New Syntax in twinBASIC: Part 2

Let's cover some more new syntax in twinBASIC, including parameterized class constructors, method overloading, and return syntax for functions.

New Syntax in twinBASIC: Part 2

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New Syntax in twinBASIC

twinBASIC introduces several new language features beyond simple VB6 compatibility.  Of course, while this syntax and these features are new compared to VB6, most of them are found in VB6's second cousin, twice-removed, VB.NET.

In Part 1, I covered the following new syntax:

  • Short-circuiting boolean operators, AndAlso and OrElse
  • The ternary operator, If()
  • Statements to skip to the next loop operation, Continue For, et al.

In this article, I'll cover the following new syntax:

  • Parameterized class constructors
  • Method overloading
  • Return syntax for functions

Parameterized class constructors

One of the features I'm most excited about in twinBASIC is its support for parameterized class constructors.  This feature is sorely lacking from VBA, requiring a workaround in the form of factory functions.

The VBA approach

Here's an example of how I handle this situation in VBA using a factory function:

'VBA: the oVehicle class module
Option Compare Database
Option Explicit

Public Make As String
Public Model As String
Public Year As Integer
The oVehicle class
Function NewVehicleObject(Make As String, _
                          Model As String, _
                          Year As Integer) As oVehicle
    Set NewVehicleObject = New oVehicle
    With NewVehicleObject
        .Make = Make
        .Model = Model
        .Year = Year
    End With
End Function
The NewVehicleObject factory function
Sub TestDealerLot()
    Dim Lot As New Collection  'of oVehicle objects
    Lot.Add NewVehicleObject("Ford", "Mustang", 1968)
    Lot.Add NewVehicleObject("Dodge", "Challenger", 1970)
    Lot.Add NewVehicleObject("Chevy", "Camaro", 1969)
    Dim Car As Object  'oVehicle
    For Each Car In Lot
        Debug.Print Car.Make, Car.Model, Car.Year
    Next Car
End Sub
Testing the class and factory function approach in VBA
Results of running the TestDealerLot routine

The twinBASIC approach

Now, let's take advantage of twinBASIC's parameterized constructor feature to rewrite the above code.  In addition to the familiar Class_Initialize() method, we can now create public methods named New() and pass zero or more variables to those methods:

Class oVehicle
    Public Make As String
    Public Model As String
    Public Year As Integer
    Public Sub New()
    End Sub
    Public Sub New(Make As String, _
                        Model As String, _
                        Year As Integer)
        Me.Make = Make
        Me.Model = Model
        Me.Year = Year
    End Sub
End Class

Module TestParamCtors
    Sub TestDealerLot()
        Dim Lot As New Collection  'of oVehicle objects
        Dim Vehicle As New oVehicle

        Set Vehicle = New oVehicle("Ford", "Mustang", 1968): Lot.Add Vehicle
        Set Vehicle = New oVehicle("Dodge", "Challenger", 1970): Lot.Add Vehicle
        Set Vehicle = New oVehicle("Chevy", "Camaro", 1969): Lot.Add Vehicle
        Dim Car As Object  'oVehicle
        For Each Car In Lot
            'These two lines of code are required while twinBASIC is in preview
            Dim Auto As oVehicle
            Set Auto = Car
            Debug.Print Auto.Make & " " & Auto.Model & " " & Auto.Year
        Next Car
    End Sub
End Module
Parameterized constructors!

Also, notice in the code below how the semantic highlighting makes it clear which Make/Model/Year values go together:

Method overloading

Method overloading allows us to declare the same routine name more than once in the same scope but with a different number and/or type of parameters.  How might this be useful?  One common scenario is looking up a database record by autonumber ID or by a unique key field.

For example:

We can use a single function name to look up a tax amount by taxpayer last name or bill ID.

Return syntax for functions

Returning a value from a function by assigning that value to an identifier with the same name as the function felt awkward to me when I first started writing VBA.  I think using the Return syntax makes code read more like plain English, which is always a good thing.

Keep in mind, though, that using the Return syntax is like including an implicit Exit Function call immediately after the Return line.  Thus, any cleanup code you might have in place would need to be called before the Return call.

Remember that the function exits immediately upon executing the Return line.

There is one last point about the new return syntax.  In order to maintain full backwards compatibility with VB6/VBA, you cannot use this new syntax within a procedure that contains GoSub subroutines.  (There's no reason to use those, anyway, so this is not much of a limitation.)

CORRECTION [2021-05-27]: Updated sample code for twinBASIC constructors from Function to Sub.  Constructors must be Sub's.

All original code samples by Mike Wolfe are licensed under CC BY 4.0