lang> Output: <lang bracmat>Object as originally created: (object= =(aMember=) (aMethod=.!(its.aMember)));

Object with additional member and method and with 'aMember' already set to some interesting value: (object=

= (anotherMember=)

   (aMember=A value)

Call both methods and output their return values. aMember contains: A value anotherMember contains: some other value</lang>


Works with: C# version 4.0

<lang csharp>// ---------------------------------------------------------------------------------------------- // // Program.cs - DynamicClassVariable // // Mikko Puonti, 2013 // // ----------------------------------------------------------------------------------------------

using System; using System.Dynamic;

namespace DynamicClassVariable {

   internal static class Program
       #region Static Members
       private static void Main()
           // To enable late binding, we must use dynamic keyword
           // ExpandoObject readily implements IDynamicMetaObjectProvider which allows us to do some dynamic magic 
           dynamic sampleObj = new ExpandoObject();
           // Adding a new property
  = 1;
           Console.WriteLine( " = {0}", );
           // We can also add dynamically methods and events to expando object 
           // More information:
           // This sample only show very small part of dynamic language features - there is lot's more
           Console.WriteLine( "< Press any key >" );


Output: = 1
< Press any key >


<lang coffeescript># CoffeeScript is dynamic, just like the Javascript it compiles to.

  1. You can dynamically add attributes to objects.
  1. First create an object very simply.

e = {} = "bar" e.yo = -> "baz" console.log, e.yo()

  1. CS also has class syntax to instantiate objects, the details of which
  2. aren't shown here. The mechanism to add members is the same, though.

class Empty

 # empty class

e = new Empty() = "bar" e.yo = -> "baz" console.log, e.yo() </lang>

Common Lisp

This version adds a new slot only to one instance, not to the whole class.

Library: Closer to MOP

<lang lisp>(defun augment-instance-with-slots (instance slots)

 (change-class instance
               (make-instance 'standard-class
                 :direct-superclasses (list (class-of instance))
                 :direct-slots slots)))</lang>


<lang lisp>CL-USER> (let* ((instance (make-instance 'foo :bar 42 :baz 69))

               (new-slots '((:name xenu :initargs (:xenu)))))
          (augment-instance-with-slots instance new-slots)
          (reinitialize-instance instance :xenu 666)
          (describe instance))
  1. <#<STANDARD-CLASS NIL {1003AEE2C1}> {1003AEE271}>

Slots with :INSTANCE allocation:

 BAR   = 42
 BAZ   = 69
 XENU  = 666</lang>

The following REPL transcript (from LispWorks) shows the definition of a class some-class with no slots, and the creation of an instance of the class. The first attempt to access the slot named slot1 signals an error as there is no such slot. Then the class is redefined to have such a slot, and with a default value of 23. Attempting to access the slot in the preëxisting instance now gives the default value, since the slot has been added to the instance. This behavior is specified in §4.3.6 Redefining Classes of the HyperSpec.

CL-USER 57 > (defclass some-class () ())

CL-USER 58 > (defparameter *an-instance*
               (make-instance 'some-class))

CL-USER 59 > (slot-value *an-instance* 'slot1)

Error: The slot SLOT1 is missing from #<SOME-CLASS 21F59E37> (of class #<STANDARD-CLASS SOME-CLASS 200BF63B>), when reading the value.
  1 (abort) Return to level 0.
  2 Return to top loop level 0.

Type :b for backtrace, :c <option number> to proceed,  or :? for other options

CL-USER 60 : 1 > :a

CL-USER 61 > (defclass some-class ()
               ((slot1 :initform 23)))

CL-USER 62 > (slot-value *an-instance* 'slot1)


<lang d>struct Dynamic(T) {

   private T[string] vars;
   @property T opDispatch(string key)() pure nothrow {
       return vars[key];
   @property void opDispatch(string key, U)(U value) pure nothrow {
       vars[key] = value;


void main() {

   import std.variant, std.stdio;
   // If the type of the attributes is known at compile-time:
   auto d1 = Dynamic!double();
   d1.first = 10.5;
   d1.second = 20.2;
   writeln(d1.first, " ", d1.second);

   // If the type of the attributes is mixed:
   auto d2 = Dynamic!Variant();
   d2.a = "Hello";
   d2.b = 11;
   d2.c = ['x':2, 'y':4];
   d2.d = (int x) => x ^^ 3;
   writeln(d2.a, " ", d2.b, " ", d2.c);
   immutable int x = d2.b.get!int;


10.5 20.2
Hello 11 ['x':2, 'y':4]

If you want Dynamic to be a class the code is similar. If the attribute names aren't known at compile-time, you have to use a more normal syntax: <lang d>import std.stdio, std.variant, std.conv;

struct Dyn {

   Variant[string] data;
   alias data this;


void main(string[] args) {

   Dyn d;
   const attribute_name = text("attribute_", args.length);
   d[attribute_name] = "something";




ELENA does not support adding a field at run-time but it can be simulated with the help of a mix-in. ELENA 3.x <lang elena>#import extensions.

class Extender :: BaseExtender {

   object prop(foo) theField.
   constructor new : anObject
       theObject := anObject.


program = [

   var anObject := 234.
   // extending an object with a field
   anObject := Extender new:anObject.
   anObject set &foo:"bar".
   console writeLine:anObject:".foo=":(anObject foo).
   console readChar.




