Re: __mul()

[Christopher Eykamp <chris@...>]

On 3/21/2010 11:12 AM, Mark Hamburg wrote:
> I haven't looked at how Lunar builds objects. Maybe it sticks the 
> methods in the metatable and has the metatable point back to itself 
> via the __index entry. 
At the risk of embarrassing myself, here's how I *think* lunar works 
when creating a 'class' called Point.  It first creates a metatable 
called Point, and adds some standard methods to it (__index, __tostring, 
__gc, new, etc.)  It then stores this table in the registry for later 
use.  It is this table to which I added the __mul method.  When lunar 
creates a Point object (using its C++ push method), it creates a 
userdata representing the object, then assigns it the metatable created 
earlier.  As for __index, here is the most salient bit of lunar code, 
from the Register function, where all this is set up:

  static void Register(lua_State *L) {
    lua_newtable(L);
    int methods = lua_gettop(L);

    luaL_newmetatable(L, T::className);
    int metatable = lua_gettop(L);

    // store method table in globals so that
    // scripts can add functions written in Lua.<--- what I want to do!
    lua_pushvalue(L, methods);<--- methods is the index of a new table
    set(L, LUA_GLOBALSINDEX, T::className);<--- Here, className would be Point

    // hide metatable from Lua getmetatable()
    lua_pushvalue(L, methods);
    set(L, metatable, "__metatable");<--- metatable is index of a new metatable

    lua_pushvalue(L, methods);
    set(L, metatable, "__index");
...

> I believe I can entirely explain your initial report based purely on 
> the diagnosis that __mul has been created as a method on the object 
> rather than as an entry in the metatable. Your report that the last 
> construct actually works would suggest that something more subtle was 
> going on in bot:getLoc().

Perhaps the initial report; however, I am pretty sure that _mul has been 
created on the metatable.  This was also suggested by another 
respondent, after which I tried the code below with the results posted 
belower (a new word!).  My knowledge of Lua is somewhat limited, but I 
can say with some confidence that _mul is on the metatable and not the 
table.

So I'm voting for something subtle is going on.

Chris

  for k,v in pairs(getmetatable(bot:getLoc())) do
     print(k,v)
  end

And got this (by the way, __mul & __add are lua funcs, the rest are 
defined in C++):

lenSquared      function: 01875200
__mul   function: 01877168
distanceTo      function: 01875330
__add   function: 01877198
distSquared     function: 01875298
len     function: 01875258
getxy   function: 018751C0
angleTo function: 018752D8
new     function: 018771C8