1. Objects & Classes

An object in Gambas is a data structure that provides properties, variables, methods and events.

Objects are accessed "by reference", i.e. by using a pointer to it, i.e. by using a variable whose value is the address of the object data structure in memory.

You can see the address of an object by using the PRINT instruction: DIM aStr AS NEW String[] PRINT aStr (String[] 0x80dccf8)

The object data structure is described by a class.

1.1. Classes

So each Gambas object has a class that describes all its public properties, methods and events. This class is a Gambas object too, whose class is the class named Class.

A static class is a class whose all members are static (see below). In Gambas, a static class is also named a module.

A static class cannot be instanciated: it would create an object with no dynamic variable, which is useless.

A virtual class is an hidden pseudo-class that you cannot explicitely manipulate.

1.2. Properties, methods and variables

Properties and methods allow to manipulate the data structure.

A property, a method or a variable can be static:

• A static variable will be shared by all instances of the same class.
• A static property or method can only modify static variables.

Private symbols can only be used from the class inside. Public symbols can be used everywhere, provided you have a reference pointing at the object.

1.3. References

There is no garbage collector in Gambas. So each object has a reference counter that is incremented each time the object is referenced by any variable, array, collection or other object, and decremented when it is released.

This reference counter is zero at object creation, and when it becomes zero again after a reference release, the object is freed.

1.4. Invalid objects

An object can become invalid. Because, for example, it is linked to an internal object not managed by Gambas that was destroyed.

Trying to use an invalid object raises an error.

1.5. Special methods

Special methods are methods declared in classes, whose name begins with an underscore character, and that are called by the interpreter in the following situations:

• When an object is created.
• When an object is freed.
• When the object class is loaded.
• When the object class is unloaded.
• When using an object as if it is an array.
• When enumerating the object.
• When using an object as if it is a function.
• When trying to use an unknown object method or property.

2. Events & Observers

2.1. Events

Events are signals sent by an object when something happens on it.

If an object raises events, it will hold a reference on its observer, or parent object.

This observer is another object that implements event handlers. An event handler is just a public method that is called each time the event is raised.

By default, the observer is the current object where the newly instanciated object is declared.

To raise events, an object must have an event name. This event name is assigned at object instanciation, when using the NEW instruction and the AS keyword, and is the prefix of all event handler methods.

Example

DIM hButton AS Button

hButton = NEW Button(ME) AS "ButtonEventName"

If no event name is specified, then the object won't raise events.

2.2. Locking Objects

An object can be locked so that it stops raising events, and can be unlocked so that it raises them again. See the Object.Lock and Object.Unlock methods.

Some events can be cancelled by the event handler, by using the STOP EVENT instruction. The effect of this cancellation depends on the event.

An object is automatically locked during the execution of its constructor: he cannot send nor receive any events.

2.3. Observers

Observers are objects that allow you to intercept events raised by other objects. They "observe" them.

You can intercept events just before they have been raised, or just after.

For each intercepted event, the observer will raise an event with the same name and the same arguments.

By using STOP EVENT inside an observer event handler, you cancel the original event.

Example

PRIVATE $hButton as Button
PRIVATE $hObserver AS Observer

PUBLIC SUB Form_Load()
  $hButton = NEW Button(ME) AS "Button"
  $hObserver = NEW Observer(hButton) AS "Observer"
END

PUBLIC SUB Observer_Click()
  DEBUG "The button has been clicked. I cancel the event!"
  STOP EVENT
END

PUBLIC SUB Button_Click()
  DEBUG "You should not see me."
END

3. Inheritance

Inheritance is the way for a class to become a specialized version of another class.

3.1. What is inherited?

The class inherits from its parent every method, property, constant and event.

You must use the ME keyword to access the inherited elements from the class inside.

3.2. Which class can be a parent class?

You can inherited any class, even a native one!

For example, you can create a custom MyListBox class that inherits ListBox but allows to associate a tag with each list item.

Note that you cannot use INHERITS in a form class file, because forms already inherits the Form class.

The inheritance tree depth cannot be greater than 16. This is a constant hard-coded inside the Gambas interpreter.

3.3. Virtual dispatching

When calling a method or accessing a property from an object reference, Gambas always uses virtual dispatching.

It means that the real class of the object is always used, and not the type of the variable that references the object - As it was in Gambas 1.0.

3.4. Inheritance and constructor

Contrary to all the object language I know, each class in the inheritance hierarchy consumes the parameters passed to the constructor.

Let's suppose we have the following inheritance tree:

MyListBox ---inherits--> ListBox ---inherits---> Control

• Control._new() does not exist.
• ListBox._new() takes one parameter: the parent control.
• MyListBox._new() takes one parameter: a name - It is just an example.

• The first will be sent to MyListBox._new().
• The second to ListBox._new().

Then you will create a MyListBox control this way:

hMyListBox = NEW MyListBox("Name", hContainer)

So the order of arguments is the following:

• Mandatory arguments are consumed first, and then optional arguments if they are available.
• The arguments of elder classes are specified first.

In Gambas 2.0, the order or arguments were reversed!

For example, if you have the following inheritance:

MyForm --> Form --> Window

with the MyForm constructor being:

Sub _new(FirstArg As String, SecondArg as Integer, Optional ThirdArg As Integer)

Note: the Form constructor takes no argument, and the Window constructor takes an optional parent argument.

The signature of the final constructor will be:

New MyForm(FirstArg As String, SecondArg As Integer, Optional Parent As Control, Optional ThirdArg As Integer)

In a more general way, the order of arguments for a three level inheritance tree is:

• Mandatory arguments of the grand-parent constructor.
• Mandatory arguments of the parent constructor.
• Mandatory arguments of the final constructor.
• Optional arguments of the grand-parent constructor.
• Optional arguments of the parent constructor.
• Optional arguments of the final constructor.

3.5. Symbol Overriding

When a symbol is overriden, the signature of the symbol in the child class must be the same as the signature of the symbol in the parent class.

The rules are:

• A dynamic symbol must be overriden by a dynamic symbol, a static one by a static one.
• A method must be overriden by a method with exactly the same signature (same arguments datatypes, same return value datatype if any).
• A read/write property must be overriden by a read/write property with the same datatype.
• A read-only property must be overriden by a read-only property with the same datatype.
• A constant can be overridden by a constant with the same datatype.

4. Components

Gambas components are external shared libraries written in C, C++ or in Gambas that add new functions and/or classes to the Gambas interpreter.

Classes are grouped according to the component they come from.

4.1. Default internal component

The interpreter includes an internal component named gb that defines all standard classes of the language.

This component is always loaded by default, and can be considered as part of the language.

4.2. Symbol tables

Each component has its own private class symbol table, so that class names do not conflict.

4.3. Global symbol table

So that components can work together, there is a global symbol table, where all classes exported by components and all classes exported by the current project are stored.

If there is a name conflict in this global symbol table, the last loaded class overrides the previous loaded class with the same name, by using inheritance. In other words, the overriding class extends the overriden one.

This last feature can be used for:

• Extending an already declared class by adding new methods or properties to it. For example, the gb.qt Application class reimplements the gb Application class.
• Overriding the methods of an already declared class. For example, the gb.form.dialog component replaces most of the static methods of the Dialog class.

4.4. Project symbol table

Your project has its own private symbol, like any component, and can export any of its classes to the global symbol table by using the EXPORT keyword.

The project classes are loaded after all components. So your exported class can override any exported classes declared in any component.