Objects

Object Oriented Programming

What about Receive objects (UDVs)?

When passing a string as parameter to a function or window for instance, you can specify as String or Receive String:

Function: MyFunction
   Parameters
      String: psParameter1
      Receive String: rpsParameter2
   Actions
      Set psParameter1 = "New string 1"
      Set rpsParameter2 = "New string 2"

The Receive String parameter can be changed inside the function.

There is no Receive for objects/UDV, how to change objects from within the function then?

Function: MyFunction
   Parameters
      cfcTest: puObject
   Actions
      Set puObject.sivName = "New string"

Well, in TD an UDV parameter is ALWAYS Receive, even when you do not specify it.
This means that in the sample code above, the puObject instance variable sivName is actually changed.
The functionality calling this function will have a changed object afterwards.

And no, there is not an option to prevent this.

It is possible to pass a Null-Object as a parameter to a function and create this object during the functions actions. So the caller of the function receives a new valid object for further use.
In C++ this technique is called "dereferencing".

Function: MyFunction
   Parameters
      cfcTest: puObject
   Actions
      Set puObject = new cfcTest
      Set puObject.sivName = "New string"

Function: AnotherFunction
   Variables
      cfcTest: uObject = OBJ_Null
   Actions
      Call MyFunction( uObject )
      Call SalMessageBox( uObject.sivName, "", MB_Ok )

But beware, the derefencing of UDV handle is not supported with window parameters. The internal architecture of passing parameters to functions is different from passing parameters to windows!

How to copy one object (instance) to another

Create a CopyFrom function in the class of the object (instance) which copies
all attributes from the source object to the destination object.

Functional Class: fcCustomer
   Instance Variables
      String: sivName
      Number: nivCustomerId
   Functions
      Function: CopyFrom
         Parameters
            fcCustomer: puSourceObject
      Actions
            Set sivName = puSourceObject.sivName
            Set nivCustomerId = puSourceObject.nivCustomerId

Having such a function in the class results in the following usage for instances.

Below you can see how to copy the contents (data) from the Customer1 object to Customer2.
After the Call, the two objects have the same data values.

Local variables
   fcCustomer: uCustomer1
   fcCustomer: uCustomer2
Actions
   Call uCustomer2.CopyFrom( uCustomer1 )

To copy objects which contains nested objects, you can reuse the CopyFrom functions
of the nested UDV's like this

Functional Class: fcAdress
   Instance Variables
      String: sivStreet
      String: sivCity
   Functions
      Function: CopyFrom
         Returns
         Parameters
            fcAdress: puSourceObject
         Actions
            Set sivStreet= puSourceObject.sivStreet
            Set sivCity= puSourceObject.sivCity

Functional Class: fcCustomer
   Instance Variables
      String: sivName
      Number: nivCustomerId
      fcAdress: uivAdress
   Functions
      Function: CopyFrom
         Returns
         Parameters
            fcCustomer: puSourceObject
         Actions
            Set sivName = puSourceObject.sivName
            Set nivCustomerId = puSourceObject.nivCustomerId
            Call uivAdress.CopyFrom( puSourceObject.uivAdress )

How to clear data of an object

1) Create a Clear function in the class of the object (instance) which sets all instance variables to initial values.

Functional Class: fcCustomer
   Instance Variables
      String: sivName
      Number: nivCustomerId
   Functions
      Function: Clear
         Returns
         Parameters
         Actions
            Set sivName = STRING_Null
            Set nivCustomerId = NUMBER_Null

! Below the code to clear an object
Call uCustomer1.Clear( )

2) For TD versions offering the new statement

! Below the code to clear (initialise) an object
Set uCustomer1 = new fcCustomer