I have a custom class module in VBA (Access) that is supposed to handle a large amount of external data. Currently I have two functions Read(name) and Write(name, value) that allows to read and set dynamic properties.
Is there a way to define a more syntactic way to read and write those data? I know that some objects in VBA have a special way of accessing data, for example the RecordSet, which allows to read and set data using myRS!property_name. Is there a way to do exactly the same for custom class modules?
The exclamation mark syntax is used to access members of a Scripting.Dictionary instance(you'll need to add a reference to Microsoft Scripting Runtime through Tools > References first). To use this syntaxyou'll need to be storing the information internally in a dictionary.
The quickest way to use it in a class is to give your class an object variable of type Scripting.Dictionary and set it up as follows:
Option Explicit
Dim d As Scripting.Dictionary
Private Sub Class_Initialize()
Set d = New Scripting.Dictionary
End Sub
Private Sub Class_Terminate()
Set d = Nothing
End Sub
Public Property Get IntData() As Scripting.Dictionary
Set IntData = d
End Property
Now you can access properties using myinstance.IntData!MyProperty = 1... but to get to where you want to be you need to use Charlie Pearson's technique for making IntData the default member for your class.
Once that's done, you can use the following syntax:
Dim m As MyClass
Set m = New MyClass
Debug.Print "Age = " & m!Age ' prints: Age =
m!Age = 27
Debug.Print "Age = " & m!Age ' prints: Age = 27
Set m = Nothing
Okay, thanks to Alain and KyleNZ I have now found a working way to do this, without having a collection or enumerable object below.
Basically, thanks to the name of the ! operator, I found out, that access via the bang/pling operator is equivalent to accessing the default member of an object. If the property Value is the default member of my class module, then there are three equivalent statements to access that property:
obj.Value("param")
obj("param")
obj!param
So to make a short syntax working for a custom class module, all one has to do is to define a default member. For example, I now used the following Value property:
Property Get Value(name As String) As String
Value = SomeLookupInMyXMLDocument(name)
End Property
Property Let Value(name As String, val As String) As String
SetSomeNodeValueInMyXMLDocument(name, val)
End Property
Normally, you could now access that like this:
obj.Value("foo") = "New value"
MsgBox obj.Value("foo")
Now to make that property the default member, you have to add a line to the Property definition:
Attribute Value.VB_UserMemId = 0
So, I end up with this:
Property Get Value(name As String) As String
Attribute Value.VB_UserMemId = 0
Value = SomeLookupInMyXMLDocument(name)
End Property
Property Let Value(name As String, val As String) As String
Attribute Value.VB_UserMemId = 0
SetSomeNodeValueInMyXMLDocument(name, val)
End Property
And after that, this works and equivalent to the code shown above:
obj("foo") = "New value"
MsgBox obj("foo")
' As well as
obj!foo = "New value"
MsgBox obj!foo
' Or for more complex `name` entries (i.e. with invalid identifier symbols)
obj![foo] = "New value"
MsgBox obj![foo]
Note that you have to add the Attribute Value.VB_UserMemId = 0 in some other editor than the VBA editor that ships with Microsoft Office, as that one hides Attribute directives for some reason.. You can easily export the module, open it in notepad, add the directives, and import it back in the VBA editor. As long as you don't change too much with the default member, the directive should not be removed (just make sure you check from time to time in an external editor).
See this other question: Bang Notation and Dot Notation in VBA and MS-Access
The bang operator (!) is shorthand for
accessing members of a Collection or
other enumerable object
If you make your class extend the Collection class in VBA then you should be able to take advantage of those operators. In the following question is an example of a user who extended the collection class:
Extend Collections Class VBA
Related
I have already asked a question here where I basically require an instance of a base class to be converted into a subclass (or a new instance of the subclass to be created using the instance of the base class' properties). The conclusion seems to be that the best way to do this is to manually assign every property I need to transfer in the constructor of the base class.
While this is feasible in some cases, it certainly is not when there are many properties to transfer, or when the base class is subject to change — every time you add a property to the base class, the constructor needs to be changed too, so this solutions is inelegant.
