Declaring Global Variables in Procs

High Maintenance:

Declare the globals to be used in each proc, using individual variables

Example: Two procs with several global variables

proc area {} {
  global height width area
  set area [expr $height * $width]
  }

proc volume {} {
  global height width depth area
  set volume [expr $height * $width * $depth]
  }

Advantage:

• All globals are displayed.

Disadvantage:

• When a new global is added, all code which might access that variable needs to have the new variable added. (Note silent bug in the proc volume two)

Use a state array to hold global variables

Example: Two procs with several global values

proc area {} {
  global values area
  set area [expr $values(height) * $values(width)]
  }

proc volume {} {
  global values volume
  set volume [expr $values(height) * $values(width) * $values(depth)]
  if {[catch {testContainerSize $volume}] } {puts "Error: $errorInfo"}
  }

Advantage:

• New indices can be added to array without modifying the global declaration in all procs.

Disadvantage:

• Still requires some modifications to global declaration. Note new error in second example.

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Lower Maintenance:

Create a list of variables to declare global.

Example: Two procs with several global values

set globalList [list values area volume errorInfo errorCode]

proc area {} {
  global globalList ; eval "global $globalList"
  set area [expr $values(height) * $values(width)]
  }

proc volume {} {
  global globalList ; eval "global $globalList"
  set volume [expr $values(height) * $values(width) * $values(depth)]
  }

Advantage:

• Single point of change to add new globals.

Disadvantage:

• All procs get all globals. Possible name-space collisions if single list is used. Different subsystems can use different global lists, however to avoid this problem.

Use a proc with uplevel to declare the globals

Example: Two procs with several global values

set globalList [list values area volume errorInfo errorCode]

proc declareGlobals {list} {
  global $list
  uplevel "global [subst $$list]"
  }

proc area {} {
  declareGlobals globalList
  set area [expr $values(height) * $values(width)]
  }

Advantage:

• Cleaner code.

Disadvantage:

• One more proc call overhead.

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