upvar ?level? otherVar myVar ?otherVar myVar ...?
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:
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.proc add2 name { upvar $name x set x [expr $x+2] }
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.
BUGS
If otherVar refers to an element of an array, then variable
traces set for the entire array will not be invoked when myVar
is accessed (but traces on the particular element will still be
invoked). In particular, if the array is env, then changes
made to myVar will not be passed to subprocesses correctly.
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Copyright © 1994-1996 Sun Microsystems, Inc.