C++-Singleton class

Any of those help Nothing, as you wrote your code--but a singleton is a class that only allows a single instance. Nothing in the second code snippet disallows instantiation of multiple instances.

it helps some times In C++11 there is a new facility to make once-only initialization reliable - std::call_once. You can find a good reference for it here. If you're working with a compiler that supports C++11, this gives you a nice platform-independent, standard way of accomplishing it.
At the platform dependent level, most operating systems provide a means of doing once-only initialization. E.g. in iOS or OS X you can use dispatch_once from Grand Central Dispatch. On Windows, there's a One-Time Initialization API available in Vista and later.

I wish did fix the issue. By default a class is singleton or not .. ,
By default a class is singleton or not .

enum You {
    INSTANCE;
}

I wish did fix the issue. When the quote says the "sole instance should be extensible by subclassing", they are talking about situations where:

help you fix your problem I am actually trying to implement something where a singleton class would extend a non-singleton class. ,
1) Is this possible in java?

public enum SomeSingleton { 
  INSTANCE;
  ...

List<Parent> someInstances = Arrays.asList(parentInstanceA, parentInstanceB, thatSingletonInstanceOfMySingleton);