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Classes

Overview​

The 4D language supports the concept of classes. In a programming language, using a class allows you to define an object behaviour with associated properties and functions.

Once a user class is defined, you can instantiate objects of this class anywhere in your code. Each object is an instance of its class. A class can extend another class, and then inherits from its functions and properties (declared and computed).

The class model in 4D is similar to classes in JavaScript, and based on a chain of prototypes.

For example, you could create a Person class with the following definition:

//Class: Person.4dm
Class constructor($firstname : Text; $lastname : Text)
 This.firstName:=$firstname
 This.lastName:=$lastname

Function get fullName() -> $fullName : Text
 $fullName:=This.firstName+" "+This.lastName

Function sayHello() -> $welcome : Text

 $welcome:="Hello "+This.fullName

In a method, creating a "Person":

var $person : cs.Person //object of Person class  
var $hello : Text
$person:=cs.Person.new("John";"Doe")
// $person:{firstName: "John"; lastName: "Doe"; fullName: "John Doe"}
$hello:=$person.sayHello() //"Hello John Doe"

Managing classes​

Class definition​

A user class in 4D is defined by a specific method file (.4dm), stored in the /Project/Sources/Classes/ folder. The name of the file is the class name.

When naming classes, you should keep in mind the following rules:

  • A class name must be compliant with property naming rules.
  • Class names are case sensitive.
  • Giving the same name to a class and a database table is not recommended, in order to prevent any conflict.

For example, if you want to define a class named "Polygon", you need to create the following file:

Project folder
 Project
  Sources
   Classes
    Polygon.4dm

Deleting a class​

To delete an existing class, you can:

  • on your disk, remove the .4dm class file from the "Classes" folder,
  • in the 4D Explorer, select the class and click or choose Move to Trash from the contextual menu.

Using the 4D interface​

Class files are automatically stored at the appropriate location when created through the 4D interface, either via the File menu or the Explorer.

File menu and toolbar​

You can create a new class file for the project by selecting New > Class... in the 4D Developer File menu or from the toolbar.

You can also use the Ctrl+Shift+Alt+k shortcut.

Explorer​

In the Methods page of the Explorer, classes are grouped in the Classes category.

To create a new class, you can:

  • select the Classes category and click on the button.
  • select New Class... from the action menu at the bottom of the Explorer window, or from the contexual menu of the Classes group.
  • select New > Class... from the contexual menu of the Explorer's Home page.

Class code support​

In the various 4D windows (code editor, compiler, debugger, runtime explorer), class code is basically handled like a project method with some specificities:

  • In the code editor:
    • a class cannot be run
    • a class function is a code block
    • Goto definition on an object member searches for class Function declarations; for example, "$o.f()" will find "Function f".
    • Search references on class function declaration searches for the function used as object member; for example, "Function f" will find "$o.f()".
  • In the Runtime explorer and Debugger, class functions are displayed with the <ClassName> constructor or <ClassName>.<FunctionName> format.

Class stores​

Available classes are accessible from their class stores. Two class stores are available:

  • cs for user class store
  • 4D for built-in class store

cs​

cs : Object

ParameterTypeDescription
classStoreObject←User class store for the project or component

The cs command returns the user class store for the current project or component. It returns all user classes defined in the opened project or component. By default, only project ORDA classes are available.

Example​

You want to create a new instance of an object of myClass:

$instance:=cs.myClass.new()

4D​

4D : Object

ParameterTypeDescription
classStoreObject←4D class store

The 4D command returns the class store for available built-in 4D classes. It provides access to specific APIs such as CryptoKey.

Examples​

You want to create a new key in the CryptoKey class:

$key:=4D.CryptoKey.new(New object("type";"ECDSA";"curve";"prime256v1"))

You want to list 4D built-in classes:

 var $keys : collection
 $keys:=OB Keys(4D)
 ALERT("There are "+String($keys.length)+" built-in classes.")

Class object​

When a class is defined in the project, it is loaded in the 4D language environment. A class is an object itself, of "Class" class. A class object has the following properties and function:

In addition, a class object can reference a constructor object (optional).

A class object itself is a shared object and can therefore be accessed from different 4D processes simultaneously.

Inheritance​

If a class inherits from another class (i.e. the Class extends keyword is used in its definition), the parent class is its superclass.

