Re: Hmmm... A different question I presume?

[Peter Shook <pshook@...>]

Ando Sonenblick wrote:
> I have a table (obviously a metatable that implements simple inheritance):
>
> {
>     __index = function(table, key)
>         return  rawget(XMethods, key) or
>                 rawget(YMethods, key) or
>                 rawget(ZMethods, key) end
> }
>
> Now, is there a way (from C code) to traverse the table to extract out the
> strings "XMethods", "YMethods" and "ZMethods" (without having to resort to
> parsing the Lua source code)?
>
> I'm hoping to implement an object inspector that will take and object and
> digest it's metatable and display in a UI the contents of the "inherited"
> tables, etc...

Hi Ando,

You could make your index event a C function with an upvalue that is a 
list of tables to search.  Then from C, you could retrieve the upvalue 
and find the names of the list elements in the table of globals.

This should give you some ideas:

print'hello'

one = { a=1,b=2,c=3 }
two = { d=99 }
three = function(t,i) print('did not find',i) return false end

print('one', one)
print('two', two)
print('three', three)

x = setmetatable({}, {__index = search{ one, two, three } })

print(x.a, x.b, x.c, x.d, x.it)


function name(value)
  for n,v in _G do
    if value == v then return n end
  end
end

for i,e in ipairs(searchlist(x)) do print(name(e), e) end


The output from this script is:

hello
one     table: 0xa045d20
two     table: 0xa0464a8
three   function: 0xa045e78
did not find    it
1       2       3       99      false
one     table: 0xa045d20
two     table: 0xa0464a8
three   function: 0xa045e78


where the search and searchlist functions are written in C like so:


#include "lua.h"
#include "lauxlib.h"

static int searcher (lua_State *L) {
  int i=1;
  do {
    lua_settop(L, 2);
    lua_rawgeti(L, lua_upvalueindex(1), i++);
    if (lua_isnil(L, -1)) return 0;  /* end of search list */
    if (lua_isfunction(L, -1)) {
      lua_pushvalue(L, 1);  /* table */
      lua_pushvalue(L, 2);  /* index */
      lua_call(L, 2, 1);
    } else if (lua_istable(L, -1) || lua_isuserdata(L, -1)) {
      if (lua_rawequal(L, 1, -1)) continue;  /* ignore recursive tables */
      lua_pushvalue(L, 2);  /* index */
      lua_gettable(L, -2);
    } else {
      luaL_error(L, "search list item must be a function, table or 
userdata");
    }
  } while (lua_isnil(L, -1));
  return 1;
}

static int search (lua_State *L) {
  luaL_checktype(L, 1, LUA_TTABLE);
  lua_settop(L, 1);
  lua_pushcclosure(L, searcher, 1);
  return 1;
}

static int searchlist (lua_State *L) {
  if (!lua_getmetatable(L, 1)) return 0;  /* no metatable */
  lua_pushliteral(L, "__index");
  lua_gettable(L, -2);
  if (lua_isnil(L, -1)) return 0;         /* no __index event */
  if (!lua_iscfunction(L, -1)) return 0;  /* not a searcher function */
  if (!lua_getupvalue(L, -1, 1)) return 0;  /* no upvalue */
  if (!lua_istable(L, -1)) return 0;      /* upvalue not a table */
  return 1;
}

int search_register (lua_State *L) {
  lua_register(L,"search", search);
  lua_register(L,"searchlist", searchlist);
  return 0;
}

- Peter