I have searched online, and can't see any reason for why this kind of type-casting isn't implemented. The arguments I have seen so far describe this operation to 'not make any sense' (making a minivan from a car was an analogy I saw), question what to do about the non-inherited variables in the subclass, or claim that there must be some better solution for what was trying to be achieved.
As far as I can see, the operation doesn't need to 'make sense' as long as it's useful, so that isn't much of a good reason. What's wrong with adding a few more properties (and perhaps methods/overriding them) to change an instance into a subclass? In the case of the non-inherited variables, that can simply be solved by allowing this kind of type-cast only a constructor is added to the subclass or by just simply setting them to their default values. After all, constructors usually call MyBase.New(...) anyway. What's the difference between using the constructor of the base (essentially creating a new instance of the base) and using an instance which is already initialised? Lastly, I don't think the third argument is well-justified — there are times when all of the other solutions are inelegant.
So finally, is there any other reason for why this kind of casting isn't allowed, and is there an elegant way to circumvent this?
Edit:
Since I don't know a lot about this topic, I think I meant to say 'convert' rather than 'cast'. I'll also add an example to show what I'm trying to succeed. The conversion would only be allowed at the initialisation of the Subclass:
Class BaseClass
Dim x as Integer
Dim y as Integer
End Class
Class Subclass1 : Inherits BaseClass
Dim z as Integer
Sub New(Byval value As Integer)
'Standard initialisation method
MyBase.New()
z = value
End Sub
Sub New(Byval value As Integer, Byval baseInstance As BaseClass)
'Type conversion from base class to subclass
baseInstance.passAllproperties()
'This assigns all properties of baseInstance belonging to BaseClass to Me.
'Properties not in BaseClass (eg. if baseInstance is Subclass2) are ignored.
z = value
End Sub
End Class
Class Subclass2 : Inherits BaseClass
Dim v As Integer
End Class
What you describe is not casting. Have you ever heard the expression"to cast something in a different light"? It means to look at the same thing in a different way or to make the same thing look different. That is the exact way that the term "cast" is used in programming. When you cast, you do NOT change the type of the object but only the type of the reference used to access the object. If you want to cast from a base type to a derived type then the object you're referring to has to actually be that derived type. If it's not then you're not performing a cast but rather a conversion.
So, why can't you convert an instance of a base type to an instance of a derived type. Well, why would you be able to? Yes, it's something that might save writing a bit of code on occasion but does it actually make sense? Let's say that you have a base type with one property and a derived type that adds another property. Let's also say that that derived type has constructors that require you to provide a value for that second property. You're suggesting that the language should provide you with a way to magically convert an instance of the base class into an instance of the derived class, which would mean it would have to slow you to circumvent that rule defined by the author via the constructors. Why would that be a good thing?
Use System.Reflection to iterate over properties and fields of the base class and apply them to the derived class. This example includes a single public property and single public field, but will also work with multiple private/protected properties and fields. You can paste the entire example into a new console application to test it.
Imports System.Reflection
Module Module1
Sub Main()
Dim p As New Parent
p.Property1 = "abc"
p.Field1 = "def"
Dim c = New Child(p)
Console.WriteLine("Property1 = ""{0}"", Field1 = ""{1}""", c.Property1, c.Field1)
Console.ReadLine()
End Sub
Class Parent
Public Property Property1 As String = "not set"
Public Property Field1 As String = "not set"
End Class
Class Child
Inherits Parent
Public Sub New(myParent As Parent)
Dim fieldInfo = GetType(Parent).GetFields(BindingFlags.NonPublic _
Or BindingFlags.Instance)
For Each field In fieldInfo
field.SetValue(Me, field.GetValue(myParent))
Next
Dim propertyInfo = GetType(Parent).GetProperties(BindingFlags.NonPublic _
Or BindingFlags.Instance)
For Each prop In propertyInfo
prop.SetValue(Me, prop.GetValue(myParent))
Next
End Sub
End Class
End Module
Output:
Property1 = "abc", Field1 = "def"
This solution is automated, so you won't need to change anything when adding or removing properties and fields in the base class.
In general, because of this:
Class TheBase
End Class
Class Derived1 : TheBase
Sub Foo()
End Sub
End Class
Class Derived2 : TheBase
Sub Bar()
End Sub
End Class
Sub Main()
Dim myDerived1 As New Derived1
' cast derived to base
Dim myTheBase = CType(myDerived1, TheBase)
' cast base to derived?
