Add a variable to a class instance at runtime: Difference between revisions

Demonstrate how to dynamically add variables to an object (a class instance) at runtime.

=={{header|ActionScript}}==

In ActionScript this can be done using an Object object

object.foo = "bar";</syntaxhighlight>

Or by creating a dynamic class

{

public dynamic class Foo

}

}</syntaxhighlight>

foo.bar = "zap";</syntaxhighlight>

=={{header|Ada}}==

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.

procedure Dynamic is

=={{header|Arturo}}==

myInstance: to :myClass ["John" "Doe"]

=={{header|AutoHotkey}}==

{{works with|AutoHotkey_L}}

e.foo := 1 </syntaxhighlight>

{{works with|BBC BASIC for Windows}}

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

REM Create a base class with no members:

=={{header|Bracmat}}==

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.

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

)

&);</syntaxhighlight>

Output:

(object=

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

=={{header|C sharp|C#}}==

{{works with|C sharp|C#|4.0}}

//

// Program.cs - DynamicClassVariable

=={{header|CoffeeScript}}==

# You can dynamically add attributes to objects.

{{libheader|Closer to MOP}}

(change-class instance

(make-instance 'standard-class

Example:

(new-slots '((:name xenu :initargs (:xenu)))))

(augment-instance-with-slots instance new-slots)

=={{header|D}}==

private T[string] vars;

Hello 11 ['x':2, 'y':4]</pre>

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:

struct Dyn {

ELENA 4.x:

class Extender : BaseExtender

'''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.dump = function ()

for n in [0: self.len()]

vect.dump()</syntaxhighlight>

Output from the above:

1: beta

2: gamma

'''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'.

self['prop'] -= value

return self.prop

=={{header|FBSL}}==

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

Needs the FMS-SI (single inheritance) library code located here:

http://soton.mpeforth.com/flag/fms/index.html

include FMS-SILib.f

=={{header|FreeBASIC}}==

' Class ... End Class

' Esta característica aún no está implementada en el compilador.

However, as in the case of Groovy and Kotlin, we can ''make it appear'' as though fields are being added at runtime by using the built-in map type. For example:

import (

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.

final x = { it + 25 }

private map = new HashMap()

Test:

a.y = 55

a.z = { println (new Date()); Thread.sleep 5000 }

=={{header|Icon}} and {{header|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).

link ximage

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

e := Empty clone

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

V__C=: 0 NB. ensure the class exists

OBJ1=:conew 'exampleclass' NB. create an instance of our class

=={{header|JavaScript}}==

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

e.foo = 1

e["bar"] = 2 // name specified at runtime</syntaxhighlight>

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:

</syntaxhighlight>Thus the value of $a has undergone the desired transition, that is, its final value is {"a":1, "b":2}.

# or equivalently:

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

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": [

{

</syntaxhighlight>

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

mutable struct Contact

name::String

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:

class SomeClass {

Latitude is prototype-oriented, so adding slots at runtime is very straightforward and common.

;; Name known at compile-time.

=={{header|Lingo}}==

put obj.foo

The following example uses a prototype for simplicity.

% we start by defining an empty object

:- object(foo).

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

| ?- foo::bar(X).

X = 1 ;

=={{header|LOLCODE}}==

<tt>BUKKIT</tt>s (the all-purpose container type) can be added to at any point during execution, and the <tt>SRS</tt> 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.

I HAS A object ITZ A BUKKIT

=={{header|Lua}}==

empty.foo = 1</syntaxhighlight>

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 {

=={{header|Mathematica}}/{{header|Wolfram Language}}==

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[a]=1;

f[b]=2;

If the name of the variable to add is known at compile time, then this is just standard class construction:

empty.foo = 1</syntaxhighlight>

If the name of the variable to add is itself in a variable, then instead of dot syntax, use normal indexing:

varName = "foo"

empty[varName] = 1</syntaxhighlight>

=={{header|Morfa}}==

To emulate adding a variable to a class instance, Morfa uses user-defined operators <tt>`</tt> and <tt>&lt;-</tt>.

import morfa.base;

=={{header|Nim}}==

{.experimental: "dotOperators".}

template `.=`(js: JsonNode, field: untyped, value: untyped) =

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 <code>objc_getAssociatedObject()</code> and <code>objc_setAssociatedObject</code> to get and set them, respectively.