Classes and singleton objects have a fixed structure which cannot be changed during runtime. However falcon does have capability to add variables/functions at runtime with Prototype based objects. Below are two of the prototype objects that allow adding variables at runtime. These are arrays and dictionaries (hashes for the perl type out there).

Array: In this example we add a function (which prints out the content of the array) and a new value. While we are not technically adding a "variable", this example is presented to show similar type of functionality. <lang falcon>vect = [ 'alpha', 'beta', 'gamma' ] vect.dump = function ()

 for n in [0: self.len()]
   > @"$(n): ", self[n]

end vect += 'delta' vect.dump()</lang> Output from the above: <lang falcon>0: alpha 1: beta 2: gamma 3: delta</lang> Dictionary: In this example we will add a variable through the use of an object from a bless'ed dictionary. We create a new variable called 'newVar' at runtime and assign a string to it. Additionally we assign an external, to the object, function (sub_func) to the variable 'sub'. <lang falcon>function sub_func( value )

 self['prop'] -= value
 return self.prop


dict = bless( [

 'prop' => 0,
 'add' => function ( value )
   self.prop += value
   return self.prop
 end ,
 'sub' => sub_func


dict[ 'newVar' ] = "I'm Rich In Data"</lang>


FBSL class instances aren't expandable with additional, directly accessible public methods at runtime once the class template is defined in the user code. But FBSL has an extremely powerful feature -- an ExecLine() function -- which permits the user to execute any additional code on the fly either privately (bypassing the main code flow) or publicly (interacting with the main code). ExecLine() can be used for a variety of applications from the fine-tuning of current tasks to designing application plug-ins or completely standalone code debuggers. The following class instance may be stuffed up at runtime with any code from simple variables to executable private methods and properties. <lang qbasic>#APPTYPE CONSOLE

CLASS Growable


DIM instructions AS STRING = "Sleep(1)" :ExecCode DIM dummy AS INTEGER = EXECLINE(instructions, 1)


METHOD Absorb(code AS STRING) instructions = code GOTO ExecCode END METHOD



DIM Sponge AS NEW Growable()

Sponge.Absorb("DIM b AS VARIANT = 1234567890: DIM result AS VARIANT = b") PRINT Sponge.Yield() Sponge.Absorb("b = ""Hello world!"": result = b") PRINT Sponge.Yield()

PAUSE</lang> Output:

Hello world!
Press any key to continue...


Works with: Forth

Works with any ANS Forth

Needs the FMS-SI (single inheritance) library code located here: <lang forth>include FMS-SI.f include FMS-SILib.f

\ We can add any number of variables at runtime by adding \ objects of any type to an instance at run time. The added \ objects are then accessible via an index number.

class foo
 object-list inst-objects \ a dynamically growable object container
 :m init: inst-objects init: ;m
 :m add: ( obj -- ) inst-objects add: ;m
 :m at: ( idx -- obj ) inst-objects at: ;m

foo foo1

 heap> string foo1 add:
 heap> fvar   foo1 add:
 s" Now is the time " 0 foo1 at: !:
 3.14159e             1 foo1 at: !:
 0 foo1 at: p: \ send the print message to indexed object 0
 1 foo1 at: p: \ send the print message to indexed object 1

main \ => Now is the time 3.14159 </lang>


Any Groovy class that implements "Object get(String)" and "void set(String, Object)" will have the apparent capability to add new properties. However, this capability will only work as expected with an appropriate implementation, backed by a Map object or something very much like a Map. <lang groovy>class A {

   final x = { it + 25 }
   private map = new HashMap()
   Object get(String key) { map[key] }
   void set(String key, Object value) { map[key] = value }


Test: <lang groovy>def a = new A() a.y = 55 a.z = { println (new Date()); Thread.sleep 5000 }

println a.x(25) println a.y (0..2).each(a.z)

println a.q</lang>


Wed Feb 23 21:33:40 CST 2011
Wed Feb 23 21:33:45 CST 2011
Wed Feb 23 21:33:50 CST 2011


All "instance variables" (or slots in Io nomenclature) are created at runtime.

<lang io>Empty := Object clone

e := Empty clone e foo := 1</lang>

Icon and Unicon

Unicon implements object environments with records and supporting procedures for creation, initialization, and methods. To modify an instance you must create a new record then copy, amend, and replace it. Strictly speaking we can't guarantee the replace as there is no way to modify the existing object and we are creating a new instance with extensions. The procedures constructor and fieldnames are needed. This example doesn't do error checking. Here extend takes three arguments, the class instance, a list of new variable names as strings, and an optional list of new values to be assigned. The new instance is returned and the object is replaced by assignment. The caveat here is that if the object was assigned to anything else we will now have two objects floating around with possible side effects. As written this isn't safe from name collisions - aside from local declarations the use of a fixed constructor name uses the global name space. There is a final caveat that needs to be observed - if future implementations of objects change then this could easily stop working.

