TIP 298: Revise Shared Value Rules for Tcl_GetBignumAndClearObj

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by Aug 21.
Author:         Don Porter <dgp@users.sf.net>
State:          Final
Type:           Project
Vote:           Done
Created:        22-Nov-2006
Keywords:       Tcl,Tcl_Obj
Tcl-Version:    8.5


This TIP proposes a revision to Tcl_GetBignumAndClearObj to make it easier to use.


Tcl_GetBignumAndClearObj was added by [237]. Because mp_int values can be big, the interface is offered to avoid making a copy when the value already in the internal representation of an unshared Tcl_Obj will no longer be needed and can be moved instead of copied.

The basic intent was fine, but in practice, callers must go through these gymnastics to use it:

 if (Tcl_IsShared(objPtr)) {
     Tcl_GetBignumFromObj(interp, objPtr, &bigValue);
 } else {
     Tcl_GetBignumAndClearObj(interp, objPtr, &bigValue);

It would make for a much more pleasant interface to move the test for a shared value into the routine itself.

Proposed Change

When passed a shared objPtr, Tcl_GetBignumAndClearObj will no longer panic, but will fall back to the copying behavior of Tcl_GetBignumFromObj. The use of the Tcl_GetBignumAndClearObj interface by a caller no longer means an assertion that objPtr is unshared, and no longer guarantees that objPtr will in fact be cleared, but merely indicates the caller will not be using the value anymore, and does not mind if it is cleared. That's all the caller should care about anyway.

Because of these changes in the implications and guarantees, the function is also renamed to Tcl_TakeBignumFromObj.

With these changes, the code above may be simplified to:

 Tcl_TakeBignumFromObj(interp, objPtr, &bigValue);


This is a incompatible change only with 8.5 alpha releases.

Reference Implementation

Available from http://sourceforge.net/tracker/index.php?func=detail&aid=1601243&group_id=10894&atid=360894 .


This document has been placed in the public domain.