upvar(3tcl)
NAME
upvar - Create link to variable in a different stack frame
SYNOPSIS
upvar ?level? otherVar myVar ?otherVar myVar ...? ______________________________________________________________________________
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 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}]
}
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 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.
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(3tcl), namespace(3tcl), uplevel(3tcl), variable(3tcl)
KEYWORDS
context, frame, global, level, namespace, procedure, variable
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