Note: Unicon can be translated via a command line switch into icon which allows for classes to be shared with Icon code (assuming no other incompatibilities exist). <lang unicon> link ximage

procedure main()

  c1 := foo(1,2)                            # instance of foo
  c1 := extend(c1,["c","d"],[8,9])          # 2 new fields
  write("new c1:\n",ximage(c1))
  c1 := extend(c1,["e"],[7])                # 1 more
  write("newest c1:\n",ximage(c1))   


class foo(a,b) # dummy class end

procedure extend(instance,newvars,newvals) #: extend a class instance

  every put(f := [],fieldnames(instance))   # copy existing fieldnames
  c := ["tempconstructor"] ||| f            # new constructor    
  every put(c,!newvars)                     # append new vars
  t := constructor!c                        # new constructor
  x := t()                                  # new instance
  every x[v := !f] := instance[v]           # same as old instance  
  x.__s := x                                # new self 
  if \newvals then 
     every i := 1 to min(*newvars,*newvals) do 
        x[newvars[i]] := newvals[i]         # add new vars = values   
  return x


ximage.icn provides ximage to dump variable contents


R_foo__state_1 := foo__state()
   R_foo__state_1.a := 1
   R_foo__state_1.b := 2
new c1:
R_tempconstructor_1 := tempconstructor()
   R_tempconstructor_1.__s := R_tempconstructor_1
   R_tempconstructor_1.__m := R_foo__methods_1 := foo__methods()
   R_tempconstructor_1.a := 1
   R_tempconstructor_1.b := 2
   R_tempconstructor_1.c := 8
   R_tempconstructor_1.d := 9
newest c1:
R_tempconstructor_1 := tempconstructor()
   R_tempconstructor_1.__s := R_tempconstructor_1
   R_tempconstructor_1.__m := R_foo__methods_1 := foo__methods()
   R_tempconstructor_1.a := 1
   R_tempconstructor_1.b := 2
   R_tempconstructor_1.c := 8
   R_tempconstructor_1.d := 9
   R_tempconstructor_1.e := 7


If you assign a value to the name which references a property of a class instance, that name within that instance gets that value.

<lang j> C=:<'exampleclass' NB. this will be our class name

  V__C=: 0                   NB. ensure the class exists
  OBJ1=:conew 'exampleclass' NB. create an instance of our class
  OBJ2=:conew 'exampleclass' NB. create another instance
  V__OBJ1,V__OBJ2            NB. both of our instances exist


  W__OBJ1                    NB. instance does not have a W

|value error

  W__OBJ1=: 0                NB. here, we add a W to this instance
  W__OBJ1                    NB. this instance now has a W


  W__OBJ2                    NB. our other instance does not

|value error</lang>


This kind of thing is fundamental to JavaScript, as it's a prototype-based language rather than a class-based one. <lang javascript>e = {} // generic object = 1 e["bar"] = 2 // name specified at runtime</lang>


Adding variables to an object at runtime is not possible in Kotlin (at least in the version targeting the JVM) which is a statically typed language and therefore the names of all class variables need to be known at compile time.

However, as in the case of Groovy, we can make it appear as though variables are being added at runtime by using a Map or similar structure. For example:

<lang scala>// version 1.1.2

class SomeClass {

   val runtimeVariables = mutableMapOf<String, Any>()


fun main(args: Array<String>) {

   val sc = SomeClass()
   println("Create two variables at runtime: ")
   for (i in 1..2) {
       println("  Variable #$i:")
       print("       Enter name  : ")
       val name = readLine()!!
       print("       Enter value : ")
       val value = readLine()!!
       sc.runtimeVariables.put(name, value)
   while (true) {
       print("Which variable do you want to inspect ? ")
       val name = readLine()!!
       val value = sc.runtimeVariables[name]
       if (value == null)
           println("There is no variable of that name, try again")
       else {
           println("Its value is '${sc.runtimeVariables[name]}'")



Sample input/output:

Create two variables at runtime:
  Variable #1:
       Enter name  : a
       Enter value : rosetta

  Variable #2:
       Enter name  : b
       Enter value : 64

Which variable do you want to inspect ? a
Its value is 'rosetta'


jq's "+" operator can be used to add a key/value pair (or to add multiple key-value pairs) to an existing object at runtime, but jq is a functional programming language, and objects themselves cannot be altered. Thus it may be helpful to introduce a variable, since the value of a variable can in effect be updated. For example: <lang jq>{"a":1} as $a | ($a + {"b":2}) as $a | $a </lang>Thus the value of $a has undergone the desired transition, that is, its final value is {"a":1, "b":2}.

