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upvar(n) Tcl Built-In Commands 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 is not # 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. An error is returned if
the name looks like an array element, such as a(b). OtherVar may refer to a scalar vari-
able, 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}]
}
If add2 is invoked with an argument giving the name of a variable, 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 out-
ermost 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 vari-
able except by exiting the procedure in which it is defined. However, it is possible to
retarget an upvar variable by executing another upvar command.
TRACES AND UPVAR
Upvar interacts with traces in a straightforward but possibly unexpected manner. If a
variable trace is defined on otherVar, that trace will be triggered by actions involving
myVar. However, the trace procedure will be passed the name of myVar, rather than the
name of otherVar. Thus, the output of the following code will be "localVar" rather than
"originalVar":
proc traceproc { name index op } {
puts $name
}
proc setByUpvar { name value } {
upvar $name localVar
set localVar $value
}
set originalVar 1
trace variable originalVar w traceproc
setByUpvar originalVar 2
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.
EXAMPLE
A decr command that works like incr except it subtracts the value from the variable
instead of adding it:
proc decr {varName {decrement 1}} {
upvar 1 $varName var
incr var [expr {-$decrement}]
}
SEE ALSO
global(n), namespace(n), uplevel(n), variable(n)
KEYWORDS
context, frame, global, level, namespace, procedure, variable
Tcl upvar(n)
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On Apache
Under GNU General Public License
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