Unable to add object to NSMutableSet (error: no visible interface)

Question

I am using XCode 5 and iOS 7 on my iPad app. I am trying to save row indexes that have been selected by the user. In -didSelectRowAtIndexPath, I use this code to mark the rows:

//  initialize singleton
SingletonSelectedCellIndexes *selectedCellIndexes = [SingletonSelectedCellIndexes sharedSelectedCellIndexes];  //  initialize

//  get the cell that was selected
UITableViewCell *theCell = [tableView cellForRowAtIndexPath:indexPath];

if ([[tableView cellForRowAtIndexPath:indexPath] accessoryType] == UITableViewCellAccessoryCheckmark) {
    [(NSMutableSet *)selectedCellIndexes addObject:indexPath];
}

Why am I getting this error on addObject to the singleton?:

No visible @interface for 'SingletonSelectedCellIndexes' declares the selector 'addObject:'

This is the code that defines the singleton (NSMutableSet):

//++++++++++++++++++++++++++++++++++++++++++++++++++++
//--  SingletonSelectedCellIndexes
@implementation SingletonSelectedCellIndexes  {

}

@synthesize selectedCellIndexes;  //  rename

//  sharedSelectedCellIndexes
+ (id)sharedSelectedCellIndexes  {

static dispatch_once_t dispatchOncePredicate = 0;
__strong static id _sharedObject = nil;
dispatch_once(&dispatchOncePredicate, ^{
    _sharedObject = [[self alloc] init];
});

return _sharedObject;
}

-(id) init {
self = [super init];
if (self) {
    selectedCellIndexes = [[NSMutableSet alloc] init];
}
return self;
}

@end

UPDATE - here is the .h file:

//--  singleton: selectedCellIndexes
@interface SingletonSelectedCellIndexes : NSObject  {
}
@property (nonatomic, retain) NSMutableSet *selectedCellIndexes;

+ (id)sharedSelectedCellIndexes;
@end

Answer 1

You need to show the .h file of your SingletonSelectedCellIndexes class before we can answer that.

Your .m file does not show an addObject method though, so I bet your .h file doesn't either.

If you want to call a method on a class, you need to declare and implement that method.