The upvar command simplifies the implementation of call-by-name procedure calling and also makes it easier to build new control constructs as Tcl procedures. For example, consider the following procedure:
proc add2 name {
upvar $name x
set x [expr $x+2]
}
Add2 is invoked with an argument giving the name of a variable,
and it adds two to the value of that variable.
Although add2 could have been implemented using uplevel
instead of upvar, upvar makes it simpler for add2
to access the variable in the caller's procedure frame.
If an upvar variable is unset (e.g. x in add2 above), the unset operation affects the variable it is linked to, not the upvar variable. There is no way to unset an upvar variable except by exiting the procedure in which it is defined. However, it is possible to retarget an upvar variable by executing another upvar command.
Copyright © 1993 The Regents of the University of California. Copyright © 1994 Sun Microsystems, Inc. Copyright © 1995 Roger E. Critchlow Jr.