gistfile1.m

#define CT_SHARED_INSTANCE_USING_BLOCK(block) \
static dispatch_once_t pred = 0; \
__strong static id _sharedObject = nil; \
dispatch_once(&pred, ^{ \
_sharedObject = block(); \
}); \
return _sharedObject;


+ (SharedThing *)sharedInstance {
    CT_SHARED_INSTANCE_USING_BLOCK(^{
        return [[self alloc] init];
    });
}