When 4D does not find a function or a property in a class, it searches it in its superclass; if not found, 4D continues searching in the superclass of the superclass, and so on until there is no more superclass (all objects inherit from the "Object" superclass).

Class keywords​

Specific 4D keywords can be used in class definitions:

  • Function <Name> to define class functions of the objects.
  • Class constructor to initialize new objects of the class.
  • property to define static properties of the objects with a type.
  • Function get <Name> and Function set <Name> to define computed properties of the objects.
  • Class extends <ClassName> to define inheritance.
  • This and Super are commands that have special

Function​

Syntax​

{shared} Function <name>({$parameterName : type; ...}){->$parameterName : type}
// code

Class functions are specific properties of the class. They are objects of the 4D.Function class. In the class definition file, function declarations use the Function keyword followed by the function name.

If the function is declared in a shared class, you can use the shared keyword so that the function could be called without Use...End use structure. For more information, refer to the Shared functions paragraph below.

The function name must be compliant with object naming rules.

note

Since properties and functions share the same namespace, using the same name for a property and a function of the same class is not allowed (an error is thrown in this case).

tip

Starting the function name with an underscore character ("_") will exclude the function from the autocompletion features in the 4D code editor. For example, if you declare Function _myPrivateFunction in MyClass, it will not be proposed in the code editor when you type in "cs.MyClass. ".

Immediately following the function name, parameters for the function can be declared with an assigned name and data type, including the return parameter (optional). For example:

Function computeArea($width : Integer; $height : Integer)->$area : Integer

Within a class function, the This command is used as the object instance. For example:

Function setFullname($firstname : Text; $lastname : Text)
 This.firstName:=$firstname
 This.lastName:=$lastname

Function getFullname()->$fullname : Text
 $fullname:=This.firstName+" "+Uppercase(This.lastName)

For a class function, the Current method name command returns: <ClassName>.<FunctionName>, for example "MyClass.myFunction".

In the application code, class functions are called as member methods of the object instance and can receive parameters if any. The following syntaxes are supported:

  • use of the () operator. For example, myObject.methodName("hello")
  • use of a "4D.Function" class member method:
Thread-safety warning

If a class function is not thread-safe and called by a method with the "Can be run in preemptive process" attribute:

  • the compiler does not generate any error (which is different compared to regular methods),
  • an error is thrown by 4D only at runtime.

Parameters​

Function parameters are declared using the parameter name and the parameter type, separated by a colon. The parameter name must be compliant with property naming rules. Multiple parameters (and types) are separated by semicolons (;).

Function add($x; $y : Variant; $z : Integer; $xy : Object)
note

If the type is not stated, the parameter will be defined as Variant.

Return value​

You declare the return parameter (optional) by adding an arrow (->) and the return parameter definition after the input parameter(s) list, or a colon (:) and the return parameter type only. For example:

Function add($x : Variant; $y : Integer)->$result : Integer
 $result:=$x+$y

You can also declare the return parameter by adding only : type and use the return expression (it will also end the function execution). For example:

Function add($x : Variant; $y : Integer): Integer
 // some code
 return $x+$y

Example 1​

property name : Text
property height; width : Integer

// Class: Rectangle
Class constructor($width : Integer; $height : Integer)
 This.name:="Rectangle"
 This.height:=$height
 This.width:=$width

// Function definition
Function getArea()->$result : Integer
 $result:=(This.height)*(This.width)
// In a project method

var $rect : cs.Rectangle
var $area : Real

$rect:=cs.Rectangle.new(50;100)  
$area:=$rect.getArea() //5000

Example 2​

This example uses the return expression:

Function getRectArea($width : Integer; $height : Integer) : Integer
 If ($width > 0 && $height > 0)
  return $width * $height
 Else
  return 0
 End if

Class Constructor​

Syntax​

// Class: MyClass
{shared} {{session} singleton} Class Constructor({$parameterName : type; ...})
// code

A class constructor function accepts optional parameters and can be used to create and initialize objects of the user class.

When you call the new() function, the class constructor is called with the parameters optionally passed to the new() function.

There can only be one constructor function in a class (otherwise an error is returned). A constructor can use the Super keyword to call the constructor of the super class.

You can create and type instance properties inside the constructor (see example). Alternatively, if your instance properties' values do not depend on parameters passed to the constructor, you can define them using the property keyword.

