通过函数参数获取对象的协议

Question

Let's say we have following C++ code:

struct ISomeInterface
{ 
  virtual ~ISomeInterface() {}   
  virtual void f() = 0;
};

class SomeClass : public ISomeInterface
{
public:
  void f() override
  {
      std::cout << "Hi";
  }
};

void getObject(ISomeInterface*& ptr)
{
  ptr = new SomeClass;
}

int main()
{
  ISomeInterface* p(nullptr);
  getObject(p);
  p->f();
  delete p;
}

It's quite straightforward and far from being perfect, but it draws the picture: getting a pointer to an interface to an object via function's parameters.

How do we get the same with Objective C protocols?

@protocol SomeProtocol <NSObject>
- (void)f;
@end

@interface SomeClass : NSObject<SomeProtocol>
- (void)f;
@end

@implementation SomeClass
- (void)f { NSLog(@"Hi"); }
@end

Thanks in advance.

Answer 1

C-style function:

id<SomeProtocol> getObject()
{
    return [SomeClass new];
}

Objective-C (class) function:

@implementation SomeOtherClass

+ id<SomeProtocol> getObject
{
    return [SomeClass new];
}

@end

Answer 2

If you actually want the reference parameter, you can do:

void getObject(id<SomeProtocol> *ptr)
{
    if (ptr) {
        *ptr = [[SomeClass alloc] init];
    }
}

int main(int argc, char *argv[])
{
    @autoreleasepool {
        id<SomeProtocol> p = nil;
        getObject(&p);
        [p f];
        return UIApplicationMain(argc, argv, nil, NSStringFromClass([AppDelegate class]));
    }
}