Objective C - “Duplicate declaration of method” compilation error

Question

I have this piece of code:

- (id) getSearchSuggestions:(NSString*)q;
- (NSOperationQueue*) getSearchSuggestions:(NSString*)q callback:(id<UserDelegate>)callback;
- (id) getSearchSuggestions;
- (NSOperationQueue*) getSearchSuggestions:(id<UserDelegate>)callback;

And I Xcode keeps showing me an error on the last line:

Duplicate declaration of method "getSearchSuggestions"

Why? It seems to me that the signatures are all different.

Answer 1

This signature:

- (id) getSearchSuggestions:(NSString*)q;

Is identical to this signature:

- (NSOperationQueue*) getSearchSuggestions:(id<UserDelegate>)callback;

All object pointers are id. So both of these are methods that take an object and return an object.

Examples of better names would be:

- (id)searchSuggestionsForQueryString:(NSString*)q; // Or ForTag, or whatever "q" is
- (NSOperationQueue*)searchOperationQueueForQuery:(NSString*)q callback:(id<UserDelegate>)callback;
- (id)fetchSearchSuggestions;
- (NSOperationQueue*)searchOperationQueueWithCallback:(id<UserDelegate>)callback;

It's not exactly clear why you return an operation queue here, but this is the kind of name you'd use for a method that did that.

Answer 2

Think of the corresponding selectors:

- (id) getSearchSuggestions:(NSString*)q;
getSearchSuggestions:

- (NSOperationQueue*) getSearchSuggestions:(NSString*)q callback:(id<UserDelegate>)callback;
getSearchSuggestions:callback:

- (id) getSearchSuggestions;
getSearchSuggestions

- (NSOperationQueue*) getSearchSuggestions:(id<UserDelegate>)callback;
getSearchSuggestions:

As you can see, the first and the last method have the same selector, hence the duplicate method declaration error. You need to disambiguate them by changing their names.

Answer 3

it is because these two have the same selector:

- (id) getSearchSuggestions:(NSString*)q;
- (NSOperationQueue*) getSearchSuggestions:(id<UserDelegate>)callback;

you must choose unique names for selectors.

Answer 4

You seem to be trying to overload methods like in Java. Objective-C doesn't have this capability (it basically doesn't work well with Objective-C's more dynamic type system). In Objective-C, the selector for a method is the entirety of how it is identified. Think of it as a message: "Call the method named getSearchSuggestions: and give it these arguments." There can't be multiple methods in the class called getSearchSuggestions: because the selector is the only thing the message dispatch system has to determine which method is called.