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 isn't # or a digit
(it defaults to 1). For each otherVar argument, upvar makes the variable by that name in the proce-dure procedure
dure 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 proce-dure: procedure:
dure:
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. |
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 cor- |
rectly.
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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