[swift-evolution] Proposal: Add @requires_super attribute

[Etan Kissling]

Or simply make `throws` and `requires_super` mutually exclusive?

The case where the base class does not `throw` but the subclass `throws` isn't possible
as it would break all clients who cast everything to the base class and assume that it doesn't throw.

Etan


> On 16 Dec 2015, at 17:46, Marc Knaup via swift-evolution <swift-evolution at swift.org> 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 <mailto: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 <mailto: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
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> 
> 
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