[swift-evolution] [swift-evolution-announce] [Review #3] SE-0117: Allow distinguishing between public access and public overridability

[王 黎明]

The problem is that: how to control/prevent the influence of customer’s code.

Question:
   How the customer’s code influence the library?

  For exam:
     func apiNNNN(v:SuperClass) { … }

     Customer call apiNNNN(v:aInstanceOfSubclass) { … }  /* some function of SuperClass be overrided */

  Is there any possibly for the following method:
    
     func apiNNNN(@denyOverridedSubclass v: SuperClass) { … }



> 在 2016年7月22日，下午4:15，Dave Abrahams via swift-evolution <swift-evolution at swift.org> 写道：
> 
> 
> on Thu Jul 21 2016, Karl <razielim-AT-gmail.com> wrote:
> 
>>> On 21 Jul 2016, at 22:13, Dave Abrahams via swift-evolution <swift-evolution at swift.org> wrote:
>>> 
>>> 
>>> on Thu Jul 21 2016, Matthew Johnson <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>> 
>> 
>>>>> On Jul 21, 2016, at 12:47 PM, Dave Abrahams via swift-evolution <swift-evolution at swift.org> wrote:
>>>>> 
>>>>> 
>>>>> on Thu Jul 21 2016, Matthew Johnson <swift-evolution at swift.org> wrote:
>>>>> 
>>>> 
>>>>>>> 	* What is your evaluation of the proposal?
>>>>>> 
>>>>>> +1 to the first design.  I think this is a great solution that
>>>>>> balances the many considerations that have been raised on all sides of
>>>>>> this issue.  `open` is 2 characters shorter than `public` so
>>>>>> complaints about boilerplate are no longer valid.  `internal` is the
>>>>>> “default” - neither `public` nor `open` are privileged as a “default”
>>>>>> for publishing API outside of a module.
>>>>>> 
>>>>>> I am interested in language enhancements such as exhaustive pattern
>>>>>> matching on classes and protocols which rely on knowledge of the full
>>>>>> class hierarchy.  Such enhancements will be far more useful if the
>>>>>> language supports non-open, non-final classes.
>>>>>> 
>>>>>> There are design techniques that would require additional boilerplate
>>>>>> if we cannot have non-open, non-final classes.
>>>>>> 
>>>>>> Most importantly, requiring library authors to choose `public` or
>>>>>> `open` provides important documentation value.  Users of the library
>>>>>> will know whether the author intends to support subclasses or not.
>>>>> 
>>>>> I think this reasoning is flawed.
>>>>> 
>>>>> If you make any methods overridable outside your module (“open”),
>>>>> obviously you mean to allow subclassing outside the module.  If you have
>>>>> no open methods, there's absolutely nothing you need to do to “support
>>>>> subclasses,” and from a design point-of-view, there's no reason to
>>>>> restrict people from subclassing.
>>>> 
>>>> I disagree.  As has been discussed when a class is not open the author
>>>> does not make a commitment to allow subclasses.  
>>> 
>>> Yes, but that argument is based on the *assumption* that disallowing
>>> subclassing is somehow an important dimension of safety or preserving
>>> API contracts, independent of overriding.  It isn't.
>                  ^^^^^^^^^^^^^^^^^^^^^^^^^
>>> 
>>> Demonstration: we don't make that argument about aggregation: you can't
>>> prevent someone from making your public struct a stored property in a
>>> type defined outside the module.  Judgements about programming style
>>> aside, in the absence of overriding, subclassing and aggregation are
>>> identical from an encapsulation point-of-view.
>>> 
>> 
>> IMO, the safety of API contracts is secondary. This is about safety of
>> implementation - my code is possibly not "resilient" enough (to to
>> speak) to handle overriding arbitrary public members. 
> 
> I wrote that subclassability is not an important element of safety
> **independent of overriding**.  If you don't allow any overriding,
> your code is always “resilient” enough to handle subclassing.
> 
> -- 
> Dave
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