multiple interface types ... how to use them... sort of

[Fred Harris]

Hi Stephen,

Not sure what an 'Abstract Syntax Tree' is, so I can't answer your question. However, the union construct makes a good choice when you need some kind of container that can hold various kinds of different things. However, they can only hold one thing at a time, and that 'thing' is indicated by the integer identifier that forms the first member of the union, and also the first member of every type added to the union.

Here just a bit ago I tried to see if interfaces could be put in a union, and it didn't seem to want to work.

[Steven Pringels 3]

Thanks Fred,

I'll check this out... btw, looking at your code... were you trying to construct a Abstract Syntax Tree for some scripting language ?

[Fred Harris]

Unions???

Hi Steven,

You might want to consider looking at a Union for something like that. The way something like that is set up is that the first member of the union is some sort of integer identifier of what the other member of the union is. The other members of the union can be structures/types of any sort or size, and the compiler will create the union structure to be large enough to hold the largest of the types that it may contain. Below is an example of how a union might be used. This example pertains to some work I was doing last summer to examine a line of VB code to figure out what it is, i.e., an assignment statement, a VB Verb such as Mid(), or perhaps athe beginning of some block logic structure such as an If Or For loop. I created an iType variable as the 1st member of the union, and added three other types to at as can be seen below. I 1st saw this setup in studying the X Window System in Linux, as that is how that system handles window messages (notice I didn't say Windows!). The 1st member is an equate for the type of message, and the other members are structures (types) for each specific message.

Type vbAssignmentStatement
  iType                        As Long
  LeftTerm                     As Term    '256
  RightTerm                    As Term    '256
  LeftTermIdentity             As Long    '  4  1=%NumericConstant, 2=%StringConstant, 3=vbObject, 4=%Integer, 5=%Long, 6=%Single, 7=%Double, etc
  RightTermIdentity            As Long    '  4  ditto
End Type

Type vbStatement
  iType                        As Long
  Statement                    As Term                         
  szReservedWord               As Asciiz*16             '16
  szObjectMethodCall           As Asciiz*32             '32
  szVariableDeclaration        As Asciiz*16
  szArguements                 As Asciiz*48             '48
  Identity                     As Long
End Type

Type vbBlockStatement
  iType                        As Long
  szStatement                  As Asciiz*32
End Type

Union vbCodeLine
  iType                        As Long
  vbAssignment                 As vbAssignmentStatement
  vbVerb                       As vbStatement
  vbBlock                      As vbBlockStatement
End Union

[Steven Pringels 3]

Hi, I wasn't able to cover the problem in one subject line...hence the
slightly mysterious description.

Ok, this is the problem. I have a method called GetField, at the
moment this method returns a iField interface. This is a generic
object that contains the majority of properties and methods related
to window controls. The method GetField is part of a class called
cForm with an interface iForm.

Now, I have a new control or iField as I called it. A Tab control.
Of course the tab control is far more different than say a label or
an Edit box control so my idea is to create a new class cTabControl
with an interface iTabControl.

Ok, so far so good. everybody following this ? Now, I also have
another class called cForm which has a method called AddField(). The
AddField method takes a parameter (ByVal ClsField as iField) this works
for Labels, Checkboxes, Editboxes but won't work for the iTabControl
interface.

So my first question. How can I pass object reference to a method where
the object can be different interfaces ?

also, How can I obtain an object reference using the method GetField(n)
, where n is the field index number, for which the reference can be
either iField or iTabControl. I'm not working with COM objects, just
PB classes.

So, in short... the iField interface is now passed as a parameter to
AddField(ByVal ClsField as iField) but I would like to pass any kind of
object to this method (iTabControl f.i. as well).

Ok, maybe it's because I'm writing this that my brain starts working. I was thinking of using a UDT with two members (pObjPtr holding the ObjPtr of the object I would like to pass and bType holding the type of the object.) Than
in stead of storing the iField in an array, I could store the UDT in an array and use a reverse method based on checking the type of object I've passed to retrieve the object using GetField. But then I use pointers instead of the
interface directly. Did I now just answer my own question or are there better approaches to this ? Like in Java one can return an object called Object which is the mother of all objects and then with InstanceOf determine the
object 'class' being returned. but that is Java and this is PB...

Thanks for reading !!

Cheers

Steven