Using the shared keyword creates a shared class, used to only instantiate shared objects. For more information, refer to the Shared classes paragraph.

Using the singleton keyword creates a singleton, used to create a single instance of the class. A session singleton creates a single instance per session. For more information, refer to the Singleton classes paragraph.

Example​

// Class: MyClass
// Class constructor of MyClass
Class Constructor ($name : Text ; $age : Integer)
 This.name:=$name
 This.age:=$age
// In a project method
// You can instantiate an object
var $o : cs.MyClass
$o:=cs.MyClass.new("John";42)  
// $o = {"name":"HelloWorld";"age":42}

property​

Syntax​

property <propertyName>{; <propertyName2>;...}{ : <propertyType>}

The property keyword can be used to declare a property inside a user class. A class property has a name and a type.

Declaring class properties enhances code editor suggestions, type-ahead features and error detection.

Properties are declared for new objects when you call the new() function, however they are not automatically added to objects (they are only added when they are assigned a value).

note

A property is automatically added to the object when it is inititalized in the declaration line.

Property names must be compliant with property naming rules.

note

Since properties and functions share the same namespace, using the same name for a property and a function of the same class is not allowed (an error is thrown in this case).

The property type can be one of the following supported types:

propertyTypeContents
TextText value
DateDate value
TimeTime value
BooleanBoolean value
IntegerLong integer value
RealReal value
PointerPointer value
PicturePicture value
BlobScalar Blob value
CollectionCollection value
VariantVariant value
ObjectObject with default class (4D.Object)
4D.<className>Object of the 4D class name
cs.<className>Object of the user class name
cs.<namespace>.<className>Object of the <namespace> component class name

If you omit the type in the declaration line, the property is created as a variant.

info

The property keyword can only be used in class methods and outside any Function or Class Constructor block.

Initializing the property in the declaration line​

When declaring a property, you have the flexibility to specify its data type and provide its value in one statement. The supported syntax is:

property <propertyName> { : <propertyType>} := <Propertyvalue>

note

When using this syntax, you cannot declare several properties in the declaration line.

You can omit the type in the declaration line, in which case the type will be inferred when possible. For example:

// Class: MyClass

property name : Text := "Smith"
property age : Integer := 42

property birthDate := !1988-09-29! //date is inferred
property fuzzy //variant

When you initialize a property in its declaration line, it is added to the class object after its instantiation with the new() function but before the constructor is called.

If a class extends another class, the properties of the parent class are instantiated before the properties of the child class.

note

If you initialize a property in its declaration line with an object or a collection in a shared class, the value is automatically transformed into a shared value:

// in a shared class
property myCollection := ["something"]
// myCollection will be a shared collection
// equivalent to:
myCollection := New shared collection("something")

Example​

// Class: MyClass

property name : Text
property age : Integer
property color : Text := "Blue"

In a method:

var $o : cs.MyClass
$o:=cs.MyClass.new() //$o:{"color" : "Blue"}
$o.name:="John" //$o:{"color" : "Blue"; "name" : "John"}
$o.age:="Smith"  //error with check syntax

Function get and Function set​

Syntax​

{shared} Function get <name>()->$result : type
// code
{shared} Function set <name>($parameterName : type)
// code

Function get and Function set are accessors defining computed properties in the class. A computed property is a named property with a data type that masks a calculation. When a computed property value is accessed, 4D substitutes the corresponding accessor's code:

  • when the property is read, the Function get is executed,
  • when the property is written, the Function set is executed.

If the property is not accessed, the code never executes.

Computed properties are designed to handle data that do not necessary need to be kept in memory. They are usually based upon persistent properties. For example, if a class object contains as persistent property the gross price and the VAT rate, the net price could be handled by a computed property.

In the class definition file, computed property declarations use the Function get (the getter) and Function set (the setter) keywords, followed by the name of the property. The name must be compliant with property naming rules.

Function get returns a value of the property type and Function set takes a parameter of the property type. Both arguments must comply with standard function parameters.

When both functions are defined, the computed property is read-write. If only a Function get is defined, the computed property is read-only. In this case, an error is returned if the code tries to modify the property. If only a Function set is defined, 4D returns undefined when the property is read.

If the functions are declared in a shared class, you can use the shared keyword with them so that they could be called without Use...End use structure. For more information, refer to the Shared functions paragraph below.

