How to use the Implements in Excel VBA
Here are some theoretical and practical contributions to the answers given, in case people arrive here who wonder what implements / interfaces are about.
As we know, VBA doesn't support inheritance, hence we might almost blindly use interfaces to implement common properties/behaviour across different classes.
Still, I think that it is useful to describe what the conceptual difference is between the two to see why it matters later on.
- Inheritance: defines an is-a relationship (a square is-a shape);
- Interfaces: define a must-do relationship (a typical example is the
drawable
interface that prescribes that drawable object must implement the methoddraw
). This means that classes originating from different root classes can implement common behaviour.
Inheritance means that a baseclass (some physical or conceptual archetype) is extended, whereas interfaces implement a set of properties/methods that define a certain behaviour.
As such, one would say that Shape
is a base class from which all other shapes inherit, one that may implement the drawable
interface to make all shapes drawable. This interface would be a contract that guarantees that every Shape has a draw
method, specifying how/where a shape should be drawn: a circle may - or may not - be drawn differently from a square.
class IDrawable:
'IDrawable interface, defining what methods drawable objects have access to
Public Function draw()
End Function
Since VBA doesn't support inheritance, we are automatically forced to opt for creating an interface IShape that guarantees certain properties/behaviour to be implemented by the generic shapes (square, circle, etc), rather than creating an abstract Shape baseclass from which we can extend.
class IShape:
'Get the area of a shape
Public Function getArea() As Double
End Function
The part where we get in trouble is when we want to make every Shape drawable.
Unfortunately, since IShape is an interface and not a base class in VBA, we cannot implement the drawable interface in the base class. It appears that VBA does not allow us to have one interface implement another; after having tested this, the compiler doesn't seem to provide the desired behaviour. In other words, we cannot implement IDrawable within IShape, and expect instances of IShape to be forced to implement IDrawable methods because of this.
We are forced to implement this interface to every generic shape class that implements the IShape interface, and luckily VBA allows multiple interfaces to be implemented.
class cSquare:
Option Explicit
Implements iShape
Implements IDrawable
Private pWidth As Double
Private pHeight As Double
Private pPositionX As Double
Private pPositionY As Double
Public Function iShape_getArea() As Double
getArea = pWidth * pHeight
End Function
Public Function IDrawable_draw()
debug.print "Draw square method"
End Function
'Getters and setters
The part that follows now is where the typical use / benefits of an interface come into play.
Let's start off our code by writing a factory that returns a new square. (This is just a workaround for our inability to send arguments directly to the constructor):
module mFactory:
Public Function createSquare(width, height, x, y) As cSquare
Dim square As New cSquare
square.width = width
square.height = height
square.positionX = x
square.positionY = y
Set createSquare = square
End Function
Our main code will use the factory to create a new Square:
Dim square As cSquare
Set square = mFactory.createSquare(5, 5, 0, 0)
When you look at the methods that you have at your disposal, you'll notice that you logically get access to all the methods that are defined on the cSquare class:
We'll see later on why this is relevant.
Now you should wonder what will happen if you really want to create a collection of drawable objects. Your app could happen to contain objects that aren't shapes, but that are yet drawable. Theoretically, nothing prevents you from having an IComputer interface that can be drawn (may be some clipart or whatever).
The reason why you might want to have a collection of drawable objects, is because you may want to render them in a loop at a certain point in the app lifecycle.
In this case I will write a decorator class that wraps a collection (we'll see why). class collDrawables:
Option Explicit
Private pSize As Integer
Private pDrawables As Collection
'constructor
Public Sub class_initialize()
Set pDrawables = New Collection
End Sub
'Adds a drawable to the collection
Public Sub add(cDrawable As IDrawable)
pDrawables.add cDrawable
'Increase collection size
pSize = pSize + 1
End Sub
The decorator allows you to add some convenience methods that native vba collections don't provide, but the actual point here is that the collection will only accept objects that are drawable (implement the IDrawable interface). If we would try to add an object that is not drawable, a type mismatch would be thrown (only drawable objects allowed!).
So we might want to loop over a collection of drawable objects to render them. Allowing a non-drawable object into the collection would result in a bug. A render loop could look like this:
Option Explicit
Public Sub app()
Dim obj As IDrawable
Dim square_1 As IDrawable
Dim square_2 As IDrawable
Dim computer As IDrawable
Dim person as cPerson 'Not drawable(!)
Dim collRender As New collDrawables
Set square_1 = mFactory.createSquare(5, 5, 0, 0)
Set square_2 = mFactory.createSquare(10, 5, 0, 0)
Set computer = mFactory.createComputer(20, 20)
collRender.add square_1
collRender.add square_2
collRender.add computer
'This is the loop, we are sure that all objects are drawable!
