“extern”前应有说明符限定符列表

//Resources.h
#import <Foundation/Foundation.h>

@interface Resources : NSObject 
{
extern NSString * const MY_CONST;
}
@end 

//Resources.m
#import "Resources.h"

@implementation Resources

NSString * const MY_CONST = @"my constant";

@end

我需要做什么?

谢谢你