Lua C API and metatable functions

Question

I'm using Lua inside a C application, and I have two tables. I want to create a third table that, while empty, will index values from my first two tables. I wrote the following simple example in Lua -

a = { one="1", two="2" }
b = { three="3", four="4" }

meta = { __index = function(t,k)
  if a[k] == nil then return b[k]
  else return a[k] end
end }

c = {}
setmetatable(c, meta)

print(c.one) -- prints "1"
print(c.four) -- prints "4"

My question is, what is the most effective way to do this from the C API? I've been able to do this by creating a new table, pushing the above Lua code chunk to that table, then calling setmetatable() on it but this seems less than optimal. Is there a better way?

Answer 1

#include <stdio.h>
#include "lua.h"

/* __index metamethod for the 'c' table (stack: 1 = table 'c', 2 = desired index) */
static int
cindex(lua_State *L)
{
    /* try the global 'a' table */
    lua_getglobal(L, "a");
    lua_pushvalue(L, 2);
    lua_gettable(L, -2);
    if (!lua_isnil(L, -1))
        return 1;

    /* try the global 'b' table */
    lua_getglobal(L, "b");
    lua_pushvalue(L, 2);
    lua_gettable(L, -2);
    if (!lua_isnil(L, -1))
        return 1;

    /* return nil */
    return 0;
}

int
main(int argc, char **argv)
{
    lua_State *L;

    L = (lua_State *) luaL_newstate();
    luaL_openlibs(L);

    /* create the global 'a' table */
    lua_createtable(L, 0, 2);
    lua_pushstring(L, "1");
    lua_setfield(L, -2, "one");
    lua_pushstring(L, "2");
    lua_setfield(L, -2, "two");
    lua_setglobal(L, "a");

    /* create the global 'b' table */
    lua_createtable(L, 0, 2);
    lua_pushstring(L, "3");
    lua_setfield(L, -2, "three");
    lua_pushstring(L, "4");
    lua_setfield(L, -2, "four");
    lua_setglobal(L, "b");

    /* create the global 'c' table and use a C function as the __index metamethod */
    lua_createtable(L, 0, 0);
    lua_createtable(L, 0, 1);
    lua_pushcfunction(L, cindex);
    lua_setfield(L, -2, "__index");
    lua_setmetatable(L, -2);
    lua_setglobal(L, "c");

    /* run the test script */
    luaL_loadstring(L, "print(c.one)\nprint(c.four)");
    if (0 != lua_pcall(L, 0, 0, 0)) {
        puts(lua_tostring(L, -1));
        return 1;
    }

    return 0;
}

Answer 2

Are you able to modify b's metatable? If so, this is more efficient:

a = { one="1", two="2" }
b = { three="3", four="4" }

setmetatable(a, { __index = b })

-- setmetatable(x, m) returns x, so you can do this:
c = setmetatable({}, { __index = a }) -- meta is here, too

print(c.one) -- prints "1"
print(c.four) -- prints "4"

When __index points to a table, it is more efficient than when it points to a function; I read somewhere that it was equivalent to 3 indirections in C. So in the worst case (c.one) there's a total of 6 indirections.