Singleton in Action?

[Kagamin]

On Sunday, 3 February 2019 at 14:42:13 UTC, Ron Tarrant wrote:
> This morning I was Googling "singleton replacement" and someone 
> on another forum said Factory would do the job. Anyone have 
> thoughts on that?

It's usually replaced with inversion of control: the service 
instance is passed as an argument to object constructors, a 
related pattern is service provider when you need to access many 
services. Though it needs a more involved code organization 
compared to singleton. As for testing, singletons can be tested 
if you provide a setter method that allows to set a custom 
singleton instance.