scope - Man Page

itcl::scope name

Creates a scoped value for the specified name, which must be a variable name.  If the name is an instance variable, then the scope command returns a name which will resolve in any context as an instance variable belonging to object.   The precise format of this name is an internal detail to Itcl. Use of such a scoped value makes it possible to use instance variables in conjunction with widgets.  For example, if you have an object with a private variable x, and you can use x in conjunction with the -textvariable option of an entry widget.  Before itcl3.0, only common variables could be used in this manner.

If the name is not an instance variable, then it must be a common variable or a global variable.  In that case, the scope command returns the fully qualified name of the variable, e.g., ::foo::bar::x.

If the name is not recognized as a variable, the scope command returns an error.

Ordinary variable names refer to variables in the global namespace. A scoped value captures a variable name together with its namespace context in a way that allows it to be referenced properly later. It is needed, for example, to wrap up variable names when a Tk widget is used within a namespace:

namespace foo {
    private variable mode 1

    radiobutton .rb1 -text "Mode #1"         -variable [scope mode] -value 1
    pack .rb1

    radiobutton .rb2 -text "Mode #2"         -variable [scope mode] -value 2
    pack .rb2
}

Radiobuttons .rb1 and .rb2 interact via the variable "mode" contained in the namespace "foo".  The scope command guarantees this by returning the fully qualified variable name ::foo::mode.

You should never attempt to craft your own scoped variable names, even if you believe you've flawlessly reverse-engineered the encoding. Instead, you should always use the scope command to generate the variable name dynamically.  Then, you can pass that name to a widget or to any other bit of code in your program.

code, namespace, variable