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 (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.
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.
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}]
}
global(3tcl),
namespace(3tcl),
uplevel(3tcl),
variable(3tcl)
context, frame, global, level, namespace, procedure, upvar, variable