OC

#import "SPManager.h"

@implementation SPManager

static SPManager *instance = nil;

+ (instancetype)shared {
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        instance = [[[self class] alloc] init];
    });
    return instance;
}

+ (instancetype)allocWithZone:(struct _NSZone *)zone {
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        instance = [super allocWithZone:zone];
    });
    return instance;
}

- (id)copyWithZone:(struct _NSZone *)zone {
    return instance;
}

@end

Swift

class SPManager {
    
    static let shared = SPManager()
    
    private init() { }
}