' but myTheBase is actually a Derived1
Dim myDerived2 As Derived2 = CType(myTheBase, Derived2)
' which function call would you like to succeed?
myDerived2.Foo()
myDerived2.Bar()
End Sub
I'm fairly new to lua and have the following problem with an assignment from a class:
We currently extend lua to support objects and inheritance. The Syntax for that is
Class{'MyClass',
attribute1 = String,
attribute2 = Number
}
Class{'MySubClass', MyClass,
attribute3 = Number
}
This works perfectly fine. The real problem lies within the next task: We should support "recursive types", that means a call like
Class{'MyClass', attribute = MyClass}
should result in an class with a field of the same type as the class. When this "class-constructor" is called the variable MyClass is nil, thats why the parameter table doesnt't have an entry attribute. How is it possible to access this attribute?
My first thought was using some kind of nil-table which gets returned every time the global __index is called with an unset key. This nil-table should behave like the normal nil, but can be checked for in the "class-constructor". The problem with this approach are comparisons like nil == unknown. This should return true, but as the __eq meta method of the nil-table is never called we cannot return true.
Is there another approach I'm currently just ignoring? Any hint is greatly appreciated.
Thanks in advance.
Edit:
Here the relevant part of the "testfile". The test by which the code is rated in class is another one and gets published later.
three = 3
print( three == 3 , "Should be true")
print( unknown == nil , "Should be true" )
Class{'AClass', name = String, ref = AClass}
function AClass:write()
print("AClass:write(), name of AClass:", self.name)
end
aclass = AClass:create("A. Class")
aclass:write()
Since MyClass is just a lookup in the global table (_G), you could mess with its metatable's __index to return a newly-defined MyClass object (which you would later need to fill with the details).
However, while feasible, such an implementation is
wildly unsafe, as you could end up with an undefined class (or worse, you may end up inadvertantly creating an infinite lookup loop. Trust me, I've been there)
very hard to debug, as every _G lookup for a non-existing variable will now return a newly created class object instead of nil (this problem could somewhat be reduced by requiring that class names start with an uppercase character)
If you go that route, be sure to also override __newindex.
How about providing the argument in string form?
Class{'MyClass', attribute = 'MyClass'}
Detect strings inside the implementation of Class and process them with _G[string] after creating the class
Or alternatively, use a function to delay the lookup:
Class{'MyClass', attribute = function() return MyClass end}
I found out how to create the properties of the class, like this:
private m_Name as string
public property get Name() as string
Name = m_Name
end sub
public property let Name(sval as string)
m_name = sval
end sub
The user will create a document and choose some fields (Name, Birthday, Phone....) inside this document, as I can't know exactly which fields will be chosen by the user, I thought create a class would be the best option.
After I create the class like above, how may I make a loop through this class to check which fields has been chosen by the user?
Any better option for my situation, please, let me know...
If I understood you correctly, you want to know which fields (out of a number of existing fields) user used/initialized?
I see several ways to do it:
1) If your variables do not have default values and must have a non-empty/non-zero values, then you can simply check if a variable is empty or zero. If it is, it hasn't been initialized.
If m_name = "" Then MsgBox "Variable is not initialized"
2) for each field you have, create a boolean fieldName_Initialized so for each field, you would have something like this:
private m_Name as string
private m_name_Initialized as Boolean
public property get Name() as string
Name = m_Name
end sub
public property let Name(sval as string)
m_name = sval
m_name_Initialized = True
end sub
3) you could have a list and add variable names to the list as they become initialized:
Make sure to add Microsoft Scripting Runtime to your References for Dictionary to work.
Dim initialized As Dictionary
Set initialized = New Dictionary
private m_Name as string
private m_name_Initialized as Boolean
public property get Name() as string
Name = m_Name
end sub
public property let Name(sval as string)
m_name = sval
initialized.Add "m_name", True
end sub
Then, to check if the var has been initialized:
If initialized.Exists("m_name") Then
' Var is initialized
4) similar to #3, except use an array of booleans. Tie specific var to a specific index, like m_name is index 0. This way you skip the hassle of controlling variable names (adds to maintenance cause as far as I know you can't get the name of the variable)
Personally, #1 is the most simple, but may not be possible in a certain situations. If #1 does not apply, I would personally pick #2, unless someone can figure out how to get a string representation of a variable name from the variable itself, then #3 is preferred.
