[cfe-dev] Specializing templates on Objective-C ARC ownership annotations

[Louis Dionne via cfe-dev]

> On Jul 31, 2018, at 19:36, Erik Pilkington <erik.pilkington at gmail.com> wrote:
> 
> CCing Akira, who knows a lot more about this. Interestingly, this works:
> 
> template <class T> struct test;
> template <class T> struct test<T * __weak> {};
> template <class T> struct test<T *> {};
> 
> test<Foo * __weak> x; // selects first specialization
> 
> But this doesn't:
> 
> template <class T> struct test;
> template <class T> struct test<T *__strong> {};
> template <class T> struct test<T *> {};
> 
> test<Foo * __strong> x; // error, specializations are ambiguous
> 
> I think the problem here is that T *'s are implicitly __strong when objc-arc is enabled, so the specializations are equivalent. Maybe we could still treat the first specialization as more specialized since it has an explicit __strong. Either way, I think you can work around this by checking if T is an interface type, in which case you know it’ll be implicitly __strong.

What I actually want is to check whether `T*` is an arbitrary ARC ownership-qualified pointer. Is there a way to do that? Clearly, the C++ standard library does not have such a type trait :-).

Louis

> 
> On 7/31/18 3:14 PM, Louis Dionne via cfe-dev wrote:
>> I'm trying to partially specialize a template on ARC-annotated pointer types, and I'm finding it doesn't work. Specifically, the following fails because the "specialization" for `T __strong*` is never considered, and we always fall back to the specialization for `T*`:
>> 
>>     // clang test.mm -fobjc-arc -ObjC++ -std=c++14
>> 
>>     #import <Foundation/NSObject.h>
>> 
>>     @interface Foo: NSObject
>>     { }
>>     @end
>> 
>>     template <typename T> struct Specialize;
>>     template <typename T> struct Specialize<T*> { static constexpr bool value = false; };
>>     template <typename T> struct Specialize<T __strong*> { static constexpr bool value = true; };
>>     static_assert(Specialize<Foo __strong*>::value, "");
>> 
>>     int main() { }
>> 
>> I have a few questions:
>> 
>> - Does this even make sense? I don't know much of Objective-C, so it's possible that treating ownership qualifiers like cv qualifiers is not even sensical.
>> - If it does make sense, why is it not supported? Is this simply a bug that needs fixing?
>> 
>> Strangely enough, I haven't found anything online regarding this, so I'm turning to this list as a last resort.
>> 
>> Thanks,
>> Louis
>> 
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>