[incr\ Tcl]

NAME

scope - capture the namespace context for a variable

SYNOPSIS

itcl::scope name

DESCRIPTION

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 string of the following form:

@itcl object varName

This is recognized in any context as an instance variable belonging to object. So with itcl3.0 and beyond, it is 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 use the @itcl syntax directly. For example, it is a bad idea to write code like this:

set {@itcl ::fred x} 3
puts "value = ${@itcl ::fred x}"

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.

KEYWORDS

code, namespace, variable

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