A Javascript-like syntax can also be used to add (or update) a key, for example:<lang jq>$a|.c = 3

  1. or equivalently:

$a|.["c"] = 3</lang>


BUKKITs (the all-purpose container type) can be added to at any point during execution, and the SRS operator permits the creation of identifiers from strings. This program and its output demonstrate both by prompting the user for a name and a value, modifying the object accordingly, and then printing the value of the new variable. <lang LOLCODE>HAI 1.3

I HAS A object ITZ A BUKKIT I HAS A name, I HAS A value

IM IN YR interface

   I HAS A option, GIMMEH option
   option, WTF?
   OMG "A"
       VISIBLE "NAME: "!, GIMMEH name
       VISIBLE "VALUE: "!, GIMMEH value
       object HAS A SRS name ITZ value, GTFO
   OMG "P"
       VISIBLE "NAME: "!, GIMMEH name
       VISIBLE object'Z SRS name

IM OUTTA YR interface

KTHXBYE</lang> Example run:

NAME: foo
NAME: foo


Logtalk supports "hot patching" (aka "monkey patching") using a category, which is a first-class entity and a fine grained units of code reuse that can be (virtally) imported by any number of objects but also used for "complementing" an existing object, adding new functionality or patching existing functionality. Complementing cateogries can be enable or disabled globally or on a per object basis.

The following example uses a prototype for simplicity.

<lang logtalk> % we start by defining an empty object

- object(foo).
   % ensure that complementing categories are allowed

:- set_logtalk_flag(complements, allow).

- end_object.

% define a complementing category, adding a new predicate

- category(bar,

:- public(bar/1). bar(1). bar(2). bar(3).

- end_category.


We can test our example by compiling and loading the two entities above and then querying the object:

<lang logtalk> | ?- foo::bar(X). X = 1 ; X = 2 ; X = 3 true </lang>


<lang lua>empty = {} = 1</lang>


Mathematica doesn't rally have classes, so it doesn't have class variables. However, many rules can be applied to a single tag, so it has some aspects similar to a class. With that definition, adding a class variable is similar to adding a rule: <lang Mathematica> f[a]=1; f[b]=2; f[a]=3; ? f</lang> Output:


Here, the two 'variables' can be seen under the single heading 'f'. And of course all of this is done at runtime.


To emulate adding a variable to a class instance, Morfa uses user-defined operators ` and <-. <lang morfa> import morfa.base;

template <T> public struct Dynamic {

   var data: Dict<text, T>;


// convenience to create new Dynamic instances template <T> public property dynamic(): Dynamic<T> {

   return Dynamic<T>(new Dict<text,T>());


// introduce replacement operator for . - a quoting ` operator public operator ` { kind = infix, precedence = max, associativity = left, quoting = right } template <T> public func `(d: Dynamic<T>, name: text): DynamicElementAccess<T> {

   return DynamicElementAccess<T>(d, name);


// to allow implicit cast from the wrapped instance of T (on access) template <T> public func convert(dea: DynamicElementAccess<T>): T {



// cannot overload assignment - introduce special assignment operator public operator <- { kind = infix, precedence = assign } template <T> public func <-(access: DynamicElementAccess<T>, newEl: T): void {[] = newEl;


func main(): void {

   var test = dynamic<int>;
   test`a <- 10;
   test`b <- 20;
   test`a <- 30;
   println(test`a, test`b);


// private helper structure template <T> struct DynamicElementAccess {

   var holder: Dynamic<T>;
   var name: text;
   public func format(formatt: text, formatter: Formatter): text
       return getFormatFunction([name])(formatt, formatter);

} </lang>

30 20


Objective-C doesn't have the ability to add a variable to an instance at runtime. However, since Mac OS X 10.6 and iOS 3.1, it has something that can accomplish a very similar purpose, called "associative references" or "associated objects", which allow you to attach additional objects onto an object without changing its class.

You can put associative references on any object. You can put multiple ones on the same object. They are indexed by a pointer key (typically the address of some dummy variable). You use the functions objc_getAssociatedObject() and objc_setAssociatedObject to get and set them, respectively.

<lang objc>#import <Foundation/Foundation.h>

  1. import <objc/runtime.h>

static void *fooKey = &fooKey; // one way to define a unique key is a pointer variable that points to itself

int main (int argc, const char *argv[]) {

   @autoreleasepool {
       id e = [[NSObject alloc] init];
       // set
       objc_setAssociatedObject(e, fooKey, @1, OBJC_ASSOCIATION_RETAIN);
       // get
       NSNumber *associatedObject = objc_getAssociatedObject(e, fooKey);
       NSLog(@"associatedObject: %@", associatedObject);
   return 0;


You can also use a selector as the key, since two selectors with the same content are guaranteed to be equal: <lang objc>#import <Foundation/Foundation.h>

  1. import <objc/runtime.h>

int main (int argc, const char *argv[]) {

   @autoreleasepool {
       id e = [[NSObject alloc] init];
       // set
       objc_setAssociatedObject(e, @selector(foo), @1, OBJC_ASSOCIATION_RETAIN);
       // get
       NSNumber *associatedObject = objc_getAssociatedObject(e, @selector(foo));
       NSLog(@"associatedObject: %@", associatedObject);
   return 0;



Octave is dynamically typed, and can have fields added in two methods:

<lang octave> % Given struct "test" test.b=1; test = setfield (test, "c", 3); </lang>


ooRexx does not directly expose instance variables to callers. Encapsulated access to instance variables is done via accesser methods for assignment and retrieval. In general, it is not possible to just dynamically add support, but it is possible to construct a class that allows for this to happen.

