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

[Alexandre F. <ale...@gm...>]

Hi again,

> 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 ?

In fact I have already spotted a suspicious point where the type is
set to NULL and the internalRep (in ptrAndLong form) is set to
{NULL,0} at the same time: it is in GetBignumFromObj:

	if (objPtr->typePtr == &tclBignumType) {
	    if (copy || Tcl_IsShared(objPtr)) {
		...
	    } else {
		UNPACK_BIGNUM(objPtr, *bignumValue);
		objPtr->internalRep.ptrAndLongRep.ptr = NULL;
		objPtr->internalRep.ptrAndLongRep.value = 0;
		objPtr->typePtr = NULL;
		if (objPtr->bytes == NULL) {
		    TclInitStringRep(objPtr, tclEmptyStringRep, 0);
		}
	    }
	    return TCL_OK;

Now I'm having a hard time understanding this: what if the string rep
is not NULL (for example if the bignum has just been printed) ? It
would seem that in this case, the object is reset to a pure string,
namely that representing the bignum. While if it was NULL (for example
just after a bignum computation), the string rep is empty. Why that
dichotomy ? And why touch the ptrAndLong ?

-Alex