I guess what you need is a kind of Nullable-behaviour. Yes you can do this ins VB6 with the datatype Variant. Then you can use the function "IsEmpty()" to check if a property was already set or not.
a little code-example:
Option Explicit
Private m_Vars()
'0 : Name
'1 : Birthday
'2 : Phone
Private Sub Class_Initialize()
ReDim m_Vars(0 To 2)
End Sub
Public Property Get Name() As String
Name = m_Vars(0)
End Property
Public Property Let Name(RHS As String)
m_Vars(0) = RHS
End Property
Public Property Get Birthday() As Date
Birthday = m_Vars(1)
End Property
Public Property Let Birthday(RHS As Date)
m_Vars(1) = RHS
End Property
Public Sub DoSomething()
Dim i As Integer
For i = 0 To UBound(m_Vars)
Dim v: v = m_Vars(i)
If IsEmpty(v) Then
MsgBox "is empty"
Else
MsgBox v
End If
Next
End Sub
If I understand correctly, the user will add tags or something to something like a document (e.g. [Name]) and you want to know how to map your class members to these tags. To do this, you dont really want to loop thru the class members, but the document tags to find out what is needed. When you find a "tag", submit it to your class to fill in the blank.
Part 1 is a parser to find the tags...my VB6 is very rusty, so pseudocode for this:
Const TagStart = "[" ' "$[" is better as it is not likely to appear
Const TagStop = "]" ' "]$" " " "
' make this a loop looking for tags
i = Instr(docScript, TagStart)
j = Instr(docScript, TagStop)
thisTag = Mid(docScript, TagStart, TagEnd )
' not correct, but the idea is to get the text WITH the Start and Stop markers
' like [Name] or [Address]
' get the translation...see below
docText = MyClass.Vocab(thisTag)
' put it back
Mid(docScript, TagStart, TagEnd) = docText
It is actually better to look for each possible legal tag (ie Instr(docScript, "[Name]")) which are stored in an array but you may have to do that in 2 loops to allow that a given tag could be requested more than once.
Part 2 supply the replacement text from MyClass:
Friend Function Vocab(tag as string) As String
Dim Ret as string
Select Case tag
Case "$[NAME]$"
ret = "Name: " & m_Name
' if the caption is part of the "script" then just:
'ret = m_Name
Case "$[ADDRESS]$"
ret = "Address: " & m_Addr
' if not found, return the tag so you can add new Vocab items
' or user can fix typos like '[NMAR]'
Case Else
ret = tag
...
End Select
Return Ret
End Function
The parsing routines in Part 1 could also be a method in your class to process the document "script" which calls a private Vocab.
Edit
a fraction of a 'script' might look like this:
Customer's Name:| $[CUST_FNAME]$ $[CUST_LNAME]$ (ignore the pipe (|) it was a table cell marker)
The parser looks thru the string to find "$[", when it does, it isolates the related tag $[CUST_FNAME]$. If you have a large number, the first part (CUST) can be used as a router to send it to the correct class. Next, call the method to get the translation:
newText = Cust.Vocab(thisTag)
Cust Class just looks at the tag and returns "Bob" or whatever and the parsing loop replaces the tag with the data:
Customer's Name:| Bob $[CUST_LNAME]$
Then just continue until all the tags have been replaced.
With "just" 22 vocab items, you could create a dedicated class for it:
Vocab.Translate(tag ...) as string
Case "$[CUST_FNAME]$"
return Cust.FirstName
...or
Are you trying to work out a way to do this via a DOC object from office? The above is more of from the ground up document composition type thing. For office I'd think you just need some sort of collection of replacement text.
HTH
If the user has a form that they do something to create their document, why not use simple checkbox controls, one for each possible field? If you want to use a loop to check for selected fields make the checkboxes a control array and loop though the array. You can assign the field name to the Tag property, then if the checkbox is checked add the field to an array.
