CODEWITHSUNDEEP
objective-cxcode
user241221
user241221

Reputation: 399

Why does adding a protocol (to id) cause semantic issues?

Let's say I have a trivial class like this:

@interface ABPair : NSObject
@property id key;
@property id value;
- (void) printSize;
@end

@implementation ABPair
- (void) printSize {
    NSLog(@"the size of your key is: %@", NSStringFromSize([self.key sizeWithAttributes: nil]));
}
@end

This compiles with no warnings (in Xcode 5), and runs successfully, and prints a reasonable value.

However, if I made this one change:

@property id<NSCopying> key;

then I get two compiler errors:

Why is the compiler able to identify the proper method (on NSString) when I provide no information at all about the type, but unable to identify the method when I say that the object must be of a protocol to which that class conforms?

Upvotes: 0

Views: 109

Answers (1)

Martin R
Martin R

Reputation: 540005

id key declares a "generic" Objective-C variable. It can point to any object, and the compiler accepts any messages sent to it. If you compile with ARC, it is only required that the message signature is known to the compiler at all (from any class).

id<myprotocol> key specifically declares a pointer to an object conforming to that protocol. The compiler accepts only messages from the <myprotocol> protocol (or other protocols that <myprotocol> inherits from).

Upvotes: 3

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