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. There must not exist a variable by the
name myVar at the time upvar is invoked.
MyVar is always treated as the name of a variable, not an
array element. Even if the name looks like an array element,
such as a(b), a regular variable is created.
OtherVar may refer to a scalar variable, an array,
or 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.
namespace eval is another way (besides procedure calls)
that the Tcl naming context can change.
It adds a call frame to the stack to represent the namespace context.
This means each namespace eval command
counts as another call level for uplevel and upvar commands.
For example, info level 1 will return a list
describing a command that is either
the outermost procedure call or the outermost namespace eval command.
Also, uplevel #0 evaluates a script
at top-level in the outermost namespace (the global namespace).
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.
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.
namespace
context, frame, global, level, namespace, procedure, variable
Copyright © 1993 The Regents of the University of California.
Copyright © 1994-1997 Sun Microsystems, Inc.
Copyright © 1995-1997 Roger E. Critchlow Jr.
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