Darin Fisher wrote:
> Christopher Blizzard wrote:
>
>> Darin Fisher wrote:
>>
>>
>>>
>>> what is an interface contract exactly? in the case of a bit field
>>> attribute, is it wrong to say that only specific bits are currently
>>> defined and that other bits are reserved for future use? the
>>> contract, in this case, is well defined for all consumers of the
>>> interface. this is a very common way of making an interface
>>> ever-so-slightly extensible, and IMO it definitely seems like it
>>> would be useful in helping us freeze several mozilla interfaces. are
>>> we worried about changes to the .xpt files? would modifying .xpt
>>> files somehow cause runtime problems?
>>>
>>> darin
>>
>>
>>
>> I think that it's wrong to say that bit fields are infinitely
>> extensible, just because they happen to be bit fields and you can pass
>> in whatever you want. The contract that we're talking about here
>> isn't just a binary one as you elude to, it also defines what you can
>> pass in and what the expected behaviour should be as a result of that
>> call. I mean, why don't we just start using varargs for everything?
>>
>> If you change the interface you have to change the behaviour of both
>> the consumer and implementor of that interface. If I've ever heard a
>> definition of an interface change, that's it.
>>
>> --Chris
>>
>
> so there can't be reserved bits in an attribute's value? it seems to me
> that there can, provided these rules are followed:
>
> 1) consumers must set all reserved bits to zero
> 2) implementors must ignore any reserved bits (they must not expect
> these bits to be zero)
> 3) all bits, which were originally reserved, must be optional bits, with
> zero being the default value ("default" meaning that the behavior of the
> interface would match that of the original definition)
>
> darin
Mike beat me over the head for a half an hour or so this afternoon and
convinced me that it was probably OK to use reserved bits on an interface.
--Chris
--
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Christopher Blizzard
http://people.redhat.com/blizzard/
Mozilla.org - we're on a mission from God. Still.
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