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Create an object/Native demonstration: Difference between revisions
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end
end</lang>
=={{header|Tcl}}==
This solution uses a dict(ionary), so requires Tcl 8.5 or better. Variable traces are used to detect write or unset access to such a protected variable, restore it to the backup value at protection time, and throw an exception
<lang Tcl>proc protect _var {
upvar 1 $_var var
set backup $var
trace add variable var {write unset} [list protect0 $backup]
}
proc protect0 {backup name1 name2 op} {
upvar 1 $name1 var
trace remove variable var {write unset} [list protect 0 $backup]
set var $backup
trace add variable var {write unset} [list protect0 $backup]
return -code error "$name1 is protected"
}
proc trying cmd { #-- convenience function for demo
puts "trying: $cmd"
if [catch {uplevel 1 $cmd} msg] {puts $msg}
}</lang>
Testing:
dict set dic 1 one
dict set dic 2 two
puts dic:$dic
protect dic
trying "dict set dic 3 three"
puts dic:$dic
trying "dict unset dic 1"
trying "unset dic"
puts dic:$dic
displays on stdout:
dic:1 one 2 two
trying: dict set dic 3 three
can't set "dic": dic is protected
dic:1 one 2 two
trying: dict unset dic 1
can't set "dic": dic is protected
trying: unset dic
dic:1 one 2 two
=={{header|zkl}}==
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