[swift-evolution] Proposal: Add @requires_super attribute

[Marc Knaup]

Sounds reasonable since even the best flow analysis cannot ensure that all
codepaths call the super implementation.

Some more edge cases:

   - Calling super asynchronously by using it in a closure
   - Referring to the super implementation by assign it to a variable and
   call it later (is that really possible? never did that)


On Wed, Dec 16, 2015 at 6:25 PM, Vester Gottfried <
vester.gottfried at gmail.com> wrote:

> I would suggest that @requires_super only checks if a call to super is
> present at all. More detailed behaviour should be part of the functions
> documentation, because I think all possibilities cannot be checked easily
> by the compiler. For example a call to super my be required to happen early
> or late inside the function. But when too early or too late is can probably
> not been forseen by the compiler.
>
> On Wed, Dec 16, 2015 at 5:46 PM, Marc Knaup <marc at knaup.koeln> wrote:
>
>> +1 always had such issues with UIViewController's lifecycle methods.
>>
>> But edge cases need to be considered like "throws" for example.
>> Do I need to call super before I throw something?
>>
>> On Wed, Dec 16, 2015 at 5:41 PM, Matthew Johnson via swift-evolution <
>> swift-evolution at swift.org> wrote:
>>
>>> +1 to this.  Anything that helps ensure inheritance is thought through
>>> carefully and used correctly is a win.
>>>
>>> On Dec 16, 2015, at 10:32 AM, Vester Gottfried via swift-evolution <
>>> swift-evolution at swift.org> wrote:
>>>
>>> Some class based libraries/frameworks expect the consumer to subclass
>>> certain classes and override specific method and require that the super
>>> implementation of an overridden method is being called.
>>>
>>> Not calling the super implementation is a common source of bugs that may
>>> be prevented if the compiler checks if super is called, like it does in
>>> some cases of init().
>>>
>>> Example:
>>>
>>> class Box {
>>>    @requires_super
>>>     func addStuff() { ... }
>>> }
>>>
>>> Overriding class Box's addStuff without calling super.addStuff() should
>>> result in an error
>>>
>>> class Chest : Box {
>>>     override addStuff() {
>>>          // ERROR: addStuff() requires call to super.addStuff()
>>>         ...
>>>     }
>>> }
>>>
>>> Objective-C developers know this as NS_REQUIRES_SUPER and I think its
>>> worth thinking about adapting it.
>>>
>>> I hope my proposal was clear and thanks for reading,
>>>
>>> Gottfried
>>> _______________________________________________
>>> swift-evolution mailing list
>>> swift-evolution at swift.org
>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>
>>>
>>>
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>>>
>>
>
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