Practical Uses for the “Curiously Recurring Template Pattern”

Question

What are some practical uses for the \"Curiously Recurring Template Pattern\"? The \"counted class\" example commonly shown just isn\'t a convincing example to me.

Answer 1

The CRTP gets a lot less curious if you consider that the subclass type that is passed to the superclass is only needed at time of method expansion. So then all types are defined. You just need the pattern to import the symbolic subclass type into the superclass, but it is just a forward declaration - as all formal template param types are by definition - as far as the superclass is concerned.

We use in a somewhat modified form, passing the subclass in a traits type structure to the superclass to make it possible for the superclass to return objects of the derived type. The application is a library for geometric calculus ( points, vectors, lines, boxes ) where all the generic functionality is implemented in the superclass, and the subclass just defines a specific type : CFltPoint inherits from TGenPoint. Also CFltPoint existed before TGenPoint, so subclassing was a natural way of refactoring this.