Threading - Learn Lua for iOS Game Development - page 83

Every time the table is accessed, the __index metafunction is invoked.

Note

So, you have to be careful when accessing a table value from inside of the __index function. It can
lead to a circular loop, causing a stack overflow, as shown in the definition following:

__index = function(tbl, key)
local a = tbl[key]
if a <=0 then a = 0 end
if a > 5 then a = 0 end
return a

a = tbl[key] , will actually trigger another __index function call, and that in turn will invoke

rawget , which retrieves the value of the table without invoking the __index metamethod. So,

local a = rawget(tbl, key)
if a<=0 then a = 0 end
if a > 5 then a = 0 end
return a
end

__newindex
This function is similar to the __index function, but is different in the way it's used to set a value; so,
when we assign a value to a table, this function is called. This gets passed three parameters: the
table, the key, and the value.

a = setmetatable({},{
__newindex = function(t, key, value)
print("Setting [" .. key .. "]=" .. value)
rawset(t, key, value )
end})
a.year = 2012
a.apps = 15

In a way similar to the preceding example where table[key]=value would invoke the __newindex
function and could get into an endless loop and cause errors, this example uses rawset to set the
value of the key.