For Each obj In collRender.getDrawables
obj.draw
Next obj
End Sub
Note that the above code adds a lot of transparency: we declared the objects as IDrawable, which makes it transparent that the loop will never fail, since the draw method is available on all objects within the collection.
If we would try to add a Person to the collection, it would throw a type mismatch if this Person class did not implement the drawable interface.
But perhaps the most relevant reason why declaring an object as an interface is important, is because we only want to expose the methods that were defined in the interface, and not those public methods that were defined on the individual classes as we've seen before.
Dim square_1 As IDrawable
Not only are we certain that square_1 has a draw
method, but it also ensure that only methods defined by IDrawable get exposed.
For a square, the benefit of this might not be immediately clear, but let's have a look at an analogy from the Java collections framework that is much clearer.
Imagine that you have a generic interface called IList
that defines a set of methods applicable on different types of lists. Each type of list is a specific class that implements the IList interface, defining their own behaviour, and possibly adding more methods of their own on top.
We declare the list as follows:
dim myList as IList 'Declare as the interface!
set myList = new ArrayList 'Implements the interface of IList only, ArrayList allows random (index-based) access
In the above code, declaring the list as IList ensures that you won't use ArrayList-specific methods, but only methods that are prescribed by the interface. Imagine that you declared the list as follows:
dim myList as ArrayList 'We don't want this
You will have access to the public methods that are specifically defined on the ArrayList class. Sometimes this might be desired, but often we just want to take benefit of the internal class behaviour, and not defined by the class specific public methods.
The benefit becomes clear if we use this ArrayList 50 more times in our code, and suddenly we find out that we're better off using a LinkedList (which allows specific internal behaviour related to this type of List).
If we complied to the interface, we can change the line:
set myList = new ArrayList
to:
set myList = new LinkedList
and none of the other code will break as the interface makes sure that the contract is fulfilled, ie. only public methods defined on IList are used, so the different types of lists are swappable over time.
A final thing (perhaps lesser known behaviour in VBA) is that you can give an interface a default implementation
We can define an interface in the following way:
IDrawable:
Public Function draw()
Debug.Print "Draw interface method"
End Function
and a class that implements the draw method as well:
cSquare:
implements IDrawable
Public Function draw()
Debug.Print "Draw square method"
End Function
We can switch between the implementations the following way:
Dim square_1 As IDrawable
Set square_1 = New IDrawable
square_1.draw 'Draw interface method
Set square_1 = New cSquare
square_1.draw 'Draw square method
This is not possible if you declare the variable as cSquare.
I can't immediately think of a good example when this might be useful, but it is technically possible if you test it.
This is an esoteric OOP concept and there's a little more you need to do and understand to use a custom collection of shapes.
You may first want to go through this answer
to get a general understanding of classes and interfaces in VBA.
Follow the below instructions
First open Notepad and copy-paste the below code
VERSION 1.0 CLASS
BEGIN
MultiUse = -1
END
Attribute VB_Name = "ShapesCollection"
Attribute VB_GlobalNameSpace = False
Attribute VB_Creatable = False
Attribute VB_PredeclaredId = False
Attribute VB_Exposed = False
Option Explicit
Dim myCustomCollection As Collection
Private Sub Class_Initialize()
Set myCustomCollection = New Collection
End Sub
Public Sub Class_Terminate()
Set myCustomCollection = Nothing
End Sub
Public Sub Add(ByVal Item As Object)
myCustomCollection.Add Item
End Sub
Public Sub AddShapes(ParamArray arr() As Variant)
Dim v As Variant
For Each v In arr
myCustomCollection.Add v
Next
End Sub
Public Sub Remove(index As Variant)
myCustomCollection.Remove (index)
End Sub
Public Property Get Item(index As Long) As cShape
Set Item = myCustomCollection.Item(index)
End Property
Public Property Get Count() As Long
Count = myCustomCollection.Count
End Property
Public Property Get NewEnum() As IUnknown
Attribute NewEnum.VB_UserMemId = -4
Attribute NewEnum.VB_MemberFlags = "40"
Set NewEnum = myCustomCollection.[_NewEnum]
End Property
Save the file as ShapesCollection.cls
to your desktop.
Make sure you are saving it with the
*.cls
extension and notShapesCollection.cls.txt
Now open you Excel file, go to VBE ALT+F11 and right click in the Project Explorer
. Select Import File
from the drop-down menu and navigate to the file.
