Re: Lua 5.1 upvalues

["Robert G. Jakabosky" <bobby@...>]

On Friday 08, Anthony Howe wrote:
> On 08/04/2011 21:16, Rob Hoelz whispered from the shadows...:
> > If you want C closures to share values, you could store a table as an
> > upvalue, store your values in there, and set the same table as an
> > upvalue for the closures.
> 
> Ugh. One person says no. Another suggests the very thing I want to try
> that prompted the question.
> 
> In 5.1 does...
> 
> 	lua_newtable(L);
> 	lua_pushvalue(L, -1);
> 	lua_pushcclosure(L, &foobar, 1);
> 	lua_pushcclosure(L, &barblat, 1);

That code should be:

lua_newtable(L);
lua_pushvalue(L, -1);
lua_pushcclosure(L, &foobar, 1);
lua_pushvalue(L, -2);
lua_pushcclosure(L, &barblat, 1);
lua_remove(L, -3);

> 
> 	-- rinse & repeat
> 
> Work? Can more than one C function share a single upvalue? I ask because
> something along these lines has been mentioned in Lua 5.2 as an
> alternative to environments or some such without any examples. So I'm
> unclear as to whether this is possible in 5.1 and/or 5.2.

Like others have said they can shared the same value, but not the same 'local' 
storage of that value.  As long as you don't try to store a new value in the 
upvalue of one C closure and expect the other C closure to see the new value.

Lua closures can share the same 'local' storage for an upvalue:
local shared_storage = 123

function foobar(new_val)
  shared_storage = new_val -- changes the shared value
end

function barblat()
  return shared_storage -- will see changes from foobar()
end

print(barblat()) -- 123
foobar(456)
print(barblat()) -- 456

-- 
Robert G. Jakabosky