Pass arguments to Constructor in VBA

How can you construct objects passing arguments directly to your own classes?

Something like this:

Dim this_employee as Employee
Set this_employee = new Employee(name:="Johnny", age:=69)

Not being able to do this is very annoying, and you end up with dirty solutions to work this around.

Answers


Here's a little trick I'm using lately and brings good results. I would like to share with those who have to fight often with VBA.

1.- Implement a public initiation subroutine in each of your custom classes. I call it InitiateProperties throughout all my classes. This method has to accept the arguments you would like to send to the constructor.

2.- Create a module called factory, and create a public function with the word "Create" plus the same name as the class, and the same incoming arguments as the constructor needs. This function has to instantiate your class, and call the initiation subroutine explained in point (1), passing the received arguments. Finally returned the instantiated and initiated method.

Example:

Let's say we have the custom class Employee. As the previous example, is has to be instantiated with name and age.

This is the InitiateProperties method. m_name and m_age are our private properties to be set.

Public Sub InitiateProperties(name as String, age as Integer)

    m_name = name
    m_age = age

End Sub

And now in the factory module:

Public Function CreateEmployee(name as String, age as Integer) as Employee

    Dim employee_obj As Employee
    Set employee_obj = new Employee

    employee_obj.InitiateProperties name:=name, age:=age
    set CreateEmployee = employee_obj

End Function

And finally when you want to instantiate an employee

Dim this_employee as Employee
Set this_employee = factory.CreateEmployee(name:="Johnny", age:=89)

Especially useful when you have several classes. Just place a function for each in the module factory and instantiate just by calling factory.CreateClassA(arguments), factory.CreateClassB(other_arguments), etc.

EDIT

As stenci pointed out, you can do the same thing with a terser syntax by avoiding to create a local variable in the constructor functions. For instance the CreateEmployee function could be written like this:

Public Function CreateEmployee(name as String, age as Integer) as Employee

    Set CreateEmployee = new Employee
    CreateEmployee.InitiateProperties name:=name, age:=age

End Function

Which is nicer.


I use one Factory module that contains one (or more) constructor per class which calls the Init member of each class.

For example a Point class:

Class Point
Private X, Y
Sub Init(X, Y)
  Me.X = X
  Me.Y = Y
End Sub

A Line class

Class Line
Private P1, P2
Sub Init(Optional P1, Optional P2, Optional X1, Optional X2, Optional Y1, Optional Y2)
  If P1 Is Nothing Then
    Set Me.P1 = NewPoint(X1, Y1)
    Set Me.P2 = NewPoint(X2, Y2)
  Else
    Set Me.P1 = P1
    Set Me.P2 = P2
  End If
End Sub

And a Factory module:

Module Factory
Function NewPoint(X, Y)
  Set NewPoint = New Point
  NewPoint.Init X, Y
End Function

Function NewLine(Optional P1, Optional P2, Optional X1, Optional X2, Optional Y1, Optional Y2)
  Set NewLine = New Line
  NewLine.Init P1, P2, X1, Y1, X2, Y2
End Function

Function NewLinePt(P1, P2)
  Set NewLinePt = New Line
  NewLinePt.Init P1:=P1, P2:=P2
End Function

Function NewLineXY(X1, Y1, X2, Y2)
  Set NewLineXY = New Line
  NewLineXY.Init X1:=X1, Y1:=Y1, X2:=X2, Y2:=Y2
End Function

One nice aspect of this approach is that makes it easy to use the factory functions inside expressions. For example it is possible to do something like:

D = Distance(NewPoint(10, 10), NewPoint(20, 20)

or:

D = NewPoint(10, 10).Distance(NewPoint(20, 20))

It's clean: the factory does very little and it does it consistently across all objects, just the creation and one Init call on each creator.

And it's fairly object oriented: the Init functions are defined inside the objects.

EDIT

I forgot to add that this allows me to create static methods. For example I can do something like (after making the parameters optional):

NewLine.DeleteAllLinesShorterThan 10

Unfortunately a new instance of the object is created every time, so any static variable will be lost after the execution. The collection of lines and any other static variable used in this pseudo-static method must be defined in a module.


When you export a class module and open the file in Notepad, you'll notice, near the top, a bunch of hidden attributes (the VBE doesn't display them, and doesn't expose functionality to tweak most of them either). One of them is VB_PredeclaredId:

Attribute VB_PredeclaredId = False

Set it to True, save, and re-import the module into your VBA project.

Classes with a PredeclaredId have a "global instance" that you get for free - exactly like UserForm modules (export a user form, you'll see its predeclaredId attribute is set to true).

