Metatables and protected calls

The idea

A table can have a metatable that defines behavior for selected operations. Setting __index to a method table lets missing fields be looked up there, which supports a simple object-like pattern. The colon call object:method(value) passes object as the first argument, matching a method whose first parameter is self. Keep per-instance data on each instance rather than placing shared mutable state accidentally in the method table.

error raises a failure, and pcall invokes a function in protected mode. Its first result reports whether the call succeeded; following results contain either the function’s return values or the error object. Check that status before treating the second result as ordinary data. Handle a failure at a boundary that can make a useful decision instead of silently converting every programming mistake into success.

local Counter = {}
Counter.__index = Counter
function Counter:add(amount) self.value = self.value + amount end
local item = setmetatable({value = 2}, Counter)
item:add(3)
local ok, result = pcall(function() return item.value end)
print(ok)
print(result)

true
5

The method is found through __index, and the colon supplies item as self. The protected function succeeds and returns the updated value after the status boolean.