Base Class and Derived Object Variants ~ LEARN MS-ACCESS TIPS AND TRICKS

Introduction.

Last week, we explored an example where a Base Class Object was passed through a Set Property Procedure, allowing it to become part of another object in memory. The passed object essentially became an extension or child object of the main object. In that earlier program, we passed the child object to the target object during the instantiation phase of our test program and then assigned values to the child object’s properties later in the code.

In the next example, we will take a slightly different approach.

For those who would like to go through the earlier Articles on the MS-Access Class Module, the links are given below:

This time, we will open both objects—ClsArea (the base class) and ClsVolume2 (the target class)—separately in our test program. We will assign values to the ClsArea base class properties before passing it to the ClsVolume2 target class object. Remember, the ClsVolume2 class has only one property, p_Height, and its Volume() method requires the Length and Width values from the base class ClsArea to calculate the volume.

Copy and paste the following sample Test Code into a Standard Module.

The SetNewVol2_2 Procedure.
```
Public Sub SetNewVol2_2()
'Method 2/2
Dim CA As ClsArea
Dim Vol As ClsVolume2

Set CA = New ClsArea
Set Vol = New ClsVolume2

CA.strDesc = "Bed Room"
CA.dblLength = 90
CA.dblWidth = 10
Stop


'Here ClsArea class Object CA is passed to the 
‘Property procedure Set CArea of ClsVolume2 object Vol
Set Vol.CArea = CA 'Pass ClsArea obj to ClsVolume2

Vol.dblHeight = 10 'assign height to ClsVolume2


Debug.Print "Description", "Length", "Width", "Area", "Height", "Volume"
With Vol.CArea
  Debug.Print .strDesc, .dblLength, .dblWidth, .Area(), Vol.dblHeight, Vol.Volume()
End With
Stop

Set CA = Nothing
Set Vol = Nothing

End Sub
```
VBA Code Review.
In the first Dim statement, CA is declared as a ClsArea object and Vol as a ClsVolume2 object. The next two statements instantiate these objects in memory.
The following three statements assign values to the properties of the ClsArea object.
A Stop statement is then used to pause code execution, allowing us to inspect the property values of the object in the Locals window.
Next, the statement Set Vol.CArea = CA assigns the ClsArea object (CA) as a child object of the ClsVolume2 object (Vol).
After that, the dblHeight property of the ClsVolume2 object is assigned the value 10.
The subsequent statements, placed before the next Stop statement, print the property values from memory to the Debug window.

Finally, the last two Set statements release both objects from memory before the program ends.
Display the Locals Window.
Inspecting the Locals Window
1. Open the Locals Window
  From the View menu in the VBA Editor, select Locals Window.
2. Run the Code
  - Click anywhere in the middle of the code window.
  - Press F5 to run the program until it pauses at the Stop statement.
  - Alternatively, press F8 to run the code step by step, which lets you observe the changes in the Locals Window at each step.
3. Expand the Objects
  Click the [+] symbol next to the object names in the Locals Window to expand and display their properties and current values.
4. Observe Object References
  - Check the CArea and p_Area object references under the Vol object.
  - At this point, their values will show as Nothing because we have not yet passed the CA object to the Vol object.
5. Continue Running the Code
  - After reviewing the Locals Window, run the code until it pauses at the next Stop statement.
  - Now, the CArea Set Property procedure assigns the p_Area object reference to the ClsArea object, linking it into the ClsVolume2 object.
Next, we will try another variation of this example using the same two classes — ClsArea and ClsVolume2 — to demonstrate a slightly different approach.

New Class Module ClsVolume3.

1. Insert a new Class Module and change its name Property Value to ClsVolume3.

2. Copy and Paste the following VBA Code into the ClsVolume3 Class Module:

Option Compare Database
Option Explicit
'Method three 
Private p_Height As Double
Public p_Area As ClsArea

Public Property Get dblHeight() As Double
    dblHeight = p_Height
End Property

Public Property Let dblHeight(ByVal dblNewValue As Double)
    p_Height = dblNewValue
End Property

Public Function Volume() As Double
    Volume = p_Area.dblLength * p_Area.dblWidth * Me.dblHeight
End Function

Private Sub Class_Initialize()
    Set p_Area = New ClsArea
End Sub

Private Sub Class_Terminate()
    Set p_Area = Nothing
End Sub

In the code, p_Height is declared as private property, while p_Area is declared as a public ClsArea object within the ClsVolume3 class. This means p_Area appears as a property of the ClsVolume3 class, with its own accessible properties and methods for direct Get/Let operations in the user program (from a standard module). Although the ClsArea object is exposed as a public property of ClsVolume3, its internal properties and methods remain encapsulated within the ClsArea class itself.

It is important to ensure that the ClsArea class is fully developed and free of errors before using it inside other classes.

The Class_Initialize() and Class_Terminate() routines handle the lifecycle of the embedded object: The ClsArea object is instantiated in Class_Initialize() when a ClsVolume3 object is created, and released from memory in Class_Terminate() When the user program ends.

The Testing Program.

The sample Test VBA Code is given below.

Copy and paste the code into the Standard Module.

Public Sub SNewVol3()
'Here ClsArea class is declared as a Public Property of ClsVolume3
Dim volm As ClsVolume3

Set volm = New ClsVolume3

volm.p_Area.strDesc = "Bed Room"
volm.p_Area.dblLength = 15 'assign length
volm.p_Area.dblWidth = 10 'assign width in clsArea
volm.dblHeight = 10 'assign height to ClsVolume2

Debug.Print "Description", "Length", "Width", "Area", "Height", "Volume"
With volm.p_Area
   Debug.Print .strDesc, .dblLength, .dblWidth, .Area, volm.dblHeight, volm.Volume
End With
Set volm = Nothing

End Sub

Display the Locals Window (View -> Locals Window), if it is not already open.

Click somewhere in the middle of the code and press F8 to execute the VBA Code one line at a time and watch the Local Window to track what happens at each step.

All the above variants of the ClsVolume Class have been written with less Code, except the first example of the ClsVolume Class.