I am attempting to create two identical objects in VB6 by assignment statements; something like this...
Dim myobj1 As Class1
Dim myobj2 As Class1
Set myobj1 = New Class1
myobj1.myval = 1
Set myobj2 = myobj1
It has become apparent that this doesn't create two objects but rather two references to the same object, which isn't what I am after. Is there any way to create a second object in this sort of way, or do I have to copy the object one member at a time...
Set myobj2 = new Class1
myobj2.mem1 = myobj1.mem1
...
?
Edit 2 Scott Whitlock has updated his excellent answer and I have incorporated his changes into this now-working code snippet.
Private Type MyMemento
Value1 As Integer
Value2 As String
End Type
Private Memento As MyMemento
Public Property Let myval(ByVal newval As Integer)
Memento.Value1 = newval
End Property
Public Property Get myval() As Integer
myval = Memento.Value1
End Property
Friend Property Let SetMemento(new_memento As MyMemento)
Memento = new_memento
End Property
Public Function Copy() As Class1
Dim Result As Class1
Set Result = New Class1
Result.SetMemento = Memento
Set Copy = Result
End Function
One then performs the assignment in the code thus...
Set mysecondobj = myfirstobj.Copy
Like many modern languages, VB6 has value types and reference types. Classes define reference types. On the other hand, your basic types like Integer are value types.
The basic difference is in assignment:
Dim a as Integer
Dim b as Integer
a = 2
b = a
a = 1
The result is that a is 1 and b is 2. That's because assignment in value types makes a copy. That's because each variable has space allocated for the value on the stack (in the case of VB6, an Integer takes up 2 bytes on the stack).
For classes, it works differently:
Dim a as MyClass
Dim b as MyClass
Set a = New MyClass
a.Value1 = 2
Set b = a
a.Value1 = 1
The result is that both a.Value1 and b.Value1 are 1. That's because the state of the object is stored in the heap, not on the stack. Only the reference to the object is stored on the stack, so Set b = a overwrites the reference. Interestingly, VB6 is explicit about this by forcing you to use the Set keyword. Most other modern languages don't require this.
Now, you can create your own value types (in VB6 they're called User Defined Types, but in most other languages they're called structs or structures). Here's a tutorial.
The differences between a class and a user defined type (aside from a class being a reference type and a UDT being a value type) is that a class can contain behaviors (methods and properties) where a UDT cannot. If you're just looking for a record-type class, then a UDT may be your solution.
You can use a mix of these techniques. Let's say you need a Class because you have certain behaviors and calculations that you want to include along with the data. You can use the memento pattern to hold the state of an object inside of a UDT:
Type MyMemento
Value1 As Integer
Value2 As String
End Type
In your class, make sure that all your internal state is stored inside a private member of type MyMemento. Write your properties and methods so they only use data in that one private member variable.
Now making a copy of your object is simple. Just write a new method on your class called Copy() that returns a new instance of your class and initialize it with a copy of its own memento:
Private Memento As MyMemento
Friend Sub SetMemento(NewMemento As MyMemento)
Memento = NewMemento
End Sub
Public Function Copy() as MyClass
Dim Result as MyClass
Set Result = new MyClass
Call Result.SetMemento(Memento)
Set Copy = Result
End Function
The Friend only hides it from stuff outside your project, so it doesn't do much to hide the SetMemento sub, but it's all you can do with VB6.
HTH
#Scott Whitlock, I was not able to make your code work but if it works it would be great.
I've created a regular module where I put the memento type
Type MyMemento
Value1 As Integer
Value2 As String
End Type
Then I create a class module called MyClass with the code
Private Memento As MyMemento
Friend Sub SetMemento(NewMemento As MyMemento)
Memento = NewMemento
End Sub
Public Function Copy() as MyClass
Dim Result as MyClass
Set Result = new MyClass
Result.SetMemento(Memento)
Set Copy = Result
End Function
Finally I try to call the copy function in another regular module like this
Sub Pruebas()
Dim Primero As MyClass, segundo As MyClass
Set Primero = New MyClass
Set segundo = New MyClass
Set segundo = Primero.Copy
End Sub
I get the message (below the picture): Error de compilacion: El tipo de agumento de ByRef no coincide
Here is an image (short of 10 points so here is the link): http://i.stack.imgur.com/KPdBR.gif
I was not able to get the message in English, I live in Spain.