Unknown Method Access

This example traps unknown method calls, then sets or retrieves the values in an encapsulated directory object. <lang ooRexx> d = .dynamicvar~new d~foo = 123 say d~foo

d2 = .dynamicvar2~new d~bar = "Fred" say d~bar

-- a class that allows dynamic variables. Since this is a mixin, this -- capability can be added to any class using multiple inheritance

class dynamicvar MIXINCLASS object
method init
 expose variables
 variables = .directory~new

-- the UNKNOWN method is invoked for all unknown messages. We turn this -- into either an assignment or a retrieval for the desired item

method unknown
 expose variables
 use strict arg messageName, arguments
 -- assignment messages end with '=', which tells us what to do
 if messageName~right(1) == '=' then do
    variables[messageName~left(messageName~length - 1)] = arguments[1]
 else do
     return variables[messageName]

-- this class is not a direct subclass of dynamicvar, but mixes in the -- functionality using multiple inheritance

class dynamicvar2 inherit dynamicvar
method init
 -- mixin init methods are not automatically invoked, so we must
 -- explicitly invoke this


Dynamic Method Definitions

An object may be written that can dynamically add methods to itself. This example is similar to the above example, but the UNKNOWN method attaches a getter/setter pair of methods for the name triggering the UNKNOWN call. On all subsequent calls, the attribute methods will get called. <lang ooRexx> d = .dynamicvar~new d~foo = 123 say d~foo

-- a class that allows dynamic variables. Since this is a mixin, this -- capability can be added to any class using multiple inheritance

class dynamicvar MIXINCLASS object
method init
 expose variables
 variables = .directory~new

-- the unknown method will get invoked any time an unknown method is -- used. This UNKNOWN method will add attribute methods for the given -- name that will be available on all subsequent uses.

method unknown
 expose variables
 use strict arg messageName, arguments
 -- check for an assignment or fetch, and get the proper
 -- method name
 if messageName~right(1) == '=' then do
    variableName = messageName~left(messageName~length - 1)
 else do
    variableName = messageName
 -- define a pair of methods to set and retrieve the instance variable.  These are
 -- created at the object scope
 self~setMethod(variableName, 'expose' variableName'; return' variableName)
 self~setMethod(variableName'=', 'expose' variableName'; use strict arg value;' variableName '= value' )
 -- reinvoke the original message.  This will now go to the dynamically added
 forward to(self) message(messageName) arguments(arguments)



Simple implementation for making runtime members - supports integer, float and string types. <lang oxygenbasic> '================= class fleximembers '=================

indexbase 0 bstring buf, *varl sys dp,en

method addVar(string name,dat)

 sys le=len buf
 if dp+16>le then
   buf+=nuls 0x100 : le+=0x100 :
 end if
 dp+=2*sizeof sys
 en+=2 'next slot

end method

method find(string name) as sys

 sys i
 for i=0 to <en step 2
   if name=varl[i] then return i+1

end method

method vars(string name) as string

 sys f=find(name)
 if f then return varl[f]

end method

method VarF(string name) as double

 return vars(name)

end method

method VarI(string name) as sys

 return vars(name)

end method

method vars(string name,dat)

 bstring varl at buf
 sys f=find(name)
 if f then varl[f]=dat

end method

method delete()

 sys i
 sys v at buf
 for i=0 to <en
    freememory v[i]
 freememory ?buf
 ? buf=0 : en=0 : dp=0

end method

end class


fleximembers a

a.addVar "p",5 a.addVar "q",4.5 a.addVar "r","123456"

print a.Vars("q")+a.vars("q") 'result 4.54.5 print a.Varf("q")+a.varf("q") 'result 9




It is not possible to add variables to instances in Oz. Every object has exactly one class and this association cannot be changed after object creation. Classes themselves are immutable.

However, classes are also first-class values and are created at runtime. Many of the tasks that are solved with "monkeypatching" in other languages, can be solved by dynamically creating classes in Oz.

<lang oz>declare

 %% Creates a new class derived from BaseClass
 %% with an added feature (==public immutable attribute)
 fun {AddFeature BaseClass FeatureName FeatureValue}
    class DerivedClass from BaseClass

 %% "FeatureName" is escaped, otherwise a new variable  %% refering to a private feature would be created

 class Base
    meth init
 Derived = {AddFeature Base foo 2}
 Instance = {New Derived init}


 {Show} %% inherited feature
 {Show} %% feature of "synthesized" class</lang>

To add a variable number of features and attributes, you can use


Works with: Perl version 5.x

<lang perl>package Empty;

  1. Constructor. Object is hash.

sub new { return bless {}, shift; }

package main;

  1. Object.

my $o = Empty->new;

