Add a variable to a class instance at runtime

From Rosetta Code
Add a variable to a class instance at runtime
You are encouraged to solve this task according to the task description, using any language you may know.

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

var object:Object = new Object(); = "bar";

Or by creating a dynamic class

public dynamic class Foo
// ...
var foo:Foo = new Foo(); = "zap";


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.

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;

Sample output:

I am Class with 1


Works with: AutoHotkey_L
e := {} := 1


It's not really intended that you should do this, but if you must you can:

      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


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.

( ( 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$("anotherMember contains:" (object..anotherMethod)$)


Object as originally created:
=(aMember=) (aMethod=.!(its.aMember)));
Object with additional member and method and with 'aMember' already set to some interesting value:
= (anotherMember=)
(aMember=A value)
Call both methods and output their return values.
aMember contains: A value
anotherMember contains: some other value


Works with: C# version 4.0
// ----------------------------------------------------------------------------------------------
// 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 >


# CoffeeScript is dynamic, just like the Javascript it compiles to.
# You can dynamically add attributes to objects.
# First create an object very simply.
e = {} = "bar"
e.yo = -> "baz"
console.log, e.yo()
# CS also has class syntax to instantiate objects, the details of which
# 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()

Common Lisp[edit]

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

Library: Closer to MOP
(defun augment-instance-with-slots (instance slots)
(change-class instance
(make-instance 'standard-class
:direct-superclasses (list (class-of instance))
:direct-slots slots)))


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))
#<#<STANDARD-CLASS NIL {1003AEE2C1}> {1003AEE271}>
Slots with :INSTANCE allocation:
BAR = 42
BAZ = 69
XENU = 666

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)


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:

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 3.4:

import extensions.
class Extender :: BaseExtender
object prop foo :: theField.
constructor new : anObject
theObject := anObject.
public program
var anObject := 234.
// extending an object with a field
anObject := Extender new(anObject).
anObject foo := "bar".
console printLine(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.

vect = [ 'alpha', 'beta', 'gamma' ]
vect.dump = function ()
for n in [0: self.len()]
> @"$(n): ", self[n]
vect += 'delta'

Output from the above:

0: alpha
1: beta
2: gamma
3: delta

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'.

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"


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.

CLASS Growable
DIM instructions AS STRING = "Sleep(1)"
DIM dummy AS INTEGER = EXECLINE(instructions, 1)
instructions = code
GOTO ExecCode
RETURN result
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()


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:

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
: main
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


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.

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 }


def a = new A()
a.y = 55
a.z = { println (new Date()); Thread.sleep 5000 }
println a.x(25)
println a.y
println a.q


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.

Empty := Object clone
e := Empty clone
e foo := 1

Icon and Unicon[edit]

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).

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
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.

   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


This kind of thing is fundamental to JavaScript, as it's a prototype-based language rather than a class-based one.

e = {}       // generic object = 1
e["bar"] = 2 // name specified at runtime


Julia does not allow direct modification of the data member variables of a type, though class methods (just Julia functions) can be added without difficulty. For special situations, such as when parsing an input file, where new data type names may be appropriate, this can be accommodated using a Dict as one of the class variables. For example, consider the below JSON input data for a program processing phone numbers, where the type of phone numbers for the person is unknown until run-time:

{"phoneNumbers": [
"type": "home",
"number": "212 555-1234"
"type": "office",
"number": "646 555-4567"
"type": "mobile",
"number": "123 456-7890"

Add the data into a class member that is declared as a Dict structure:

mutable struct Contact
person = Contact("Jane Doe", Dict())
person.phonenumber["home"] = "212 555-1234"


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:

// 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:

{"a":1} as $a | ($a + {"b":2}) as $a | $a
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:
$a|.c = 3
# or equivalently:
$a|.["c"] = 3


obj = script("MyClass").new()
-- "FOO"
-- add new property 'bar'
obj.setProp(#bar, "BAR")
-- "BAR"


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.

HAI 1.3
I HAS A name, I HAS A value
IM IN YR interface
I HAS A option, GIMMEH option
option, WTF?
object HAS A SRS name ITZ value, GTFO
VISIBLE object'Z SRS name
IM OUTTA YR interface

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.

% 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).
:- end_category.

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

| ?- foo::bar(X).
X = 1 ;
X = 2 ;
X = 3


empty = {} = 1

M2000 Interpreter[edit]

Adding y member to an object with a x member which made by a class alfa (a global function). We can make m as a copy of this new group (which is in a container, in a(3)). We can make a pointer to A(3) and handle the new member.

