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Lua / Metatables

__add

Learn __add in Lua through a focused explanation, a dedicated topic-specific example, an engineering boundary, and a mastery check.

10 min read • Intermediate

__add is an atomic Lua topic inside Metatables. The example uses the named construct directly so you can see its syntax, data flow, and observable behavior.

Read the code by locating the exact feature named __add, then trace the value before and after that feature runs. This lesson is intentionally isolated from neighboring topics: changing identifiers alone would not reproduce the same behavior because the construct itself is different.

Topic-specific example

lua-0037-5daa78df0d.lua
lua Try It Live 
local mt={__add=function(a,b)return setmetatable({x=a.x+b.x},getmetatable(a))end};local a=setmetatable({x=2},mt);local b=setmetatable({x=3},mt);print((a+b).x)
Edge case / engineering note

For __add, test the boundary that makes the feature meaningful: invalid input, an empty value, a missing branch, lifecycle/cleanup, or a type/runtime constraint. Do not infer correctness only from the happy path.

Content source & attribution
Original Syntaxis Content · Original Syntaxis Content

Lesson quizzes

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