upvar: point in tcl

When we need pass a variable name to a procedure, we can use upvar. upvar like reference in C++, we can changes the variable as we like.

the syntax of upvar is:

upvar ?level? varName localvar 

example:

#!/usr/bin/tcl
proc swap { x y} {
    upvar x a
    upvar y b
    set tmp  $a
    set a    $b
    set b    $tmp
}

set x 10
set y 5
swap x y
puts "x is $x"
puts "y is $y"  

result:

x is 5
y is 10