googletesting: Where Have All the Singletons Gone?

Where Have All the Singletons Gone?

Thursday, August 21, 2008

Miško HeverySingletons are Pathological Liars

Collaborator Graph: This is the graph of objects that would be emitted if you serialized your application. This shows which objects are aware of which others. (through object's fields)

Construction Graph: This graph shows which object created which other ones.

Call Graph: This graph shows which other methods each method calls. A stack-trace would be a single slice through this graph.

How to Think About the new Operator

Every object only has references to what it needs directly! No passing around of objects which are not directly needed by the code. There is no global state at all. Dependencies are obvious since each object only asks for what it needs.

If an object needs a reference to a new dependency it simply declares it. This change only affects the corresponding factory, and as a result, it is very isolated.

All of the new operators end up in the factories; application logic is devoid of new operators.

You group all of the objects with the same lifetime into a single factory (If the factory gets too big you can break it up into more classes, but you can still think of it as a single factory)

The problem of "how do I ensure that I only have one of something" is nicely sidestepped. You instantiate only a single ApplicationFactory in your main, and as a result, you only instantiate a single instance of all of your singletons.