<cat>Dev.Lua</cat> <title>Concepts - Userdata</title> # Userdata Userdata is a type primarily used in the **Lua C API** that points to C data - such as objects - and comes in two flavors, full & light. <br/> ## Full Userdata When people mention `userdata` they likely refer to `full userdata`. Most of the time you will encounter full userdata objects, such as panels & entities that function similar to tables. Create empty userdata with <page>Global.newproxy</page>. <br/> ### Details Full userdata can have metatables, however it be indexed, Full userdata can have metatables, however it won't be indexed, and will not work with functions in the table library. Arbitrary values can be assigned to types like entities because of the entity userdatum's metatable, which uses the `__newindex` metamethod to set values in the entity's internal Lua table. Correspondingly, the `__index` metamethod checks the entity's internal Lua table for the key, and returns its value. It is through these functions that types like entities can be indexed as if they are tables. Full userdata is garbage collected, and Lua lets full userdata know it's about to be collected by calling the userdata's `__gc` metamethod, passing the userdatum itself as an argument. This allows for any resources used by the userdata to be freed on the C side, such as deleting an object or releasing a file handle. <br/> ## Light Userdata Light userdata represents a pointer as a value in Lua. <br/> ### Details Unlike full userdata, light userdata is not garbage collected, has no individual metatable, and is only equal to other light userdata when the pointers they hold are equal. This is not inherently used in Garry's Mod. See the [Programming in Lua documentation](https://www.lua.org/pil/28.5.html) for more information.