The type of the computed property is defined by the $return type declaration of the getter. It can be of any valid property type.

Assigning undefined to an object property clears its value while preserving its type. In order to do that, the Function get is first called to retrieve the value type, then the Function set is called with an empty value of that type.

Example 1​

//Class: Person.4dm
property firstName; lastName : Text

Class constructor($firstname : Text; $lastname : Text)
 This.firstName:=$firstname
 This.lastName:=$lastname

Function get fullName() -> $fullName : Text
 $fullName:=This.firstName+" "+This.lastName

Function set fullName( $fullName : Text )
 $p:=Position(" "; $fullName)
 This.firstName:=Substring($fullName; 1; $p-1)
 This.lastName:=Substring($fullName; $p+1)
//in a project method
$fullName:=$person.fullName // Function get fullName() is called
$person.fullName:="John Smith" // Function set fullName() is called

Example 2​

Function get fullAddress()->$result : Object

 $result:=New object

 $result.fullName:=This.fullName
 $result.address:=This.address
 $result.zipCode:=This.zipCode
 $result.city:=This.city
 $result.state:=This.state
 $result.country:=This.country

Class extends <ClassName>​

Syntax​

// Class: ChildClass
Class extends <ParentClass>

The Class extends keyword is used in class declaration to create a user class which is a child of another user class. The child class inherits all functions of the parent class.

Class extension must respect the following rules:

  • A user class cannot extend a built-in class (except 4D.Object and ORDA classes which are extended by default for user classes).
  • A user class cannot extend a user class from another project or component.
  • A user class cannot extend itself.
  • It is not possible to extend classes in a circular way (i.e. "a" extends "b" that extends "a").
  • It is not possible to define a shared user class extended from a non-shared user class.

Breaking such a rule is not detected by the code editor or the interpreter, only the compiler and check syntax will throw an error in this case.

An extended class can call the constructor of its parent class using the Super command.

Example​

This example creates a class called Square from a class called Polygon.

//Class: Square

//path: Classes/Square.4dm

Class extends Polygon

Class constructor ($side : Integer)

 // It calls the parent class's constructor with lengths
 // provided for the Polygon's width and height
 Super($side;$side)
 // In derived classes, Super must be called before you
 // can use 'This'
 This.name:="Square"



 Function getArea() -> $area : Integer
  $area:=This.height*This.width

Class function commands​

The following commands have specific features when they are used within class functions:

Super​

The Super command allows calls to the superclass, i.e. the parent class of the function. It can be called in the class constructor or in a class function code.

For more details, see the Super command description.

This​

The This command returns a reference to the currently processed object. In most cases, the value of This is determined by how a class function is called. Usually, This refers to the object the function was called on, as if the function were on the object.

Example:

//Class: ob

Function f() : Integer
 return This.a+This.b

Then you can write in a method:

$o:=cs.ob.new()
$o.a:=5
$o.b:=3
$val:=$o.f() //8

For more details, see the This command description.

Class commands​

Several commands of the 4D language allows you to handle class features.

OB Class​

OB Class ( object ) -> Object | Null​

OB Class returns the class of the object passed in parameter.

OB Instance of​

OB Instance of ( object ; class ) -> Boolean​

OB Instance of returns true if object belongs to class or to one of its inherited classes, and false otherwise.

Shared classes​

You can create shared classes. A shared class is a user class that instantiates a shared object when the new() function is called on the class. A shared class can only create shared objects.

Shared classes also support shared functions that can be called without Use...End use structures.

The .isShared property of Class objects allows to know if the class is shared.

info

Creating a shared class​

To create a shared class, add the shared keyword before the Class Constructor. For example:

	//shared class: Person
shared Class Constructor($firstname : Text; $lastname : Text)
 This.firstName:=$firstname
 This.lastName:=$lastname

//myMethod
var $person := cs.Person.new("John"; "Smith")
OB Is shared($person) // true
cs.Person.isShared //true

Shared functions​

If a function defined inside a shared class modifies objects of the class, it should call Use...End use structure to protect access to the shared objects. However, to simplify the code, you can define the function as shared so that it automatically triggers internal Use...End use when executed.

To create a shared function, add the shared keyword before the Function keyword in a shared class. For example:

	//shared class Foo
shared Class Constructor()
  This.variable:=1

shared Function Bar($value : Integer)
  This.variable:=$value //no need to call use/end use
note

If the shared function keyword is used in a non-shared user class, it is ignored.

