Re: cpp to lua binding question

["Peter Shook" <pshook@...>]

Hi Georges,

setmetatable returns its first argument.  I didn't realize that the manual 
no longer mentions this.
This is an omission in the new manual.  Perhaps this is intentional?

setmetatable is just the Lua 5 version of the old settag function.  The old 
manual says:  settag returns the value of its first argument (the table).

Sometimes the manual doesn't tell you the whole story.
When you really want to know how something works, have a look at the code in 
src/lib/lbaselib.c

static int luaB_setmetatable (lua_State *L) {
 int t = lua_type(L, 2);
 luaL_checktype(L, 1, LUA_TTABLE);
 luaL_argcheck(L, t == LUA_TNIL || t == LUA_TTABLE, 2,
                   "nil or table expected");
 if (luaL_getmetafield(L, 1, "__metatable"))
   luaL_error(L, "cannot change a protected metatable");
 lua_settop(L, 2);
 lua_setmetatable(L, 1);
 return 1;
}

The  return 1 is a giveaway that it returns something.  The settop(L, 2) 
leaves only the arguments on the stack.  Then lua_setmetatable(L, 1) pops 
the metatable off the stack, so only the original table is left on top of 
the stack and that is what is returned by 'setmetatable'.


By the way, this is a better implementation of Class.  There is no sense in 
creating a new metatable for each new object; it's better to keep the one 
metatable as an upvalue/closure value.

local function Class(methods, mt)
 methods = methods or {}
 mt = mt or {__index = methods}
 function methods:new(obj)
   return setmetatable(obj or {}, mt)
 end
 return methods
end


Sorry for leaving out the semicolons and parentheses.  This probably makes 
more sense to you:

cc = cs.texture.t1;
print('cc', cc);
cc:load('one');

cc = cs.mesh.m2;
print('cc', cc);
cc:load('two');

Cheers,

- Peter

Georges Mertens wrote:
> Feel your function : 'function Class(methods)' will help me with  OO in 
> Lua.
>
> My question : what is the efect of :
>    return setmetatable(obj or {}, {__index = methods})
> What is the value of 'setmetatable' which is returned. Thought 
> 'setmetatable' sets second argument as metatable for the first argument, 
> but no retun value.
>
> Also : have a problem with the syntax of the statements at the end. See a 
> lot of white spaces but no '()'
> cc = cs.texture.t1 print('cc',cc) cc:load'one'
> cc = cs.mesh.m2    print('cc',cc) cc:load'two'
> cc = cs.sound.s3   print('cc',cc) cc:load'three'
> cc = cs.sound.s1   print('cc',cc) cc:load'four'
> Would you be so kind to shed some light ...
>
> Thank you very much for your kind attention.
>
> Best Regards,
>
> Georges.

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