v2 Objects

In its simplest form, an object stores a collection of values that can be accessed by a key, like a name or a list index. You can think of an object like a box that groups a bunch of variables together, where the variable names are the keys and the variable contents are the values. A collection of keys that map to values is called a key-value store. Commonly, key-value stores are described as being like dictionaries or phone books. In the case of a dictionary, the word you are looking up is the key and the definition of the word is the value. For a phone book, the name you are looking up is the key and the phone number is the value.

Most AutoHotkey v2 objects have two key-value stores, which is an upgrade from v1 where all objects had a single store. These two stores are the property store and the item store.

The property store holds values that assist with you writing your code, called properties. For example, the Length property will give you a value that shows how many items are in a list. The property store uses keys that you will hard-code in your script, like how you might hard code variable names in expressions. The property store is accessed by dot notation, for example: object.Key where Key is the literal text for the key name. The code MsgBox object.Length will check the property store of object for the Length property.

The item store holds keys and values, and is designed to hold data where the keys could be supplied by a variable. For example, if you are accessing items from the item store in a loop you might use the loop index variable A_Index for the key. The item store is accessed by bracket notation, for example: object["Key"] where "Key" is quoted text, a variable, or some other expression.

AutoHotkey v2 has many built-in object types. For now we'll focus on these three: Basic objects, Map objects, and Array objects.

Basic objects are the foundation for all other types of objects in AutoHotkey. They have very few properties defined, and do not define an item store.

Basic objects are created using either curly braces ({}), or by creating a new instance of the Object class (Object()).

Basic objects should be used when you need to store a collection of keys and values, where the keys do not change and will be hard coded into the script.

Arrays are based on basic objects, and are used to store a list of items, numbered (indexed) starting at 1.

Arrays are created using either square brackets ([]), or by creating a new instance of the Array class (Array()). Between the brackets, or the parentheses of the call to Array(), you can put a comma delimited list of items to save to the array's item store.

Arrays have a variety of built-in properties / methods that can be used to interact with the list of items.

Unlike in AutoHotkey v1, arrays are not sparse. In v1, you could specify any new index for an array and assign a value into it. However, in v2 you cannot specify new indexes and all indexes are contiguous to existing indexes. So while in v1 you could take an array with three items and assign a new item at index 54, in v2 you can only assign indexes between 1 and array.Length. If you want to extend the array, you must use the Push method.

Maps are based on basic objects, and are used to store unordered items where the keys can be text, numbers, other objects.

Maps are created by creating a new instance of the Map class (Map()). When creating an instance of the Map class, you can provide any amount of keys and values, in the form Map(Key1, Value1, Key2, Value2, Key3, Value3).

Maps have a variety of built-in properties / methods that can be used to interact with the set of items.

It's important to know that AutoHotkey imagines an object as being separate from any variables that may contain it. Every object has a secret identifying number that acts like a label on a box, called its "pointer". A variable that "contains" an object really just contains that pointer, so that whenever the variable is used AutoHotkey knows where in memory to find the box. Accessing an object whose pointer is contained by a variable is called "referencing" that object, and the object pointer itself is sometimes called "a reference".

The object pointer system is an implementation detail that you don't need to worry about most of the time, but there are a few common situations where it impacts script behavior. The biggest impact is that when you make a second variable B := A where A "contained" an object, you haven't actually copied the object but instead copied the object pointer. With two copies of the object pointer, you now have two ways to reference the same object. If you make changes to the object by referencing it from variable B, those changes will still be present if you reference it from variable A.

This behavior is also seen when passing objects as parameters to functions. When you pass an object as a parameter that function receives a copy of the object pointer rather than a copy of the object itself. If that function makes changes to the object those changes are reflected outside of the function as well.

In AutoHotkey v1, there were several global collections of names that were kept separate. There was a collection of command names, a collection of label names, a collection of function names, and a collection of variable names. AutoHotkey v2 has mostly merged these collections, folding them all into the just variable name collection. Label-based subroutines have been replaced in favor of functions. Commands have been replaced in favor of functions. And, critically, functions have been redesigned to all be stored inside global variables.

Allowing functions to be saved inside variables and passed around like data is known as having first-class functions. In AutoHotkey, it is achieved by using function objects, which are objects that can run code when you use the call syntax: name(). Both user-defined functions and built-in functions are implemented this way, with function definition syntax creating a global variable by the function's name to hold the function object.

