Question

I’m using userdata for passing things around with metatables in Lua and calling c++. I’m wondering if maybe light userdata can help me with a few problem cases I have.

A light userdata is a void *, but you can’t do anything with it in lua, at least thats my understanding you can just pass it around and back to C somehow – by a function call?.

How do you use a light userdata? If anyone has some use cases they’ve used them for it would help me get my head around them.

tia

Answer 1

I think Programming In Lua explains it quite well:

The real use of light userdata comes from equality. As a full userdata is an object, it is only equal to itself. A light userdata, on the other hand, represents a C pointer value. As such, it is equal to any userdata that represents the same pointer. Therefore, we can use light userdata to find C objects inside Lua.

As a typical example, suppose we are implementing a binding between
Lua and a Window system. In this binding, we use full userdata to
represent windows. (Each userdatum may contain the whole window
structure or only a pointer to a window created by the system.) When
there is an event inside a window (e.g., a mouse click), the system
calls a specific callback, identifying the window by its address. To
pass the callback to Lua, we must find the userdata that represents
the given window. To find this userdata, we can keep a table where the
indices are light userdata with the window addresses and the values
are the full userdata that represent the windows in Lua. Once we have
a window address, we push it into the API stack as a light userdata
and use the userdata as an index into that table. (Note that the table
should have weak values. Otherwise, those full userdata would never be
collected.)

The page also explains that light userdata is not garbage collected so memory management must be done by your code. It seems to me the use cases are actually fairly limited which is probably why you can’t find many examples. I did find one example:

I have a routine that constructs a gui object (tree view) from a lua
table. I then have a seperate lua table indexed by the window handle
of each tree node, as a lightuserdata. This simplifies the c/lua
interface – i can use the c handle to lookup the appropriate lua table
object.

// save a window handle
lua_pushlightuserdata(L, hKey);
lua_pushvalue(L,filt); // copy of filter table entry
lua_settable(L, treeindex);


// find data for a window handle
lua_pushstring(L, "__treeitems");
lua_gettable(L, LUA_GLOBALSINDEX);
lua_pushlightuserdata(L, hKey);
lua_gettable(L, -2);