What is userdata and lightuserdata in Lua?

A userdata is a garbage-collected value of an arbitrary size and content. You create one from the C API, with lua_newuserdata() which creates and pushes it on the stack and gives you a pointer to its content to initialize as you see fit from C.

A userdata is a garbage-collected value of an arbitrary size and content. You create one from the C API, with lua_newuserdata(), which creates and pushes it on the stack and gives you a pointer to its content to initialize as you see fit from C. It is very comparable to calling malloc().

A key distinction from malloc() is that you never need to call free(), rather you just allow the last reference to it to evaporate and the garbage collector will reclaim its storage eventually. The are most useful for holding data that is useful from C, but which must be managed from Lua. They support individual metatables, which are the key feature that allows binding C or C++ objects to Lua.

You simply populate its metatable with methods written in C that access, modify, and/or use the content of the userdata, and the result is an object that is accessible from Lua. A good example of this is the io library, which stores C FILE pointers in userdata, and provides bindings that implement the familiar read, write and similar methods. By implementing an __gc metamethod, the io library makes sure that one of its file objects closes the associated FILE when it is collected.

A light userdata is how you represent a pointer to something as a value in Lua. You create one by calling lua_pushlightuserdata() with the pointer that is its value. They are managed by Lua much the same way that a number is.

They are useful when you need to name a C object in a way that the name can be passed around within Lua, but the object's lifetime is not managed by Lua. Like numbers are equal when they have the same value, light userdata compare equal when they hold the same pointer. Like numbers, they exist as long as they are on the stack or stored in a variable, and they do not have individual metatables and they are not garbage collected.

1 for detail description about memory management :) – Eonil Jun 20 at 1:50.

Well, userdata is data from the C-side that can be used from within Lua. Such as for example file handles as io. Input are userdata (try print(type(io.

Input))). You'll need it yourself if you start messing about with the Lua C-API (or use the newproxy function, which gives you an empty userdatum, upon which you can set a metatable (see dden Features lua-users.org/wiki/ddenFeatures) on the Lua-users wiki). A good introduction is: lua.org/pil/28.html As for the C functions stuff: yea you can just register C functions as functions to be called from within Lua, but it won't get you other data types, pointers to data on the C side, etc etc.

You can use userdata Whenever you have some amount of data that you want do be managed by the lua gc. For example you could use it for C++ objects. Some examples for c++-objects with userdata: You could save one in userdata then you can forget about it in C++ as it will be managed by lua.So you could reference it in luavariables and pass it to functions that call a memberfunction of the c++ object.

(there are ways to generalize that of course like putting a generic function object in the userdata, bind that as upvalue to a C-closure and register that c-closure on a luaobject that represents the lua-side of the c++ object, that involves userdata as well). If you don't want the lua gc to manager your objects and just want to reference your c++ object from lua you can store a pointer to it as light userdata.

I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.