Pass arguments to Constructor in VBA
Solution 1:
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.
Solution 2:
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.
Solution 3:
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 = "Jane" ' 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.