design patterns - What is your threshold to use factory instead of a constructor to create an object?

Question

What is your threshold to use factory instead of a constructor to create an object?

1. You always use factory.
2. You use factories only if you have invariant checks other than checking for nulls.
3. You always use constructors
4. You rarely use factories... what are those cases??

pros and cons

Update: I am applying factory pattern from Domain Driven Design in my project. And one of the reason behind creating factories is to reduce noise in the domain model.

Thanks

Answer 1

I use a factory if I have an abstract base class (or an interface), and several concrete derived classes, and there is some logic according to which one of concrete classes is created. I implement that logic in the factory.