  1. Set runtime variable (key => value).

$o->{'foo'} = 1;</lang>

Perl 6

Works with: Rakudo version 2015.12

You can add variables/methods to a class at runtime by composing in a role. The role only affects that instance, though it is inheritable. An object created from an existing object will inherit any roles composed in with values set to those at the time the role was created. If you want to keep changed values in the new object, clone it instead. <lang perl6>class Bar { } # an empty class

my $object =; # new instance

role a_role { # role to add a variable: foo,

  has $.foo is rw = 2;   # with an initial value of 2


$object does a_role; # compose in the role

say $; # prints: 2 $ = 5; # change the variable say $; # prints: 5

my $ohno =; # new Bar object

  1. say $; # runtime error, base Bar class doesn't have the variable foo

my $this = $; # instantiate a new Bar derived from $object say $; # prints: 2 - original role value

my $that = $object.clone; # instantiate a new Bar derived from $object copying any variables say $; # 5 - value from the cloned object</lang> That's what's going on underneath, but often people just mix in an anonymous role directly using the but operator. Here we'll mix an attribute into a normal integer. <lang perl6>my $lue = 42 but role { has $.answer = "Life, the Universe, and Everything" }

say $lue; # 42 say $lue.answer; # Life, the Universe, and Everything</lang> On the other hand, mixins are frowned upon when it is possible to compose roles directly into classes (as with Smalltalk traits), so that you get method collision detection at compile time. If you want to change a class at run time, you can also use monkey patching:

<lang perl6>use MONKEY-TYPING; augment class Int {

   method answer { "Life, the Universe, and Everything" }

} say 42.answer; # Life, the Universe, and Everything</lang> This practice, though allowed, is considered to be Evil Action at a Distance.


<lang php>class E {};

$e=new E();


$e->{"foo"} = 1; // using a runtime name $x = "foo"; $e->$x = 1; // using a runtime name in a variable</lang>


In general, all instance variables in PicoLisp are dynamically created at runtime. <lang PicoLisp>: (setq MyObject (new '(+MyClass))) # Create some object -> $385605941

(put MyObject 'newvar '(some value)) # Set variable

-> (some value)

(show MyObject) # Show the object

$385605941 (+MyClass)

  newvar (some value)

-> $385605941</lang>


Pike does not allow adding variables to existing objects, but we can design a class that allows us to add variables. <lang Pike>class CSV {

   mapping variables = ([]);
   mixed `->(string name)
       return variables[name];
   void `->=(string name, mixed value)
       variables[name] = value;
   array _indices()
       return indices(variables);


object csv = CSV(); csv->greeting = "hello world"; csv->count = 1; csv->lang = "Pike";

indices(csv); Result: ({ /* 3 elements */




In Pop11 instance variables (slots) are specified at class creation time and there is no way to add new slot to an instance after its class was created. However, for most practical purposes one can obtain desired effect in different way. Namely, except for a few low-level routines slots in Pop11 are accessed via getter and setter methods. Getters and setters are like ordinary methods, but are automatically defined and "know" low level details of slot access. Pop11 allows dynamic definition of methods, and one can add new methods which work as "getter" and "setter" but do not store data directly in instance. One possibility is to have one instance variable which contains a hastable (this is essentially what Perl solution is doing). Another possibility (used below) is to create na external hashtable. Adding new slots typically make sense if slot name is only known at runtine, so we create method definition (as a list) at runtime and compile it using the 'pop11_compile' procedure.

<lang pop11>lib objectclass;

define :class foo; enddefine;

define define_named_method(method, class);

   lvars method_str = method >< ;
   lvars class_str = class >< ;
   lvars method_hash_str = 'hash_' >< length(class_str) >< '_'
                             >< class_str >< '_' >< length(method_str)
                             >< '_' >< method_str;
   lvars method_hash = consword(method_hash_str);
       lvars ^method_hash = newassoc([]);
       define :method ^method(self : ^class);
       define :method updaterof ^method(val, self : ^class);
           val -> ^method_hash(self);


define_named_method("met1", "foo"); lvars bar = consfoo(); met1(bar) =>  ;;; default value -- false "baz" -> met1(bar); met1(bar) =>  ;;; new value</lang>


PowerShell allows extending arbitrary object instances at runtime with the Add-Member cmdlet. The following example adds a property Title to an integer: <lang powershell>$x = 42 `

    | Add-Member -PassThru `
       NoteProperty `
       Title `
       "The answer to the question about life, the universe and everything"</lang>

Now that property can be accessed:

PS> $x.Title
The answer to the question about life, the universe and everything

or reflected:

PS> $x | Get-Member

   TypeName: System.Int32

Name        MemberType   Definition
----        ----------   ----------
CompareTo   Method       int CompareTo(System.Object value), ...
Equals      Method       bool Equals(System.Object obj), bool...
GetHashCode Method       int GetHashCode()
GetType     Method       type GetType()
GetTypeCode Method       System.TypeCode GetTypeCode()
ToString    Method       string ToString(), string ToString(s...
Title       NoteProperty System.String Title=The answer to th...

While trying to access the same property in another instance will fail:

PS> $y = 42
PS> $y.Title

(which simply causes no output).