Function objects come inside global read-only variables by default, but can be passed around just like any other object. As shown above, it's easy to put the function object into a different variable even if the new variable has a different name. Additionally, AutoHotkey allows you to skip defining the global read-only variable by defining some functions directly inside an expression:

By itself, this syntax is usually seen when defining OnEvent type callbacks. It allows you to skip defining a function that might only be called in one place:

CloseCallback() {
    MsgBox "You tried to close the GUI"
}
g := Gui()
g.OnEvent("Close", CloseCallback)
 
; Can be rewritten as:
 
g := Gui()
g.OnEvent("Close", () => MsgBox("You tried to close the GUI"))
 
; Or in v2.1:
 
g := Gui()
g.OnEvent("Close", () {
    MsgBox "You tried to close the GUI"
})

However, where things start to get really interesting is when you put function objects into other objects. Just like a function object can be stored inside a regular variable and then that variable becomes callable, a function object can be stored as an object property and then that property becomes callable. A callable property on an object is called a method.

When you call a function stored as an object property, AutoHotkey does a little trick with the parameter list. If you have MyObject with a property FunctionProperty that contains a function object, calling MyObject.FunctionProperty(1, 2, 3) will automatically translate into (roughly) Temp := MyObject.FunctionProperty then Temp(someObject, 1, 2, 3) where Temp(…) is a regular call to the function. You see, the object that contains the property is passed as a first parameter to the function.

AutoHotkey objects are *prototype* based, but AutoHotkey's docs don't really do a proper job of explaining what that means or how it works. Prototype-based Object-Oriented-Programming (OOP) is a way of arranging objects containing function objects so that the emergent behavior is similar to non-prototype OOP languages (think C++ or Java).

The first part of this arrangement was function objects being nested inside regular objects. The second part is prototyping. Prototyping is the generic term for allowing one object to borrow the properties of another object (the prototype object). In AutoHotkey, this is achieved using the "base" mechanism. By adding a base to your object, whenever you try to access a property on your object that does not exist AutoHotkey will then check the base object to see if it exists there instead.

AutoHotkey's class syntax is so-called sugar syntax. Sugar syntax is an easier to read and write shorthand for code that is too verbose to work with directly. The implication of calling class syntax as sugar syntax is that you can do almost everything that the class keyword does entirely without using it. There are a few minor exceptions that we will go over later.

Class syntax is used to simultaneously define two things: a "prototype object" and a "class object". A prototype object is used the *base* object for class instances. When you create an object like myObject := MyClass(), the value of myObject ends up looking something like myObject := {base: MyClass.Prototype}. The prototype object is the object that holds all the method functions that you can call on the class instance.

Remembering the fundamental of how functions stored in objects are called, it would mean that in this following example, when testMethod is called the value of this will be equal to myObject not MyClass.Prototype.

The "class object" created by class syntax starts pretty simple: an object with a Prototype field. But then AHK adds onto that with an "instance factory". Instance factory is a term that I don't think the AHK docs ever uses, but it really should because that's what it would be called in any sane language.

An instance factory is a function that creates instances of a class. An instance factory for an AHK class works something like this:

classFactory(someClass) {
    instance := {base: someClass.Prototype}
    instance.__Init()
    if HasMethod(instance, "__New") {
        instance.__New()
    }
    return instance
}

The instance factory gets put onto the class object as its "Call" method. With the class factory put onto the class object like this, you can create instances by calling the class object directly:

This code invokes "Call" automatically, like (MyClass.Call)(MyClass), which invokes classFactory and returns the instance object.

That's the vast majority of what class syntax does. In that last example, we manually created this class:

When defining a class, it allows you to specify static and non-static properties. You can do exactly the same with the manually written code. Static properties get added to the class object. Non-static properties get added to the instance by the __Init method called by the instance factory:

As mentioned previously, there are a few unique features of the class syntax that are not easily replicated.

The first is definition hoisting. Definition hoisting is the ability to define a class (or other construct) below the point where it will be referenced. This allows you to write a class definition at the bottom of your script, but still use it in the auto-execution section. Function definitions are also hoisted in this way.

The second difference is that the variable defined using class syntax to hold the class object is made read-only. If you define a class manually like any other object, that class name can be overwritten later. But if you define it with class syntax, trying to overwrite the global variable that holds the class object will result in the exception "This Class cannot be used as an output variable."