On 6/23/2011 1:08 AM, Ben Robinson wrote:

The Singleton pattern does have its uses because it guarantees a single instance of a class. However, it suffers from three weaknesses:

1) Encapsulation: Singleton provides global access to the class. Some may view this as a feature, but advocates of unit testing, and dependency injection would disagree. 2) Initialization: Singleton restricts the class to the default constructor. This makes Singleton unusable for classes which require non-default constructors. 3) Lifetime: Singleton introduces lifetime management complexities (for a detailed discussion, see Chapter 6 of Modern C++ Design by Andrei * Alexandrescu*).

I have formalized what I believe is a novel design pattern, which guarantees a single instance of a class, but which does not suffer 1-3 above. And unlike Monostate, this pattern I propose does instantiate a genuine instance of any class, using any one of its available constructors. I hope this inquiry peaks your interest.

I've often wondered about singleton's bad rap (other than the difficulty of making them reliable in C++). I use a singleton from the boost vault for hiding implementation details of globally accessible interfaces (e.g. an interface to access information about elements of the periodic table when I don't hard-code all the constants). This usage is unit testable as long as the implementation details can be thoroughly tested through the interface. It could support DI too if the dependencies are given to the singleton's methods instead of at construction. I don't understand weakness #2. My usage of singletons makes initialization an implementation detail. Construction is not part of the singleton's interface because constructing it different ways from different parts of the program would mean having more than a single instance. #3 is a indeed a bitch with C++. The boost vault solution I use makes it quite easy to chain singletons though. Nevertheless, my interest is piqued. Thanks, -Matt