Author:         Will Duquette <will@wjduquette.com>
Author:         miguel sofer <msofer@users.sf.net>
State:          Final
Type:           Project
Vote:           Done
Created:        19-Jun-2005
Post-History:   
Tcl-Version:    8.5

Abstract

This TIP proposes a new namespace subcommand, namespace upvar, to efficiently alias namespace variables into the current scope.

Rationale

A pure-Tcl object system which defines a namespace to contain the variables for each object instance must either duplicate the object's method code in each instance namespace, or define the method code such that it exists in one namespace but accesses data from another. The Snit package http://www.wjduquette.com/snit does the latter. Instance variables are declared automatically within each method body using code like this, where "selfns" is a variable containing the name of the instance namespace:

 upvar ${selfns}::myvar myvar

The fully-qualified variable name "${selfns}::myvar" must be recomputed each time the method is called, which is a significant source of method-call overhead. This TIP proposes a mechanism for avoiding many of these costs while also allowing people to write clearer code.

With namespace upvar, the code would look like this:

 namespace upvar $selfns myvar myvar

The speed gains come from:

• it avoids building and then destroying a Tcl_Obj for the fully qualified name

• it avoids parsing the fully qualified name into its namespace/tail components, creating and then destroying the corresponding Tcl_Objs

• it may reuse a cached namespace in the internal representation of ${selfns}

In addition, the programmer's intention is easier to see in a command

 namespace upvar $selfns var1 var1 var2 var2 var3 var3

than in the currently necessary

 upvar 0 ${selfns}::var1 var1 ${selfns}::var2 var2 ${selfns}::var3 var3

where the fact that all variables come from the same namespace is not so obvious.

Specification

The syntax of the new subcommand is as follows:

namespace upvar ns otherVar myVar ?otherVar myVar ...?

The semantics are identical to the following upvar call:

upvar 0 ns::otherVar myVar ?ns::otherVar myVar...?

That is, the variable otherVar in namespace ns (as resolved from the local scope) is aliased to variable myVar in the local scope.

Reference Implementation

A reference implementation of namespace upvar is being developed at SF patch #1275435 https://sourceforge.net/tracker/index.php?func=detail&aid=1275435&group_id=10894&atid=310894 .

Copyright

This document has been placed in the public domain.