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       itclvars - variables used by [incr Tcl]


       The  following  global  variables  are created and managed automatically by the [incr Tcl]
       library.  Except where noted below, these variables should normally be  treated  as  read-
       only by application-specific code and by users.

              When  an  interpreter  is created, [incr Tcl] initializes this variable to hold the
              name of a directory containing the  system  library  of  [incr Tcl]  scripts.   The
              initial value of itcl::library is set from the ITCL_LIBRARY environment variable if
              it exists, or from a compiled-in value otherwise.

              When an interpreter is created, [incr Tcl] initializes this variable  to  hold  the
              current  patch  level  for  [incr Tcl].   For  example, the value "2.0p1" indicates
              [incr Tcl] version 2.0 with the first set of patches applied.

              When an interpreter is created  containing  Tcl/Tk  and  the  [incr Tcl]  namespace
              facility, this variable controls a "backward-compatibility" mode for widget access.

              In  vanilla Tcl/Tk, there is a single pool of commands, so the access command for a
              widget is the same as  the  window  name.   When  a  widget  is  created  within  a
              namespace,  however,  its access command is installed in that namespace, and should
              be accessed outside of the namespace using a qualified name.  For example,

                     namespace foo {
                         namespace bar {
                             button .b -text "Testing"
                     foo::bar::.b configure -background red
                     pack .b

              Note that the window name ".b" is still used in conjunction with commands like pack
              and  destroy.   However, the access command for the widget (i.e., name that appears
              as the first argument on a command line) must be more specific.

              The "winfo command" command can be used to query the fully-qualified access command
              for any widget, so one can write:

                     [winfo command .b] configure -background red

              and  this  is  good practice when writing library procedures.  Also, in conjunction
              with the bind command, the "%q" field can be used in place of "%W"  as  the  access

                     bind Button <Key-Return> {%q flash; %q invoke}

              While  this  behavior  makes  sense from the standpoint of encapsulation, it causes
              problems with existing Tcl/Tk applications.  Many existing applications are written
              with  bindings  that use "%W".  Many library procedures assume that the window name
              is the access command.

              The itcl::purist variable controls a backward-compatibility mode.  By default, this
              variable  is  "0",  and  the  window  name  can be used as an access command in any
              context.  Whenever the unknown procedure stumbles across a widget name,  it  simply
              uses  "winfo  command" to determine the appropriate command name.  If this variable
              is set to "1", this backward-compatibility mode is  disabled.   This  gives  better
              encapsulation, but using the window name as the access command may lead to "invalid
              command" errors.

              When an interpreter is created, [incr Tcl] initializes this variable  to  hold  the
              version number of the form x.y.  Changes to x represent major changes with probable
              incompatibilities and changes to y represent small enhancements and bug fixes  that
              retain backward compatibility.


       itcl, variables