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Okay, so can anyone tell me where I am going wrong here:

-- 'key' is a userdata which is created in my C++ code.
function set_visible(key, visible)
	if visible then 
		all_visible[key] = true
		all_visible[key] = nil

	-- Show some output for debugging
	for k,v in pairs(all_visible) do

-- Objects is a table that contains other tables of which the field
'key' is the same sort of userdata
function show_all_visible()
	-- Show some output for debugging.  Verify that the all_visible
table is consistant between f() calls
	for k,v in pairs(all_visible) do

	for i, v in ipairs(objects) do
		print(v.key, all_visible[v.key])
		if all_visible[v.key] == true then

local key = 'some userdata'

set_visible(key, true)

What I was expecting to happen is the all_visible[key] value to be set
to true, and the show_stuff() call to be made for the object with that
key.  However, this never happened.  The debug output showed that
all_visible[key] was set to true in set_visible, and was still true when
outputting that table at the start of show_all_visible(), however the
output in the ipairs loop in show_all_visible was showing
all_visible[v.key] to be nil.

So what special rules are there for using userdata as a key in a table?
Is it actually allowed?  Am I being a complete numpty?

(I got round the problem, by calling tostring(key) everywhere I used it
as a key, but I don't like that as a solution)

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