NB: You needed to save the code in a
.cls
file first and then import it because VBEditor does not allow you to use Attributes. The attributes allow you to specify the default member in the iteration and use the for each loop on custom collection classes
See more:
Now Insert 3 class modules. Rename accordingly and copy-paste the code
cShape this is your Interface
Public Function GetArea() As Double
End Function
Public Function GetInertiaX() As Double
End Function
Public Function GetInertiaY() As Double
End Function
Public Function ToString() As String
End Function
cCircle
Option Explicit
Implements cShape
Public Radius As Double
Public Function GetDiameter() As Double
GetDiameter = 2 * Radius
End Function
Public Function GetArea() As Double
GetArea = Application.WorksheetFunction.Pi() * (Radius ^ 2)
End Function
''Inertia around the X axis
Public Function GetInertiaX() As Double
GetInertiaX = Application.WorksheetFunction.Pi() / 4 * (Radius ^ 4)
End Function
''Inertia around the Y axis
''Ix = Iy in a circle, technically should use same function
Public Function GetInertiaY() As Double
GetInertiaY = Application.WorksheetFunction.Pi() / 4 * (Radius ^ 4)
End Function
Public Function ToString() As String
ToString = "This is a radius " & Radius & " circle."
End Function
'interface functions
Private Function cShape_getArea() As Double
cShape_getArea = GetArea
End Function
Private Function cShape_getInertiaX() As Double
cShape_getInertiaX = GetInertiaX
End Function
Private Function cShape_getInertiaY() As Double
cShape_getInertiaY = GetInertiaY
End Function
Private Function cShape_toString() As String
cShape_toString = ToString
End Function
cRectangle
Option Explicit
Implements cShape
Public Length As Double ''going to treat length as d
Public Width As Double ''going to treat width as b
Public Function GetArea() As Double
GetArea = Length * Width
End Function
Public Function GetInertiaX() As Double
GetInertiaX = (Width) * (Length ^ 3)
End Function
Public Function GetInertiaY() As Double
GetInertiaY = (Length) * (Width ^ 3)
End Function
Public Function ToString() As String
ToString = "This is a " & Width & " by " & Length & " rectangle."
End Function
' interface properties
Private Function cShape_getArea() As Double
cShape_getArea = GetArea
End Function
Private Function cShape_getInertiaX() As Double
cShape_getInertiaX = GetInertiaX
End Function
Private Function cShape_getInertiaY() As Double
cShape_getInertiaY = GetInertiaY
End Function
Private Function cShape_toString() As String
cShape_toString = ToString
End Function
You need to Insert
a standard Module
now and copy-paste the below code
Module1
Option Explicit
Sub Main()
Dim shapes As ShapesCollection
Set shapes = New ShapesCollection
AddShapesTo shapes
Dim iShape As cShape
For Each iShape In shapes
'If TypeOf iShape Is cCircle Then
Debug.Print iShape.ToString, "Area: " & iShape.GetArea, "InertiaX: " & iShape.GetInertiaX, "InertiaY:" & iShape.GetInertiaY
'End If
Next
End Sub
Private Sub AddShapesTo(ByRef shapes As ShapesCollection)
Dim c1 As New cCircle
c1.Radius = 10.5
Dim c2 As New cCircle
c2.Radius = 78.265
Dim r1 As New cRectangle
r1.Length = 80.87
r1.Width = 20.6
Dim r2 As New cRectangle
r2.Length = 12.14
r2.Width = 40.74
shapes.AddShapes c1, c2, r1, r2
End Sub
Run the Main
Sub and check out the results in the Immediate Window
CTRL+G
Comments and explanation:
In your ShapesCollection
class module there are 2 subs for adding items to the collection.
The first method Public Sub Add(ByVal Item As Object)
simply takes a class instance and adds it to the collection. You can use it in your Module1
like this
Dim c1 As New cCircle
shapes.Add c1
The Public Sub AddShapes(ParamArray arr() As Variant)
allows you to add multiple objects at the same time separating them by a ,
comma in the same exact way as the AddShapes()
Sub does.
It's quite a better design than adding each object separately, but it's up to you which one you are going to go for.
Notice how I have commented out some code in the loop
Dim iShape As cShape
For Each iShape In shapes
'If TypeOf iShape Is cCircle Then
Debug.Print iShape.ToString, "Area: " & iShape.GetArea, "InertiaX: " & iShape.GetInertiaX, "InertiaY:" & iShape.GetInertiaY
'End If
Next
If you remove comments from the 'If
and 'End If
lines you will be able to print only the cCircle
objects. This would be really useful if you could use delegates in VBA but you can't so I have shown you the other way to print only one type of objects. You can obviously modify the If
statement to suit your needs or simply print out all objects. Again, it is up to you how you are going to handle your data :)