[manual] the table library and metatables
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- Subject: [manual] the table library and metatables
- From: Stephan Hennig <sh-list@...>
- Date: 2016年1月27日 18:09:29 +0100
Hi,
section "Changes in the Libraries" has this:
The Table library now respects metamethods for setting
and getting elements.
a) Considering the place of this sentence, where will this information
go in the Lua 5.4 manual? Will it just go away (as long as Lua 5.4
doesn't introduce a new incompatibility to that behaviour)? I think
this information is worth a sentence in one of the preceding
sections.
b) In the current form, the above mentioned sentence is vague. What is
setting or getting an element? Does table.insert count as "setting
an element"? Does table.concat count as "getting an element"? What
about table.unpack? A test shows that none of these functions, that
I'd consider part of "the table library", respect a table's __index
metamethod.
-- A table with three entries.
local t = { 1, 2, 3 }
-- We add a fourth one that is accessible via
-- the __index metamethod.
local mt = {
__index = { [4] = 4 },
}
setmetatable(t, mt)
-- How many elements can we iterate over?
local i = 1
while t[i] ~= nil do-- 1 ... 4
print(i, t[i])
i = i + 1
end
print('table.concat: ', table.concat(t))-- Where's the four?
print('table.unpack: ', table.unpack(t))-- Where's the four?
-- OK, what about table.insert?
print('inserting element "x"')
table.insert(t, 'x')-- Where does the four go?
-- How many elements can we iterate over?
local i = 1
while t[i] ~= nil do-- 1 ... 4, again
print(i, t[i])
i = i + 1
end
What metamethods do each of the functions in the table library respect?
Best regards,
Stephan Hennig