Singleton classes​

A singleton class is a user class that only produces a single instance. For more information on the concept of singletons, please see the Wikipedia page about singletons.

Singletons types​

4D supports three types of singletons:

  • a process singleton has a unique instance for the process in which it is instantiated,
  • a shared singleton has a unique instance for all processes on the machine.
  • a session singleton is a shared singleton but with a unique instance for all processes in the session. Session singletons are shared within an entire session but vary between sessions. In the context of a client-server or a web application, session singletons make it possible to create and use a different instance for each session, and therefore for each user.

Singletons are useful to define values that need to be available from anywhere in an application, a session, or a process.

info

Singleton classes are not supported by ORDA-based classes.

The following table indicates the scope of a singleton instance depending on where it was created:

Singleton created onScope of process singletonScope of shared singletonScope of session singleton
4D single-userProcessApplicationApplication or Web/REST session
4D ServerProcess4D Server machineClient/server session or Web/REST session or Stored procedure session
4D remote modeProcess (note: singletons are not synchronized on the twin process)4D remote machine4D remote machine or Web/REST session

Once instantiated, a singleton class (and its singleton) exists as long as a reference to it exists somewhere in the application running on the machine.

Creating and using singletons​

You declare singleton classes by adding appropriate keyword(s) before the Class constructor:

  • To declare a (process) singleton class, write singleton Class Constructor().
  • To declare a shared singleton class, write shared singleton Class constructor().
  • To declare a session singleton class, write session singleton Class constructor().
note
  • Session singletons are automatically shared singletons (there's no need to use the shared keyword in the class constructor).
  • Singleton shared functions support onHttpGet keyword.

The class singleton is instantiated at the first call of the cs.<class>.me property. The instantiated class singleton is then always returned when the me property is used.

If you need to instantiate a singleton with parameters, you can also call the new() function. In this case, it is recommended to instantiate the singleton in some code executed at application startup.

The .isSingleton property of Class objects allows to know if the class is a singleton.

The .isSessionSingleton property of Class objects allows to know if the class is a session singleton.

Examples​

Process singleton​

	//class: ProcessTag
singleton Class Constructor()
 This.tag:=Random

To use the singleton:

	//in a process
var $mySingleton := cs.ProcessTag.me //First instantiation
	//$mySingleton.tag = 5425 for example  
...  
var $myOtherSingleton := cs.ProcessTag.me
	//$myOtherSingleton.tag = 5425

	//in another process
var $mySingleton := cs.ProcessTag.me //First instantiation
	//$mySingleton.tag = 14856 for example  
...  
var $myOtherSingleton := cs.ProcessTag.me  
	//$myOtherSingleton.tag = 14856

Shared singleton​

//Class VehicleFactory

property vehicleBuilt : Integer

shared singleton Class constructor()
  This.vehicleBuilt := 0 //Number of vehicles built by the factory

shared Function buildVehicle ($type : Text) -> $vehicle : cs.Vehicle

  Case of
    : $type="car"
      $vehicle:=cs.Car.new()
    : $type="truck"
      $vehicle:=cs.Truck.new()
    : $type="sport car"
      $vehicle:=cs.SportCar.new()
    : $type="motorbike"
      $vehicle:=cs.Motorbike.new()
  Else
    $vehicle:=cs.Car.new()
  End case
  This.vehicleBuilt+=1

You can then call the cs.VehicleFactory singleton to get a new vehicle from everywhere in the application on your machine with a single line:

$vehicle:=cs.VehicleFactory.me.buildVehicle("truck")

Since the buildVehicle() function modifies the cs.VehicleFactory singleton (by incrementing This.vehicleBuilt) you need to add the shared keyword to it.

Session singleton​

In an inventory application, you want to implement an item inventory using session singletons.

//class ItemInventory

property itemList : Collection:=[]

session singleton Class constructor()

shared function addItem($item:object)
    This.itemList.push($item)

By defining the ItemInventory class as a session singleton, you make sure that every session and therefore every user has their own inventory. Accessing the user's inventory is as simple as:

//in a user session
$myList := cs.ItemInventory.me.itemList
//current user's item list

See also​

Singletons in 4D (blog post)
Session Singletons (blog post).