Making class conform to protocol with category for existing methods

I have a protocol named MyProtocol.
MyProtocol has an required method:

- (NSUInteger)length;

And some other methods.

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  • Now i want to make the NSString class conform to MyProtocol with a category. Like so:

    @interface NSString (NSStringWithMyProtocol) <MyProtocol>

    In this category i implement all methods excluding the ‘length’ method, because i want the original NSString implementation.
    I do not want to override it in this particular class.

    Now i get a warning because of an incomplete implementation of MyProtocol in the category.

    I know there are a few solutions to solve this.

    1. Make the method optional
    2. Pointer Swizzling
    3. Adding subclass to class which is conform to the protocol. Then leave out the implementation.

    I do not want to use these options because they result in a bad design for the rest of my code.
    Option 3 is bad, because of existing direct subclasses will not be conform to the protocol.

    Does anybody know how to remove the warning without implementing the length method?

    NOTE: The class, category and protocol are just examples. I did encounter this problem with other classes which i could not post about.

    EDIT: Added the third option.

    Full Code:

    The protocol:

    @protocol MyProtocol <NSObject>
    - (void) myMethod;
    - (NSInteger) length;

    The category header:

    #import <Foundation/Foundation.h>
    #import "MyProtocol.h"
    @interface NSString (MyProtocol) <MyProtocol>

    The category implementation:

    @implementation NSString (MyProtocol)
    - (void)myMethod {

    This results in the following warnings.

    Incomplete implementation
    Method in protocol not implemented

    In this screenshot you can see my warning:
    Screen shot

    I tried compiling with LLVM GCC 4.2 and the Apple LLVM 3.0 compiler.
    I also compiled on xcode 4.0.2 and Xcode 4.2.
    I’m on OS X 10.6.8.

    2 Solutions Collect From Internet About “Making class conform to protocol with category for existing methods”

    I cannot reproduce this issue. Can you post code that demonstrates it? The following compiles without warnings on 10.7.

    #import <Foundation/Foundation.h>
    @protocol MyProtocol <NSObject>
    - (NSUInteger)length;
    @interface NSString (NSStringWithMyProtocol) <MyProtocol>
    int main (int argc, const char * argv[]) {
      @autoreleasepool {
        id<MyProtocol> foo = @"foo";
        NSLog(@"%@", foo);    
      return 0;

    You might consider this a bit of a hack, and I don’t know if it will even work, but how about declaring it as a read only property

    @property (readonly, assign) NSUInteger length;

    and then in the implementation of your category, make it @dynamic