Question

There are heaps of examples of how to clone a Lua table in Lua, however I wasn’t able to find any example of how to do it with the native Lua C API. I tried to do it by hand twice, but ended up with a real (although working) mess.

Does anyone have any tips or links on how to elegantly do a shallow copy of a Lua table in the C API?

Answer 1

What you need to do is define the Lua function, and then break it down into the associated API calls.

shallow_copy = function(tab)
    local retval = {}
    for k, v in pairs(tab) do
        retval[k] = v
    end
    return retval
end

So we’re going to need to take the index of a table on the stack and the lua_State.

void shallow_copy(lua_State* L, int index) {

/*Create a new table on the stack.*/

        lua_newtable(L);

/*Now we need to iterate through the table. 
Going to steal the Lua API's example of this.*/

        lua_pushnil(L);
        while(lua_next(L, index) != 0) {
/*Need to duplicate the key, as we need to set it
(one pop) and keep it for lua_next (the next pop). Stack looks like table, k, v.*/


            lua_pushvalue(L, -2);
/*Now the stack looks like table, k, v, k. 
But now the key is on top. Settable expects the value to be on top. So we 
need to do a swaparooney.*/

            lua_insert(L, -2);

    /*Now we just set them. Stack looks like table,k,k,v, so the table is at -4*/



    lua_settable(L, -4);

/*Now the key and value were set in the table, and we popped off, so we have
table, k on the stack- which is just what lua_next wants, as it wants to find
the next key on top. So we're good.*/

        }
    }

Now our copied table sits on the top of the stack.

Christ, the Lua API sucks.