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I had some luck with using a NAN value with a metatable for numbers. So now I have:
NIL+x -> NIL
NIL[x] -> NIL
NIL[x]=5 -> no error
#NIL -> 0
NIL>20 -> false

But I'm still having 2 problems:

1) NIL==NIL gives false. That's because the __eq metamethod is not being called for numbers at all, and normally NAN!=NAN. Is there a trick to get this to work? It is important because otherwise there is no convenient way to detect when the value is really NIL. Basically I want to do:
if (obj.mass==NIL) ...

2) Since I'm replacing the __index and __newindex for numbers, when I try to index a regular number I'm getting:
bad argument #1 to '?' (can't index a number value)

instead of the nicer standard message produced by the luaG_typeerror function:
attempt to index global 'a' (a number value)

Is there a way to use the luaG_typeerror myself? It requires a const TValue * argument and I don't know how to get it in my function.


Duncan Cross wrote:
Unfortunately I don't think there is a way to get what you want
without a custom modification of Lua, in a way that could potentially
make comparisons slightly slower.

The only thing I'd mention is that you could use "or" to specify a
default - instead of the cumbersome 'if obj.mass and obj.mass > 20'
you could do 'if (obj.mass or 0) > 20'.


On Sun, May 31, 2009 at 7:41 PM, Ivo Beltchev <> wrote:

I am embedding Lua in my application and I want to expose some objects to
the Lua scripts. Different objects have different sets of properties. For
example object1 can have speed and length and object2 can have mass and

Currently if the script wants to check if an object has mass>20 it has to

if (obj.mass and obj.mass>20)...

because obj.mass may be nil and then comparing it to 20 fails. So for
properties that don't exist I want to create a special object (NIL) that can
be safely used in expressions:
NIL+x -> NIL
NIL..x -> NIL
NIL[x] -> NIL
#NIL -> 0
NIL==NIL -> true
NIL==nil -> true (not so sure about this)
NIL>20 -> false

You get the idea.

Then I can simply write:
if (obj.mass>20) ...

However the __lt metamethod is not being called when I compare NIL to a
number. It is used only if the two values are of the same type. Otherwise
Lua throws an error.

Any ideas how to do this? A NAN value almost does what I want but it can't
be indexed, and also NAN~=NAN.