[cfe-dev] Introducing attribute objc_pure_virtual_method

[Ted Kremenek]

On Oct 29, 2013, at 9:55 PM, Louis Gerbarg <lgerbarg at gmail.com> wrote:

> On Tue, Oct 29, 2013 at 7:32 PM, Ted Kremenek <kremenek at apple.com> wrote:
> On Oct 29, 2013, at 3:16 PM, variadictemplate . <variadic.template at googlemail.com> wrote:
> 
>> currently there is no possibility in objective-c to mark a method as "pure virtual" like in C++. I think the "pure-virtual"-construct enables an elegant and clean way to describe and specify APIs, although if it may be a rarely used construct.
>> A workaround for objective-c is to declare the method within the base-class and add to it's implementation an assertion like assert(false && "has to be implemented by subclasses"). But i think it would be nicer when the compiler gives me a hint that there is something wrong, instead of crashing the app at runtime.
>> 
>> So, with my attached patch I'm introducing an attribute for that case, named objc_pure_virtual_method. It can only be attached to methods of a class-declaration (the other way round: extensions, categories and protocols are excluded). If a subclass does not implement the method, it will cause a warning. If the subclass redeclares the method within it's @interface-declaration and also adds the objc_pure_virtual_method, no warning will be generated. If the call of an init-method to an instance of a class with pure virtual methods is detected, an error is generated.
>> 
>> I'm not sure if the init-call to a subclass of an abstract class, that did not implement all pure virtual methods, should also produce an error, as this case already produces warnings of unimplemented methods - in my eyes, this is sufficient.
>> 
>> Surely there are some issues with my current implementation, as i'm not so sure if i have chosen the right places to add the logic and about the names / warnings i have chosen and I'm happy to hear any suggestion / criticism.
> 
> 
> 
> I have also been in a situation where I have done the assert(false && "has to be implemented by subclasses") thing, but it is very rare. In Objective C subclassing is (compared to most OOP languages) infrequent and shallow. Since most the framework code in Cocoa/CocoaTouch/GNUStep chooses to use delegation as opposed subclassing it is generally possible to achieve the same sort of compile time warnings by having @required methods in a formal protocol on a delegate.
> 
> While I am generally in favor of adding annotations, I wonder if it is worthwhile to add an annotation for something that is really not a common idiomatic pattern in the language?
> 
> (2) In a subclass, if a method is redeclared as being “non-virtual”, should we also guarantee that it is implemented in the @implementation?  That may be tricky of course with categories.
>  
> I suspect you can get into similar situations today where the compiler does not have enough info to do things with @required in formal protocols and categories. I am sure you can if you are using class_addMethod to provide the IMPs at runtime. The language already provides @dynamic for dealing with that with respect to properties. Extending @dynamic to methods would work for the proposed annotation (if it makes sense to do the annotation at all) and solve the issue with formal protocols (though in practice I am not sure I have seen the cases ever come up, they are sort of pathological).
> 
> Louis

Thanks Louis.  I was also thinking about this some more this evening and exchanged some thoughts with others, and I think I agree with you that this doesn’t seem worth adding to the language.

Also, it seems like this could easily be modeled using protocols.  The subclass could implement a protocol with the required method, and use -Wprotocol to catch when something isn’t implemented.

varadictemplate: Do you see a compelling need for this that cannot be modeled using protocols?  The class adopting the protocol is saying “all methods are implemented” for that protocol, which is natural way to express not only that a particular method is implemented, but that a group of methods are implemented.

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