<lang python>class empty(object):


e = empty()</lang>

If the variable (attribute) name is known at "compile" time (hard-coded):

<lang python> = 1</lang>

If the variable name is determined at runtime: <lang python> setattr(e, name, value)</lang>

Note: Somewhat counter-intuitively one cannot simply use e = object(); = 1 because the Python base object (the ultimate ancestor to all new-style classes) will raise attribute exceptions. However, any normal derivatives of object can be "monkey patched" at will.

Because functions are first class objects in Python one can not only add variables to instances. One can add or replace functionality to an instance. Doing so is tricky if one wishes to refer back to other instance attributes since there's no "magic" binding back to "self." One trick is to dynamically define the function to be added, nested within the function that applies the patch like so:

<lang python>class empty(object):

   def __init__(this): = "whatever"

def patch_empty(obj):

   def fn(self=obj):
   obj.print_output = fn

e = empty() patch_empty(e) e.print_output()

  1. >>> whatever</lang>
Note: The name self is not special; it's merely the pervasive Python convention. In this example I've deliberately used this in the class definition to underscore this fact. The nested definition could use any name for the "self" object. Because it's nested the value of the object is evaluated at the time that the patch_empty() function is run and thus the function being patched in has a valid reference to the object into which it is being inserted. Other arguments could be passed as necessary. Such techniques are not recommended; however they are possible.


<lang rebol> REBOL [ Title: "Add Variables to Class at Runtime" Author: oofoe Date: 2009-12-04 URL: ]

As I understand it, a REBOL object can only ever have whatever
properties it was born with. However, this is somewhat offset by the
fact that every instance can serve as a prototype for a new object
that also has the new parameter you want to add.
Here I create an empty instance of the base object (x), then add the
new instance variable while creating a new object prototyped from
x. I assign the new object to x, et voila', a dynamically added

x: make object! [] ; Empty object.

x: make x [ newvar: "forty-two" ; New property. ]

print "Empty object modifed with 'newvar' property:" probe x

A slightly more interesting example

starfighter: make object! [ model: "unknown" pilot: none ] x-wing: make starfighter [ model: "Incom T-65 X-wing" ]

squadron: reduce [ make x-wing [pilot: "Luke Skywalker"] make x-wing [pilot: "Wedge Antilles"] make starfighter [ model: "Slayn & Korpil B-wing" pilot: "General Salm" ] ]

Adding new property here.

squadron/1: make squadron/1 [deathstar-removal-expert: yes]

print [crlf "Fighter squadron:"] foreach pilot squadron [probe pilot] </lang>


<lang Red>person: make object! [

 name: none
 age:  none


people: reduce [make person [name: "fred" age: 20] make person [name: "paul" age: 21]] people/1: make people/1 [skill: "fishing"]

foreach person people [

 print reduce [person/age "year old" person/name "is good at" any [select person 'skill "nothing"]]



We can add an attribute (or a group of attributes) to the object state using addattribute() function <lang ring>o1 = new point addattribute(o1,"x") addattribute(o1,"y") addattribute(o1,"z") see o1 {x=10 y=20 z=30} class point</lang>

x: 10.000000
y: 20.000000
z: 30.000000


<lang ruby>class Empty end

e = class << e

 attr_accessor :foo

end = 1 puts # output: "1"

f = = 1 # raises NoMethodError </lang>

"class << e" uses the singleton class of "e", which is an automatic subclass of Empty that has only this single instance. Therefore we added the "foo" accessor only to "e", not to other instances of Empty. Another way of adding a method to a singleton is: <lang ruby>yes_no = "Yes"

def yes_no.not

 replace( self=="Yes" ? "No": "Yes")


  1. Demo:

p yes_no.not # => "No" p yes_no.not # => "Yes" p "aaa".not # => undefined method `not' for "aaa":String (NoMethodError)</lang>


Works with: Scala version 2.10

Since version 2.10 Scala supports dynamic types. Dynamic types have to implement trait Dynamic and implement methods selectDynamic and updateDynamic.

<lang scala>import language.dynamics import scala.collection.mutable.HashMap

class A extends Dynamic {

 private val map = new HashMap[String, Any]
 def selectDynamic(name: String): Any = {
   return map(name)
 def updateDynamic(name:String)(value: Any) = {
   map(name) = value


Sample output in the REPL:

<lang scala>scala> val a = new A a: A = A@7b20f29d

scala> = 42 Any = 42

scala> res10: Any = 42</lang>


<lang ruby>class Empty{}; var e = Empty(); # create a new class instance e{:foo} = 42; # add variable 'foo' say e{:foo}; # print the value of 'foo'</lang>


Slate objects are prototypes: <lang slate>define: #Empty -> Cloneable clone. define: #e -> Empty clone. e addSlotNamed: #foo valued: 1.</lang>


the following addSlot function creates an anonymus class with the additional slot, defines accessor methods and clones a new instance from the given object which becomes the old one. This preserves object identity. (by the way: if we remember and reuse these temp classes, we get the core of Google's fast JavaScript interpreter implementation ;-)

Works with: Smalltalk/X

(should work with all Smalltalks, though)

