Variants - why?

I have a question regarding the general nature of variants, and forgive me
if it sounds very basic but I would like to be clear on it. Regarding
variants, they are described as "self-defining structures that can contain
different types of data". Now, responsible programming practice suggests
you specify the type of data the variant holds (i.e. "double"). Well, if
that's the case, why are variants even needed? For example, a Vlisp point
has to be a variant containing an array of doubles. Why doesn't
Vlisp/ActiveX just allow the data type to be a simple array of doubles
without being "wrapped" in this "extra" data structure?

Thanks for any input,
LT

 

I think this has to do with the structure of the language..
In VLisp, there is no 'place' to define data types..e.g. no DIM statement area. The 'variant' structure allows for many 'types' of data to be stored and used. Arrays of point values are just (one) type.

Bob

 

Thanks, Bob. That basically answers my question. It still seems like extra
steps to me, though. The fact that the user has to *always* wrap his data
type with a variant type tell me that maybe Vlisp should just automatically
handle creating the variant without the programmer having to be burdened
with it. Either that or perhaps Vlisp *should* have a dim statement.

Thanks again,
LT

area. The 'variant' structure allows for many 'types' of data to be stored
and used. Arrays of point values are just (one) type.

 

LT,

A variant wrapper isn't required for a point (your example) depending on which vlisp
functions are used. For instance:

Command: (setq vobj (vlax-ename->vla-object
(car (entsel "Select line: "))))

Select line: #<VLA-OBJECT IAcadLine 0ab1df44>

Command: (vla-get-startpoint vobj)
#<variant 8197 ...> ;returns a variant

Command: (vlax-get vobj 'startpoint)
(7.90699 44.7369 0.0) ;returns a list of reals

Likewise:

;requires a variant argument
Command: (vla-put-startpoint vobj (vlax-3D-point '(7.0 34.0 0.0)))
nil

;accepts a list of reals
Command: (vlax-put vobj 'startpoint '(7.0 34.0 0.0))
nil

Try using the vlax-get, vlax-put and vlax-invoke functions. The arguments supplied
and the data types returned are pretty much the same as vanilla LISP. Note, these
functions require quoted symbols, such as 'StartPoint.

HTH
Joe Burke

 

Very nice. Thanks, Joe - I will add this to my arsenal.

LT

 

You're welcome.

Joe Burke

 

Command: (setq vobj (vlax-ename->vla-object

just to add,
(vlax-get-property vobj 'startpoint) returns a variant
as unfortunately it's show in help

Bruno Toniutti