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[Theodolite]

n lua you can do this:function() to pass this as the first argument to function(), so I'm wondering, if I create a lua table in C/whatever like this

Code:

lua_newtable(ls)

lua_pushstring(ls,"fart")
lua_pushcfunction(ls,cat_fart)
lua_settable(ls,-3)

lua_pushstring(ls,"x")
lua_pushinteger(ls,415)
lua_settable(ls,-3)

lua_setglobal(ls,"cat")

In lua I'll be able to access cat.x , but if I wanted to do for example, cat:fart() , how do I need to write my cat_fart c function so that, when it's called with the cat table as the first argument, it can access whatever it is in that table? Like (pseudocode)

Code:

static int cat_fart(lua_state l)
{
  // take the table passed an argument    <- how do I do this 
  // change the x value to something else <- and this?
};

[Overkill]

Well, chances are if you're binding a C object to Lua, you want to use userdata, not tables. But I can explain either.

Tables aren't going to handle well for any dynamic objects, like things you malloc/new into existance, because Lua doesn't magically know to free your stuff when it's done.


Anyway, that said, to get a table from Lua in a C function, it's already on the stack. You just need to use positive stack indexes to your table functions. Index 1 is the first argument, index 2 is the second argument, and so on. If you wanna push it to the top of the stack and copy the reference to it, use lua_pushvalue.


Userdata are like tables, but they have few extra things at their disposal to make them more flexible. One thing is a __gc metamethod, so that when the userdata is no longer is used, it's automatically garbage collected.

As far as the C API is concerned, using Lua tables vs userdata is pretty much identical, save for the fact that userdata have pointers associated with them. (use luaL_checkuserdata to get good typechecking in your userdata's methods)

You usually make a wrapper userdata that has a pointer to the C/C++ object you want to use. And, you use lua_newuserdata to create the wrapper structure, which Lua manages internally. Then you allocate the object you want to keep a hold of, and put a reference in your wrapper struct. Then you set a __gc metamethod (and make a C function to go with it) so that when the object escapes scope, it'll correctly delete/free your wrapper struct's reference.

Hopefully some of that made sense, I've been replying rather fast. If you need clarification, just yell back.

[Theodolite]

Anyway, that said, to get a table from Lua in a C function, it's already on the stack. You just need to use positive stack indexes to your table functions. Index 1 is the first argument, index 2 is the second argument, and so on. If you wanna push it to the top of the stack and copy the reference to it, use lua_pushvalue.

What I was having trouble with was exactly that but I found out a way to do what I wanted without userdata. Thanks anyway.