table of contents
other languages
conflicting packages
upvar(3tcl) | Tcl Built-In Commands | upvar(3tcl) |
NAME¶
upvar - Create link to variable in a different stack frameSYNOPSIS¶
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 (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}] }
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
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, upvar, variableTcl |