Basically, a class defines some data elements and the methods (sub-routines) that manipulate that data.

In the case of a database, the methods are Forms, Reports, Queries, and other non-data pieces of the application. Typically, these methods read, write, print, and display the data stored in the tables.

The Tables are the class data definitions - each table is like a variable length array of related data. However, classes never store data, they just define its structure.

An object is an instance of a class. Only the object actually stores data, the class just defines how the data is stored. If several objects exist, each one has its own data, but there is only one copy of the methods.

There are significant differences between Paradox and MS Access. The following comments assume that froms and reports are developed on a separate development system and then delivered to the live system. (Never ever develop software on the live system.)

Paradox

Uses an alias to access tables. Since these aliases can be re-defined programmatically, they can be used as object pointers. Since each database is implemented as a subdirectory, updating a method is as simple as copying a file to the directory.

MS Access

ODBC pointers can be used to access any table EXCEPT MS Access tables. Must use OpenDatabase to connect to a database via code. In Forms, the Record Source property can only be a Table or a Query. Since each database is implemented as an mdb file, there is no way to copy a single method to an existing design. As a result, you need to replace the entire mdb file. If the data and methods where in the same file, this could never be done.

"Temp DocRef Old"
"Temp DocRef Main"
"Temp DocRef Compare"
"Temp DocRef Friends"
"Temp DocRef Root"

method Add_Aliases() Logical
; Assign aliases used by forms, reports, and queries
var
  DirStr        String
  BasicPathStr  String
  tempStr       String
endVar

  BasicPathStr = getAliasPath("Reference Documents") + "\\" + 
                 globalParameters["Relative Path"]

  DirStr = Document_UI_Field.Get_Document_Directory()
  tempStr = BasicPathStr + "\\" + globalParameters["Old Version"] +
                           "\\" + DirStr

  if globalParameters["Old Version"] <> "" then
    if not Add_One_Alias("Temp DocRef Old", tempStr) then
      return False
    endif
  endif

  tempStr = BasicPathStr + "\\" + globalParameters["Current Version"] +
                           "\\" + DirStr

  if not Add_One_Alias("Temp DocRef Main", tempStr) then
    return False
  endif

  tempStr = BasicPathStr + "\\" + globalParameters["Current Version"] +
                           "\\compare"

  if not Add_One_Alias("Temp DocRef Compare", tempStr) then
    return False
  endif

  tempStr = BasicPathStr + "\\" + globalParameters["Current Version"] +
                           "\\friends"

  if not Add_One_Alias("Temp DocRef Friends", tempStr) then
    return False
  endif

  if not Add_One_Alias("Temp DocRef Root", BasicPathStr) then
    return False
  endif

return True

endmethod Add_Aliases


method Add_One_Alias(const AliasStr     String,
                     const DirectoryStr String) Logical
var 
  tempStr String
endvar

; If the alias is already defined, don't do it again
; This will allow the form to work when other objects 
; (reports, forms, queries ...) are open 
; using the same alias definitions

  tempStr = getAliasPath(AliasStr)

  if tempStr = upper(DirectoryStr) then return True endif

  if not addAlias(AliasStr, "Standard", DirectoryStr) then
    msginfo(string("Alias \":", AliasStr, ":\" in use"),
             "You must close the existing form before " +
             "requesting a new one.\n\n" + DirectoryStr)
    return False
  endif

  return True

endmethod Add_One_Alias

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