upvar(n) Tcl upvar(n)
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NAME
upvar - Create link to variable in a different stack frame
SYNOPSIS
upvar ?level? otherVar myVar ?otherVar myVar ...?
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DESCRIPTION
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
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upvar(n) Tcl upvar(n)
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. |
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.
SEE ALSO
namespace(n)
KEYWORDS
context, frame, global, level, namespace, procedure,
variable
Page 2 (printed 2/19/99)
