Question

I’m brand new to Lua/LuaPlus and trying to figure out how metatables work.

In this code taken from the manual:

LuaObject metaTableObj = state->GetGlobals().CreateTable("MultiObjectMetaTable");
metaTableObj.SetObject("__index", metaTableObj);
metaTableObj.RegisterObjectFunctor("Print", &MultiObject::Print);

In the first line we create a new table, but the second line is a little confusing. In this table we just created, we are setting the element with key __index equal to the table itself. Why is __index chosen as a key and why set an element of the table to be equal to the table itself?

And then in the next section of code:

MultiObject obj1(10);
LuaObject obj1Obj = state->BoxPointer(&obj1);
obj1Obj.SetMetaTable(metaTableObj);
state->GetGlobals().SetObject("obj1", obj1Obj);

We create a C++ object, associate its address with a LuaObject via the BoxPointer call, and set the metatable so that we can use the Print function.

But for the last line, is that just creating a global Lua variable called “obj1”? At this point “obj1” and “MultiObjectMetaTable” will be global Lua variables?

Answer 1

This is not standard Lua, it looks like you’re using some C++ wrapper that I’m unfamiliar with, but I can make some guesses

In the first line we create a new table, but the second line is a
little confusing. In this table we just created, we are setting the
element with key __index equal to the table itself. Why is __index
chosen as a key and why set an element of the table to be equal to the
table itself?

__index is a special key when using metatables. If I have a table t and I try to index into it with a key of foo for example, naturally, I’ll get back the value associated with that key. But lets say there’s nothing there. Normally if you try to index into a spot that has nothing, you’ll get nil back.

But not if you have a metatable with the special key __index in it! If you have a metatable with an __index function or table, it’ll use that to find you your value. If you have a table assigned to __index as you do here, it’ll look into that table and return the value at the key you provided. This allows you get inheritance-like behavior. i.e. if table t doesn’t have this value, default to the value in this other table instead.

But for the last line, is that just creating a global Lua variable called "obj1"? At this point "obj1" and "MultiObjectMetaTable" will be global Lua variables?

As I mentioned, this is not standard Lua, so I’m not totally sure what’s happening there. (Mixing C++ and Lua can get tricky though, so while you’re still learning Lua it’s probably better that you stick to the C interface so you can understand whats really happening. Once you understand that you can move on to more automated solutions)