Metamethod

Metamethod is a term used in Lua to describe a special kind of function stored in a metatable that defines or overrides the behavior of operations and other language constructs on values that carry that metatable. Metamethods are not methods of the value itself; they are functions that the Lua runtime can invoke automatically to implement operator overloading and related behavior.

How metamethods work: When Lua evaluates an operation such as addition, indexing, or a function call, it checks the involved values for an appropriate metamethod. If a suitable metamethod is present in a metatable—for example, add for addition or index for table access—it is called with the operands to produce a result. If no metamethod applies, Lua uses the default behavior or raises an error. Metamethods can also customize string conversion with tostring or function invocation with call.

Common metamethods include add, sub, mul, div, mod, pow, and unm for arithmetic; eq, lt, and le for comparisons; index for retrieving fields; newindex for assigning fields; call to make a value callable; tostring for string representation; and concat for string concatenation. Some versions also introduce additional metamethods to control iteration and other specialized behavior.

Implementation and usage: A metatable is a regular table attached to a value via setmetatable, and metamethod keys are the corresponding function values. Metamethods enable patterns such as object-like behavior, operator overloading, or proxy objects while preserving the underlying type. They require careful design to avoid unintended recursion and performance costs, and their availability and specifics can vary between languages that implement similar concepts.