Re: [TCLCORE] Empty list is not a list -- using twoPtrValue for NULL type

[miguel s. <mig...@gm...>]

Alexandre Ferrieux wrote:
> Hello,
> 
> While implementing mutability, I realize that the empty list at some
> places (notably when doing Tcl_SetListObj with no elements) is not a
> list at all, it's just an object with an empty string rep and a NULL
> type.
> 
> That NULL type is nasty for me, because so far I relied on the intrep
> to store the mutable flag (I don't want to increase the size of
> Tcl_Objs).
> 
> Now, I could handle that case differently if I could find an unused
> field in Tcl_Obj's of NULL type (pure strings). Is there one ? Is the
> internalRep.twoPtrValue.ptr(1 or 2) usable for this purpose to denote
> the fact that a NULL-type object is mutable ?
> 
> (BTW it's primarily intended for mutable empty lists, but could be
> generalized to mutable strings)

Those fields are typically filled with garbage, so they will not really 
be usable. Except if you start setting them to NULL at each Tcl_NewObj. 
Probably not a great idea. But ...

Before setting the mutable flag on an empty list, you can force 
shimmering to listType using tclListType's setFromAnyProc. After that 
you have a List internal rep, and you can do your thing.

If this is a completely new empty list, a call to Tcl_NewListObj(1,NULL) 
will give you back an empty list with List internal rep - empty but with 
enough space allocated for 1 element.

HTH

Miguel