Would you be so kind to provide with an example in VBA Excel?, I have been really trying to make this work.
Thanks for your work
===============================================
EDIT: Problem Solved:
The problem was on line "Result.SetMemento(Memento)", in VBA it needed to be called with "Call"
Public Function Copy() As MyClass
Dim Result As MyClass
Set Result = New MyClass
Call Result.SetMemento(Memento)
Set Copy = Result
End Function
It works great, thanks Scott Whitlock, you are a genius
or do I have to copy the object one member at a time...
Unfortunately yes.
It is possible (but technically very very difficult) to write a COM server in C++ that - using the IDispatch interface - will copy the value of each property, but really this is High Temple programming, if I had to do it, I don't I know if I could do it, but I'd be looking at something like 10 days work ( and I know how COM is implemented in C++, I'd also need to investigate to see if ATL framework has anything to help etc).
I worked with Vb3, 4,5 & 6 for something like 10 years (hands on, 5 days a week) and never found a good way to do this, beyond manually implementing serialisation patterns like Mementos and Save & Store, which really just boiled down to fancy ways of copying each member, one at a time.
VB6 classes have no parameterized constructors. What solution have you chosen for this? Using factory methods seems like the obvious choice, but surprise me!
I usually stick to factory methods, where I put the "constructors" for related classes in the same module (.BAS extension). Sadly, this is far from optimal since you can't really limit access to the normal object creation in VB6 - you just have to make a point of only creating your objects through the factory.
What makes it worse is having to jump between the actual object and your factory method, since organization in the IDE itself is cumbersome at best.
How about using the available class initializer? This behaves like a parameterless constructor:
Private Sub Class_Initialize()
' do initialization here
End Sub
I use a mix of factory functions (in parent classes) that then create an instance of the object and call a Friend Init() method.
Class CObjects:
Public Function Add(ByVal Param1 As String, ByVal Param2 As Long) As CObject
Dim Obj As CObject
Set Obj = New CObject
Obj.Init Param1, Param2
Set Add = Obj
End Function
Class CObject:
Friend Sub Init(ByVal Param1 As String, ByVal Param2 As Long)
If Param1 = "" Then Err.Raise 123, , "Param1 not set"
If Param2 < 0 Or Param2 > 15 Then Err.Raise 124, , "Param2 out of range"
'Init object state here
End Sub
I know the Friend scope won't have any effect in the project, but it acts as a warning that this is for internal use only.
If these objects are exposed via COM then the Init method can't be called, and setting the class to PublicNotCreatable stops it being created.
This solution is far from perfect, however, this is what I ended up doing for some of my legacy projects. It's somewhat memory-efficient for storage, but just looking up something definitely takes more time than otherwise required, especially for the huge structs.
Suppose you need to implement the following,
type user
id as long
name as String
desc as String
end type
But notice that VB6 natively supports Strings. So Instead of using structs, have "fake-struct" functions such as
Public Function user_raw(ByVal id as Long, ByRef name as String, ByRef desc as String) as String
Const RAW_DELIM as String = "|"
user_raw = cstr(id) & RAW_DELIM & name & RAW_DELIM & desc
End Function
So as a result, you will have passable-by-value (hence stackable into arrays, or multiline Strings), "fake-structs" made of strings such as
1|Foo|The boss of VB
2|Bar|Foo's apprentice
So the construct also counts as a crude way to perform to_string(). On the slightly good side, such storage is almost instantly convertible to the Markdown table syntax.
Later on, you can retrieve data from the struct by doing something like,
Public const RAW_DELIM as String = "|"
Public Const POS_USER_ID = 0
Public Const POS_USER_NAME = 1
Public Const POS_USER_DESC = 2
'...
public function get_raw_variable(byref user as String, byval var as integer) as String
dim buf() as String
buf = SPLIT(user, "|")
Assert ubound(buf) <= var
get_raw_variable = buf(var)
end function
The retrieval is sure inefficient, but is about the best you can have. Good luck