#import <objc/runtime.h>

You can also use a selector as the key, since two selectors with the same content are guaranteed to be equal:

#import <objc/runtime.h>

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

% Given struct "test"

test.b=1;

===Unknown Method Access===

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

d = .dynamicvar~new

d~foo = 123

===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.

d = .dynamicvar~new

d~foo = 123

=={{header|OxygenBasic}}==

Simple implementation for making runtime members - supports integer, float and string types.

'=================

class fleximembers

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)

=={{header|Perl}}==

{{works with|Perl|5.x}}

# Constructor. Object is hash.

Attempting to fetch/store "jelly" on a non-dynamic class would trigger a fatal error, unless said field had been explictly defined.

Needs 0.8.1+

<span style="color: #008080;">class</span> <span style="color: #000000;">wobbly</span> <span style="color: #000000;">dynamic</span>

<span style="color: #000080;font-style:italic;">-- (pre-define a few fields/methods if you like)</span>

=={{header|PHP}}==

$e=new E();

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

runtime.

-> $385605941

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

=={{header|Pike}}==

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

{

mapping variables = ([]);

it using the 'pop11_compile' procedure.

define :class foo;

=={{header|PowerShell}}==

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

| Add-Member -PassThru `

NoteProperty `

=={{header|Python}}==

pass

e = empty()</syntaxhighlight>

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

If the variable name is determined at runtime:

'''Note:''' Somewhat counter-intuitively one cannot simply use ''e = object(); e.foo = 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:

def __init__(this):

this.foo = "whatever"

{{works with|Rakudo|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.

my $object = Bar.new; # new instance

say $that.foo; # 5 - value from the cloned object</syntaxhighlight>

That's what's going on underneath, but often people just mix in an anonymous role directly using the <tt>but</tt> operator. Here we'll mix an attribute into a normal integer.

say $lue; # 42

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" }

=={{header|REBOL}}==

REBOL [

Title: "Add Variables to Class at Runtime"

=={{header|Red}}==

name: none

age: none

=={{header|Ring}}==

We can add an attribute (or a group of attributes) to the object state using addattribute() function

addattribute(o1,"x")

addattribute(o1,"y")

=={{header|Ruby}}==

end

"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:

def yes_no.not

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

import scala.collection.mutable.HashMap

Sample output in the REPL:

a: A = A@7b20f29d

=={{header|Sidef}}==

var e = Empty(); # create a new class instance

e{:foo} = 42; # add variable 'foo'

=={{header|Slate}}==

Slate objects are prototypes:

define: #e -> Empty clone.

e addSlotNamed: #foo valued: 1.</syntaxhighlight>

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 :=

].</syntaxhighlight>

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

addSlot value:p value:'z'.

p z:30.

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.

addSlot: slotName

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 addSlot:'z'. "instance specific added slot"

p z:30.

CG: commented the bad example; maybe the original author wants to fix it / comment on it.

instanceVariableNames: 'aVar'

classVariableNames: ''

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

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

The code below uses the fact that each object is implemented as a namespace, to add a ''time'' variable to an instance of ''summation'':

% oo::class create summation {

constructor {} {

%</syntaxhighlight>

An alternative approach is to expose the (normally hidden) <code>varname</code> method on the object so that you can get a handle for an arbitrary variable in the object.

constructor {} {

variable v 0

=={{header|Wren}}==

Although Wren is dynamically typed, it is not possible to add new variables (or fields as we prefer to call them) to a class at run time. We therefore follow the example of some of the other languages here and use a map field instead.

class Birds {

=={{header|XBS}}==

Object.Hello = "World";

log(Object.Hello);</syntaxhighlight>

=={{header|zkl}}==

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.

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{{omit from|Brainf***}}

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