Version 11 of Pass by reference

Updated 2009-01-10 14:52:13 by dkf

KPV After having read a criticism of tcl that it doesn't have true pass by reference--you have to fake it with using upvar -- I thought I'd write a tcl-only clone of proc called xproc that will automatically give you pass by reference ala C++ syntax. Specifically, you say proc myproc {arg1 &arg2 arg3 &arg4} {body} and arg2 and arg4 will automatically be pass by reference.

 proc xproc {pname arglist body} {
     set preamble ""
     foreach arg $arglist {
         set arg [lindex $arg 0]
         if {[string match "&*" $arg]} {
             set barearg [string range $arg 1 end]
             append preamble "upvar 1 \[set [list $arg]\] [list $barearg]\n"
         }
     }
     proc $pname $arglist "$preamble#original body follows:\n$body"
 }

(Remark by AvL: I simplified the xproc-implementation, and made it more robust against nasty variable names) Here's a sample usage:

 xproc myproc {&arr} {
     foreach n [array names arr] {
         puts "arr($n) => $arr($n)"
     }
 }
 array set myarray {1 one 2 two 3 three}
 myproc myarray

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RS: Nice! I'd like to speak in defense of upvar, which is no black magic, but just a scoping mechanism. Like everything, references in Tcl are strings, implemented as variable names, which are valid in their scope. Now as proc variables normally have local scope (which is a good thing!), you have to dereference variables from other (only upper) scopes if required, and that is precisely what upvar does. And: even seeming innocent statements like

 set i 0

already contain a reference - to the variable "i". In general, every time you mention a variable name without prefixing it with $, you're working with a reference. (See Dangers of creative writing for a list of commands that use references, and possibly create new variables).

[AvL: I've removed a suggestion of a change that has meanwhile been included, to avoid confusion.]

See Implicit upvar for a later alternative to xproc, and use_refs

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