What Are class modules, and how are they diffrrent from regular modules?

[dimava]

well... the question is in my title

Classes differ from standard modules in the way their data is stored. There's never more than one copy of a standard module’s data. This means that when one part of your program changes a public variable in a standard module, and another part of your program subsequently reads that variable, it will get the same value. Class module data, on the other hand, exists separately for each instance of the class (that is, for each object created from the class). By the same token, data in a standard module has program scope — that is, it exists for the life of your program — while class module data for each instance of a class exists only for the lifetime of the object; it’s created when the object is created, and destroyed when the object is destroyed.
Finally, variables declared Public in a standard module are visible from anywhere in your project, whereas Public variables in a class module can only be accessed if you have an object variable containing a reference to a particular instance of a class. All of the above are also true for public procedures in standard modules and class modules

[dimava]

thanks megatron

[yzfr1owner]

Another difference is that you can raise events from a class, but cannot from a module.

[dimava]

thanks

i suppose you could think of a class module as a User-Defined Type, which has its own functions and subroutines.

[Sam Finch]

I think megatron's highlighted the main differences but I would quite like to add a little more, the first question people usually have about class modules is "what's the point in having a module that you have to declare an instance of and can only get at its functions if you have an instance of it declares in the current scope?" which is a good question. the key point is that class modules are about mixing your data and your code together. let's have an example

Say you are working with angles and you and the user like to measure angles in degrees and minutes (a minute is a 60th of a degree) but VB works in radians (VBs trigonomic functions take their arguments in radians) , how do you solve this problem?

one way would be to have a standard module with functions that convert degreea and minutes to radians and vice versa and one for converting degrees and minutes into a string (so you can display it to the user) and one that converts radians into a degrees and minutes string etc. but this way you would be calling these functions all the time and your code would be absolutley cluttered with calls to these functions, not only is is hard to write but it soon becomes almost unreadable and very hard to debug.

no prizes for guessing that the solution is a class module.

so what we are going to do is have a class module that represents an angle, it stores the value of this angle and all it's functions work on this angle (ie they don't take the input parameters that the module would have to, instead of having a radians function which takes degrees and minutes as parameters the class has a radians property which returns/sets the value of the angle)

now by a mind blowing coincidence I happen to be working on a project that involves a class just like this one. Here's the code.

Option Explicit


Dim m_Degrees As Integer
Dim m_Minutes As Integer



Public Property Let Degrees(New_degrees As Integer)

'we have to do the mod function twice to catch -ve numbers
m_Degrees = ((New_degrees Mod 360) + 360) Mod 360
End Property

Public Property Get Degrees() As Integer
Degrees = m_Degrees
End Property


Public Property Let Minutes(New_Minutes As Integer)
m_Minutes = New_Minutes Mod 60
Me.Degrees = Me.Degrees + Fix(New_Minutes / 60)
End Property

Public Property Get Minutes() As Integer
Minutes = m_Minutes
End Property


'This is the Value of the Angle in Radians
'It is set as the Default Property.
'This allows us to add angles and perform trig operations on it directly
'It is hidden to keep syntax consistent
Public Property Let Radians(New_Radians As Double)
Me.Degrees = 0
Me.Minutes = (New_Radians * 10800 / PI)
End Property

Public Property Get Radians() As Double
Dim dblTemp As Double

dblTemp = Me.Degrees + (CDbl(Me.Minutes) / 60)

Radians = dblTemp * PI / 180

End Property


Public Property Get StringValue() As String
If Me.Minutes Then

    StringValue = Me.Degrees & "°" & Me.Minutes & "'"

Else

    StringValue = Me.Degrees & "°"
    
End If
End Property

(N.B. I have PI declared as a public constant in a standard module)

ok statrt a new project add a class module to it call it clsAngle and pase the code in

before you do anything else go to the tools menu, his procedure attributes, select radians from the combo box, hit advanced and turn on the user interface default check box

now you have an angle variable you can treat like a normal variable but it's much better, try some code like this to see what I mean

Dim alpha As New clsAngle 'NB, classes used as variables like this  have to be declares with the new keyword

Dim beta As New clsAngle

'set alpha to 60.5 degrees
alpha.Degrees = 60
alpha.Minutes = 30

'set beta to 29.5 degrees
beta.Degrees = 29
beta.minutes = 30


Msgbox "alpha = " & alpha.StringValue & ".  Sin(alpha) = " & Sin(alpha)

Msgbox "beta = " & beta.StringValue & ".  Sin(beta) = " & Sin(beta)


alpha = alpha + beta

Msgbox "Sin(" & alpha.StringValue & ") = " & Sin(alpha)

so we can treat an instance ofour class module like a variable (because we set the radians property as the user interface default we can automaticly access the radians property, which allows us to add the angles together and plug them into trigonomic formulae very easily)

so that's the difference between a class module and a standard module.ik

Ok if you have a table called Customer which is used over multiple projects, forms, modules etc. Rather than loading the table at each iteration, writing coding to test updates etc, create a customer class. Chuck all your update rules into, and you only have to define the table once.

So for example customer name is a required field. You would have a property called "name", and a method called "update"
which checks that name has a value

Therefore programmatically:

customer.name = txtname
customer.update

would handle it all for ya. Get into it, will save you alot of coding time.

Big advantage.....if there is a bug in a table update, it can only occur in one place.

[agent]

An ActiveX control without a user interface, and is created at runtime, not design time. (vb5)

Can you create an ActiveX control (or an intrsinc control) at runtime (and create the user interface on the form) in vb6?

[kedaman]

set youobject=createobject(classname)