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.