Module checkit {
class alfa {
\\ a class is a global function which return a group
Dim a(5)=alfa()
Print a(3).x=5
For a(3) {
group anyname { y=10}
\\ merge anyname to this (a(3))
Print a(3).y=10
Print Valid(a(2).y)=false
\\ make a copy of a(3) to m
Print m.y=20, a(3).y=10
\\ make a pointer to a(3) in n
Print n=>y=10
Print a(3).y=30
\\ now n points to a(2)
Print Valid(n=>y)=false ' y not exist in a(2)
Print n is a(2) ' true
\\ we don't have types for groups
Print valid(@n as m)=false ' n haven't all members of m
Print valid(@m as n)=true ' m have all members of n


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:

? f



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 <-.

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);
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.

#import <Foundation/Foundation.h>
#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:

#import <Foundation/Foundation.h>
#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:

% Given struct "test"
test = setfield (test, "c", 3);


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[edit]

This example traps unknown method calls, then sets or retrieves the values in an encapsulated directory object.

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[edit]

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.

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.

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.

%% 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

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


Works with: Perl version 5.x
package Empty;
# Constructor. Object is hash.
sub new { return bless {}, shift; }
package main;
# Object.
my $o = Empty->new;
# Set runtime variable (key => value).
$o->{'foo'} = 1;

Perl 6[edit]

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.

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
#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

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.

my $lue = 42 but role { has $.answer = "Life, the Universe, and Everything" }
say $lue; # 42
say $lue.answer; # Life, the Universe, and Everything

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:

augment class Int {
method answer { "Life, the Universe, and Everything" }
say 42.answer; # Life, the Universe, and Everything

This practice, though allowed, is considered to be Evil Action at a Distance.


class E {};
$e=new E();
$e->{"foo"} = 1; // using a runtime name
$x = "foo";
$e->$x = 1; // using a runtime name in a variable


In general, all instance variables in PicoLisp are dynamically created at runtime.

: (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


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

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";
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.

lib objectclass;
define :class foo;
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


PowerShell allows extending arbitrary object instances at runtime with the Add-Member cmdlet. The following example adds a property Title to an integer:

$x = 42 `
| Add-Member -PassThru `
NoteProperty `
Title `
"The answer to the question about life, the universe and everything"

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).


class empty(object):
e = empty()

If the variable (attribute) name is known at "compile" time (hard-coded): = 1

If the variable name is determined at runtime:

   setattr(e, name, value)

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:

class empty(object):
def __init__(this): = "whatever"
def patch_empty(obj):
def fn(self=obj):
obj.print_output = fn
e = empty()
# >>> whatever
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.


rebol [
Title: "Add Variables to Class at Runtime"

; 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
; variable.
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]


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

o1 = new point
see o1 {x=10 y=20 z=30}
class point
x: 10.000000
y: 20.000000
z: 30.000000


class Empty
e =
class << e
attr_accessor :foo
.foo = 1
puts # output: "1"
f = = 1 # raises NoMethodError

"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:

yes_no = "Yes"
def yes_no.not
replace( self=="Yes" ? "No": "Yes")
p yes_no.not # => "No"
p yes_no.not # => "Yes"
p "aaa".not # => undefined method `not' for "aaa":String (NoMethodError)


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.

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:

scala> val a = new A
a: A = A@7b20f29d
scala> = 42 Any = 42
res10: Any = 42


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


Slate objects are prototypes:

define: #Empty -> Cloneable clone.
define: #e -> Empty clone.
e addSlotNamed: #foo valued: 1.


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)
|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).

p := Point x:10 y:20.
addSlot value:p value:'z'.
p z:30.
p z.
p z:40.
p inspect

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.

!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.

then, again create a 2D Point object, add a z slot, change and retrieve the z-value, finally inspect it (and see the slots).

p := Point x:10 y:20.
p addSlot:'z'.
p z:30.
p z.
p z:40.
p inspect

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.
Object subclass: #Monkey
instanceVariableNames: 'aVar'
classVariableNames: ''
poolDictionaries: ''
category: nil !
!Monkey class methodsFor: 'new instance'!
| o |
o := super new.
o init.
!Monkey methodsFor: 'init instance'!
aVar := 0
initWith: value
aVar := value
!Monkey methodsFor: 'set/get the inst var(s)'!
setVar: var
aVar := var
"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
aMonkey setX: 10 .
aMonkey inspect .
(aMonkey x) printNl .

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:

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:

% package require TclOO
% oo::class create summation {
constructor {} {
variable v 0
method add x {
variable v
incr v $x
method value {{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
% # Prove it's really part of the object...
% $s value time

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.

% oo::class create summation {
constructor {} {
variable v 0
method add x {
variable v
incr v $x
method value {{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
% # Show that it is only in one object...
% $s2 value time
can't read "time": no such variable


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.