1upvar(n) Tcl Built-In Commands upvar(n)
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8 upvar - Create link to variable in a different stack frame
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11 upvar ?level? otherVar myVar ?otherVar myVar ...?
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16 This command arranges for one or more local variables in the current
17 procedure to refer to variables in an enclosing procedure call or to
18 global variables. Level may have any of the forms permitted for the
19 uplevel command, and may be omitted (it defaults to 1). For each oth‐
20 erVar argument, upvar makes the variable by that name in the procedure
21 frame given by level (or at global level, if level is #0) accessible in
22 the current procedure by the name given in the corresponding myVar
23 argument. The variable named by otherVar need not exist at the time of
24 the call; it will be created the first time myVar is referenced, just
25 like an ordinary variable. There must not exist a variable by the name
26 myVar at the time upvar is invoked. MyVar is always treated as the
27 name of a variable, not an array element. An error is returned if the
28 name looks like an array element, such as a(b). OtherVar may refer to
29 a scalar variable, an array, or an array element. Upvar returns an
30 empty string.
31 The upvar command simplifies the implementation of call-by-name proce‐
33 dure calling and also makes it easier to build new control constructs
34 as Tcl procedures. For example, consider the following procedure:
35 proc add2 name {
37 upvar $name x
38 set x [expr {$x + 2}]
39 }
40 If add2 is invoked with an argument giving the name of a variable, it
42 adds two to the value of that variable. Although add2 could have been
43 implemented using uplevel instead of upvar, upvar makes it simpler for
44 add2 to access the variable in the caller's procedure frame.
45 namespace eval is another way (besides procedure calls) that the Tcl
47 naming context can change. It adds a call frame to the stack to repre‐
48 sent the namespace context. This means each namespace eval command
49 counts as another call level for uplevel and upvar commands. For exam‐
50 ple, info level 1 will return a list describing a command that is
51 either the outermost procedure call or the outermost namespace eval
52 command. Also, uplevel #0 evaluates a script at top-level in the out‐
53 ermost namespace (the global namespace).
54 If an upvar variable is unset (e.g. x in add2 above), the unset opera‐
56 tion affects the variable it is linked to, not the upvar variable.
57 There is no way to unset an upvar variable except by exiting the proce‐
58 dure in which it is defined. However, it is possible to retarget an
59 upvar variable by executing another upvar command.
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62 Upvar interacts with traces in a straightforward but possibly unex‐
63 pected manner. If a variable trace is defined on otherVar, that trace
64 will be triggered by actions involving myVar. However, the trace pro‐
65 cedure will be passed the name of myVar, rather than the name of other‐
66 Var. Thus, the output of the following code will be “localVar” rather
67 than “originalVar”:
68 proc traceproc { name index op } {
70 puts $name
71 }
72 proc setByUpvar { name value } {
73 upvar $name localVar
74 set localVar $value
75 }
76 set originalVar 1
77 trace variable originalVar w traceproc
78 setByUpvar originalVar 2
79 If otherVar refers to an element of an array, then variable traces set
81 for the entire array will not be invoked when myVar is accessed (but
82 traces on the particular element will still be invoked). In particu‐
83 lar, if the array is env, then changes made to myVar will not be passed
84 to subprocesses correctly.
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87 A decr command that works like incr except it subtracts the value from
88 the variable instead of adding it:
89 proc decr {varName {decrement 1}} {
91 upvar 1 $varName var
92 incr var [expr {-$decrement}]
93 }
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96 global(n), namespace(n), uplevel(n), variable(n)
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99 context, frame, global, level, namespace, procedure, upvar, variable
100Tcl upvar(n)