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 if the first letter of the first
20 otherVar isn't # or a digit (it defaults to 1). For each otherVar
21 argument, upvar makes the variable by that name in the procedure frame
22 given by level (or at global level, if level is #0) accessible in the
23 current procedure by the name given in the corresponding myVar argu‐
24 ment. The variable named by otherVar need not exist at the time of the
25 call; it will be created the first time myVar is referenced, just like
26 an ordinary variable. There must not exist a variable by the name
27 myVar at the time upvar is invoked. MyVar is always treated as the
28 name of a variable, not an array element. Even if the name looks like
29 an array element, such as a(b), a regular variable is created. Other‐
30 Var may refer to a scalar variable, an array, or an array element.
31 Upvar returns an empty string.
32 The upvar command simplifies the implementation of call-by-name proce‐
34 dure calling and also makes it easier to build new control constructs
35 as Tcl procedures. For example, consider the following procedure:
36 proc add2 name {
37 upvar $name x
38 set x [expr $x+2]
39 }
40 add2 is invoked with an argument giving the name of a variable, and it
41 adds two to the value of that variable. Although add2 could have been
42 implemented using uplevel instead of upvar, upvar makes it simpler for
43 add2 to access the variable in the caller's procedure frame.
44 namespace eval is another way (besides procedure calls) that the Tcl
46 naming context can change. It adds a call frame to the stack to repre‐
47 sent the namespace context. This means each namespace eval command
48 counts as another call level for uplevel and upvar commands. For exam‐
49 ple, info level 1 will return a list describing a command that is
50 either the outermost procedure call or the outermost namespace eval
51 command. Also, uplevel #0 evaluates a script at top-level in the out‐
52 ermost namespace (the global namespace).
53 If an upvar variable is unset (e.g. x in add2 above), the unset opera‐ │
55 tion affects the variable it is linked to, not the upvar variable. │
56 There is no way to unset an upvar variable except by exiting the proce‐ │
57 dure in which it is defined. However, it is possible to retarget an │
58 upvar variable by executing another upvar command. │
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61 Upvar interacts with traces in a straightforward but possibly unex‐ │
62 pected manner. If a variable trace is defined on otherVar, that trace │
63 will be triggered by actions involving myVar. However, the trace pro‐ │
64 cedure will be passed the name of myVar, rather than the name of other‐ │
65 Var. Thus, the output of the following code will be localVar rather │
66 than originalVar: │
67 proc traceproc { name index op } { │
68 puts $name │
69 } │
70 proc setByUpvar { name value } { │
71 upvar $name localVar │
72 set localVar $value │
73 } │
74 set originalVar 1 │
75 trace variable originalVar w traceproc │
76 setByUpvar originalVar 2 │
77 } │
78 If otherVar refers to an element of an array, then variable traces set │
80 for the entire array will not be invoked when myVar is accessed (but │
81 traces on the particular element will still be invoked). In particu‐ │
82 lar, if the array is env, then changes made to myVar will not be passed │
83 to subprocesses correctly.
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86 A decr command that works like incr except it subtracts the value from
87 the variable instead of adding it:
88 proc decr {varName {decrement 1}} {
89 upvar 1 $varName var
90 incr var [expr {-$decrement}]
91 }
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95 global(n), namespace(n), uplevel(n), variable(n)
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99 context, frame, global, level, namespace, procedure, variable
100Tcl upvar(n)