Question

This has been driving me nuts for over a week. Below are two snippets of Lua code from a robot player in the game Bitfighter (written in C++, using a variant of LuaWrapper for binding).

When I first start the Lua script, both work exactly as expected. But after some minutes of intense object creation and destruction, variant 2 stops working, and gives me the following error:

robot.lua:253: attempt to call missing or unknown method 'getEnergy' (a nil value)

To my eyes these should function identically. Who can explain the difference?

Notes: target is a (heavy) userdata representing a C++ object. getEnergy and getHealth are properly registered C++ functions. I can reproduce this differing behavior easily. This is Lua 5.1, using the luavec mod.

Variant 1 – always works

local mt = getmetatable(target)
local pow = mt.getEnergy(target) + mt.getHealth(target)

Variant 2 – starts failing after script has been running for an arbitrary amount of time

 local pow = target:getEnergy() + target:getHealth()

Answer 1

To track what happens when it stops working you can wrap the call in pcall and explore what happened with the target value:

local ok, res = pcall(function() return target:getEnergy() + target:getHealth() end)
if not ok then
  local s = "Invalid target value: "..tostring(target).." "..type(target).."\n"
  for k, v in pairs(target) do s = s.."target "..tostring(k).." "..tostring(v).."\n" end
  for k, v in pairs(getmetatable(target)) do s = s.."meta "..tostring(k).." "..tostring(v).."\n" end
  -- add anything else that helps you figure out what happened to target
  error(res..s)
end
local pow = res