<lang smalltalk>|addSlot p|

addSlot :=

 [:obj :slotName |
   |anonCls newObj|
   anonCls := obj class 
           subclass:(obj class name,'+') asSymbol 
           poolDictionaries: category:nil 
   anonCls compile:('%1 ^  %1' bindWith:slotName).
   anonCls compile:('%1:v %1 := v' bindWith:slotName).
   newObj := anonCls cloneFrom:obj.
   obj become:newObj.

create a 2D Point object, add a z slot, change and retrieve the z-value, finally inspect it (and see the slots). <lang smalltalk>p := Point x:10 y:20. addSlot value:p value:'z'. p z:30. p z. p z:40. p inspect</lang>

The above used a block to perform this operation in privacy. In a real world application, the addSlot code would be added as an extension to the Object class, as in. <lang smalltalk>!Object methodsFor:'adding slots'!

addSlot: slotName

   |anonCls newObj|
   anonCls := self class 
           subclass:(self class name,'+') asSymbol 
           poolDictionaries: category:nil 
   anonCls compile:('%1 ^  %1' bindWith:slotName).
   anonCls compile:('%1:v %1 := v' bindWith:slotName).
   newObj := anonCls cloneFrom:self.
   self become:newObj.</lang>

then, again create a 2D Point object, add a z slot, change and retrieve the z-value, finally inspect it (and see the slots). <lang smalltalk>p := Point x:10 y:20. p addSlot:'z'. p z:30. p z. p z:40. p inspect</lang>

This example is incorrect. Please fix the code and remove this message.

Details: It extends the class (adds a new instance var and new method that will exists even in brand new future instances of that class), not only the particular instance. -- The description of the problem must be reworded then, as it

asks for adding variables to the class, not the instance.

CG: no, you should read the code carefully: the original class remains unchanged, meaning that future instances of the class ("Point" in the above example) are not affected. Only the one affected instance gets a new class (which is anonymous, as it gets installed into the environment "nil", which means: "nowhere"). Actually, there is no way to get more new instances of that new instance's class because the class of it is only referenced by the changed p. (well, it could be done by using reflection, as with "p class new"). So, the above EXACTLY does what was asked for, whereas the example below is wrongly changing the original class.

<lang smalltalk>Object subclass: #Monkey

 instanceVariableNames: 'aVar'
 category: nil !

!Monkey class methodsFor: 'new instance'! new

 | o |
 o := super new.
 o init.


!Monkey methodsFor: 'init instance'! init

 aVar := 0

! initWith: value

 aVar := value


!Monkey methodsFor: 'set/get the inst var(s)'! setVar: var

 aVar := var

! getVar



"Create a new instance" Smalltalk at: #aMonkey put: (Monkey new) !

"set the 'original' instance var to 12" aMonkey setVar: 12 .

"let's see what's inside" aMonkey inspect .

"add a new instance var" Monkey addInstVarName: 'x'.

"let's see what's inside now" aMonkey inspect .

"let us create a new method for x" !Monkey methodsFor: 'about x'! setX: val

  x := val

! x



aMonkey setX: 10 . aMonkey inspect . (aMonkey x) printNl .</lang>

Output is:

An instance of Monkey
  aVar: 12
An instance of Monkey
  aVar: 12
  x: nil
An instance of Monkey
  aVar: 12
  x: 10


We can use the same associated object mechanism as in Objective-C:

<lang swift>import Foundation

let fooKey = UnsafeMutablePointer<UInt8>.alloc(1)

class MyClass { } let e = MyClass()

// set objc_setAssociatedObject(e, fooKey, 1, .OBJC_ASSOCIATION_RETAIN)

// get if let associatedObject = objc_getAssociatedObject(e, fooKey) {

 print("associated object: \(associatedObject)")

} else {

 print("no associated object")



Works with: Tcl version 8.6


Library: TclOO

The code below uses the fact that each object is implemented as a namespace, to add a time variable to an instance of summation: <lang Tcl>% package require TclOO % oo::class create summation {

  constructor {} {
      variable v 0
  method add x {
      variable v
      incr v $x
  method value Template:Var v {
      variable $var
      return [set $var]
  destructor {
      variable v
      puts "Ended with value $v"



% set s [summation new] % # Do the monkey patch! % set [info object namespace $s]::time now now % # Prove it's really part of the object... % $s value time now %</lang> An alternative approach is to expose the (normally hidden) varname method on the object so that you can get a handle for an arbitrary variable in the object. <lang tcl>% oo::class create summation {

  constructor {} {
      variable v 0
  method add x {
      variable v
      incr v $x
  method value Template:Var v {
      variable $var
      return [set $var]
  destructor {
      variable v
      puts "Ended with value $v"



% set s [summation new] % set s2 [summation new] % oo::objdefine $s export varname % # Do the monkey patch... % set [$s varname time] "now" % $s value time now % # Show that it is only in one object... % $s2 value time can't read "time": no such variable</lang>


Once created, class structure is fixed. However, using reflection, you can blow apart the class structure, add what ever and recompile the class (at run time). The REPL does this to store intermediate user results (defined classes, functions, variables, etc). It is ugly, slow and left as an exercise to the reader who cares.