Metatables

Metatables are a feature of Lua that allow developers to customize the behavior of tables by associating them with another table called the metatable. Through a metatable, you can control how a table responds to operations such as indexing, assignment, or arithmetic, enabling patterns like prototype-based inheritance and operator overloading.

Lua consults the metatable when performing many operations. The __index metamethod defines lookup behavior for missing keys, while __newindex handles assignments to non-existent keys. The __call metamethod lets a table be invoked like a function, and __tostring controls how the value is converted to text. Arithmetic and comparison operations are overridden with metamethods such as __add, __sub, __eq, and others. The length operator uses __len, and iteration can be customized with __pairs and __ipairs in supported versions. The __metatable field can be used to hide the actual metatable from external queries via getmetatable.

Common uses include implementing simple object-oriented patterns, providing default values for missing keys, creating read-only wrappers, and building proxy objects that intercept and manage access to underlying data. __index can be a function or another table, allowing delegation or inheritance chains.

Notes and limitations: metatables apply to tables and userdata, not primitive values. Some metamethods require symmetric definitions for both operands in an operation. The __gc metamethod only applies to userdata. To prevent external code from discovering the metatable, the __metatable field can be set to a string or value exposed as a placeholder. getmetatable and setmetatable are the primary primitives for querying and assigning metatables.