A lot of people just happily use the predeclared instance to store state. That's wrong - it's like storing instance state in a static class!

Instead, you leverage that default instance to implement your factory method:

[Employee class]

'@PredeclaredId
Option Explicit

Private Type TEmployee
    Name As String
    Age As Integer
End Type

Private this As TEmployee

Public Function Create(ByVal emplName As String, ByVal emplAge As Integer) As Employee
    With New Employee
        .Name = emplName
        .Age = emplAge
        Set Create = .Self 'returns the newly created instance
    End With
End Function

Public Property Get Self() As Employee
    Set Self = Me
End Property

Public Property Get Name() As String
    Name = this.Name
End Property

Public Property Let Name(ByVal value As String)
    this.Name = value
End Property

Public Property Get Age() As String
    Age = this.Age
End Property

Public Property Let Age(ByVal value As String)
    this.Age = value
End Property

With that, you can do this:

Dim empl As Employee
Set empl = Employee.Create("Johnny", 69)

Employee.Create is working off the default instance, i.e. it's considered a member of the type, and invoked from the default instance only.

Problem is, this is also perfectly legal:

Dim emplFactory As New Employee
Dim empl As Employee
Set empl = emplFactory.Create("Johnny", 69)

And that sucks, because now you have a confusing API. You could use '@Description annotations / VB_Description attributes to document usage, but without Rubberduck there's nothing in the editor that shows you that information at the call sites.

Besides, the Property Let members are accessible, so your Employee instance is mutable:

empl.Name = "Booba" ' Johnny no more!

The trick is to make your class implement an interface that only exposes what needs to be exposed:

[IEmployee class]

Option Explicit

Public Property Get Name() As String : End Property
Public Property Get Age() As Integer : End Property

And now you make Employee implement IEmployee - the final class might look like this:

[Employee class]

'@PredeclaredId
Option Explicit
Implements IEmployee

Private Type TEmployee
    Name As String
    Age As Integer
End Type

Private this As TEmployee

Public Function Create(ByVal emplName As String, ByVal emplAge As Integer) As IEmployee
    With New Employee
        .Name = emplName
        .Age = emplAge
        Set Create = .Self 'returns the newly created instance
    End With
End Function

Public Property Get Self() As IEmployee
    Set Self = Me
End Property

Public Property Get Name() As String
    Name = this.Name
End Property

Public Property Let Name(ByVal value As String)
    this.Name = value
End Property

Public Property Get Age() As String
    Age = this.Age
End Property

Public Property Let Age(ByVal value As String)
    this.Age = value
End Property

Private Property Get IEmployee_Name() As String
    IEmployee_Name = Name
End Property

Private Property Get IEmployee_Age() As Integer
    IEmployee_Age = Age
End Property

Notice the Create method now returns the interface, and the interface doesn't expose the Property Let members? Now calling code can look like this:

Dim empl As IEmployee
Set empl = Employee.Create("Immutable", 42)

And since the client code is written against the interface, the only members empl exposes are the members defined by the IEmployee interface, which means it doesn't see the Create method, nor the Self getter, nor any of the Property Let mutators: so instead of working with the "concrete" Employee class, the rest of the code can work with the "abstract" IEmployee interface, and enjoy an immutable, polymorphic object.


Using the trick

Attribute VB_PredeclaredId = True

I found another more compact way:

Option Explicit
Option Base 0
Option Compare Binary

Private v_cBox As ComboBox

'
' Class creaor
Public Function New_(ByRef cBox As ComboBox) As ComboBoxExt_c
  If Me Is ComboBoxExt_c Then
    Set New_ = New ComboBoxExt_c
    Call New_.New_(cBox)
  Else
    Set v_cBox = cBox
  End If
End Function

As you can see the New_ constructor is called to both create and set the private members of the class (like init) only problem is, if called on the non-static instance it will re-initialize the private member. but that can be avoided by setting a flag.


Another approach

Say you create a class clsBitcoinPublicKey

In the class module create an ADDITIONAL subroutine, that acts as you would want the real constructor to behave. Below I have named it ConstructorAdjunct.

Public Sub ConstructorAdjunct(ByVal ...)

 ...

End Sub

From the calling module, you use an additional statement

Dim loPublicKey AS clsBitcoinPublicKey

Set loPublicKey = New clsBitcoinPublicKey

Call loPublicKey.ConstructorAdjunct(...)

The only penalty is the extra call, but the advantage is that you can keep everything in the class module, and debugging becomes easier.


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