@interface Settings : NSObject
+ (instancetype)shared;
- (id)objectForKeyedSubscript:
    (NSString *)name;
- (void)setObject:(id)value
    forKeyedSubscript:(NSString *)name;
@end

@implementation Settings {
    NSMutableDictionary *_values;
}

//subscripts are instance-only: the
//static version goes through a
//shared instance
+ (instancetype)shared {
    static Settings *instance;
    static dispatch_once_t once;
    dispatch_once(&once, ^{
        instance = [Settings new];
    });
    return instance;
}

- (instancetype)init {
    self = [super init];
    if (self) {
        _values =
            [NSMutableDictionary dictionary];
    }
    return self;
}

- (id)objectForKeyedSubscript:
    (NSString *)name {
    return _values[name];
}

- (void)setObject:(id)value
    forKeyedSubscript:(NSString *)name {
    _values[name] = value;
}
@end

Settings.shared[@"host"] = @"192.168.100.1";
Settings.shared[@"port"] = @3306;

NSString *host = Settings.shared[@"host"];
//host is "192.168.100.1"