WRSides.h

#import <Foundation/Foundation.h>

typedef enum _WRSideIdentifier {
  kWRSideLeft  =  1 << 0,
  kWRSideRight =  1 << 1,
  kWRSideBottom = 1 << 2,
  kWRSideTop    = 1 << 3
} WRSideIdentifier

@interface WRSides : NSObject

+ (id)sidesNamed:(WRSideIdentifier)identifiers;

- (void)objectForSide:(WRSideIdentifier)identifier;

@end

WRSides.m

#import "WRSides.h"

@interface WRSides()

@property (nonatomic, retain) NSMutableDictionary *sideDictionary;

@end

@implementation WRSides

- (id)init;
{
    if (!(self = [super init]))
        return nil;
    
    sideDictionary = [[NSMutableDictionary dictionary] retain];

    return self;
}

+ (id)sidesNamed:(WRSideIdentifier)identifiers;
{
    WRSides *sides = [[[self alloc] init] autorelease];
    
    if (identifiers & kWRSideLeft) [sides.sideDictionary setObject:[MyLeftSide object] forKey:[NSNumber numberWithInt:kWRSideLeft];
    //... etc ...

    return sides;
}

- (void)objectForSide:(WRSideIdentifier)identifier;
{
    return [self.sideDictionary objectForKey:[NSNumber numberWithInt:identifier]];
}

@end