upvar - Create link to variable in a different stack frame
upvar ?level? otherVar myVar ?otherVar myVar ...?
This command arranges for one or more local variables in the current
procedure to refer to variables in an enclosing procedure call or
to global variables.
Level may have any of the forms permitted for the uplevel
command, and may be omitted if the first letter of the first otherVar
isn't # or a digit (it defaults to 1).
For each otherVar argument, upvar makes the variable
by that name in the procedure frame given by level (or at
global level, if level is #0) accessible
in the current procedure by the name given in the corresponding
myVar argument.
The variable named by otherVar need not exist at the time of the
call; it will be created the first time myVar is referenced, just like
an ordinary variable.
MyVar may not refer to an element of an array, but otherVar
may refer to an array element.
Upvar returns an empty string.
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.
context, frame, global, level, procedure, variable
Copyright © 1993 The Regents of the University of California.
Copyright © 1994 Sun Microsystems, Inc.
Copyright © 1995 Roger E. Critchlow Jr.