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On Tue, Apr 15, 2008 at 2:21 PM, Ralph Hempel
<> wrote:
> Dirk Feytons wrote:
> >
> > I recently discovered one disadvantage of such an approach: relational
> > operators are only possible on operands of the same type. In other
> > words, you can't compare such a userdata object with a regular number
> > since it is by definition false. This might or might not be a problem,
> > depending on your specific needs but it's certainly something to keep
> > in mind.
> >
>  These can be implemented in the metatable as well. The Lua VM
>  determines that one side or the other of the comparison is a
>  userdata and then calls the operator in the metatable for that
>  userdata.
>  It is up to the C implementation of the userdata to either
>  issue an error message or make an attempt to cast the other
>  side of the operator if necessary.
>  It's probably best in this application to have a completely
>  separate userdata that is incompatible with "regular" Lua numbers.
>  At least you'll get decent error messages if you do mix up
>  the operands :-)

Are you really sure because this contradicts everything I observed
(and worked around).
The VM throws an error if the types of the operands are not equal; see
for instance lessequal() in lvm.c. Even if both operands are userdata
they have to have the same metamethods before the VM even calls any
metamethod; see for instance call_orderTM() in lvm.c.