Tcl 8.7/Tk8.7a5 Documentation > Tcl Commands > classvariable

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classvariable — create link from local variable to variable in class


package require tcl::oo

classvariable variableName ?...?


The classvariable command is available within methods. It takes a series of one or more variable names and makes them available in the method's scope; those variable names must not be qualified and must not refer to array elements. The originating scope for the variables is the namespace of the class that the method was defined by. In other words, the referenced variables are shared between all instances of that class.

Note: This command is equivalent to the command typevariable provided by the snit package in tcllib for approximately the same purpose. If used in a method defined directly on a class instance (e.g., through the oo::objdefine method definition) this is very much like just using:

namespace upvar [namespace current] $var $var

for each variable listed to classvariable.


This class counts how many instances of it have been made.

oo::class create Counted {
    initialise {
        variable count 0

    variable number
    constructor {} {
        classvariable count
        set number [incr count]

    method report {} {
        classvariable count
        puts "This is instance $number of $count"

set a [Counted new]
set b [Counted new]
$a report
         This is instance 1 of 2
set c [Counted new]
$b report
         This is instance 2 of 3
$c report
         This is instance 3 of 3


global, namespace, oo::class, oo::define, upvar, variable


class, class variable, variable
Copyright © 2011-2015 Andreas Kupries
Copyright © 2018 Donal K. Fellows