return Derived* from make_classifier not SupervisedClassifier

[Peter]

needs ticket:318

The SupervisedClassifier? interface requires inherited classes to have a function make_classifier returning a SupervisedClassifier?*. Some classes does this and some returns Derived*. What is the difference?

Well, Derived* can be converted to SupervisedClassifer?* but the other way around is not true. Say that you have a template with a Classifier e.g. EnsembleBuilder?. You wanna make a copy of your classifier prototype and typically you can do that using copy constructor

T* sc2 = new T(*sc);

However this will not work if T is SupervisedClassifer?. In that case you would need to use the make_classifier function, which works as a dynamic copy constructor. You change the code to

T* sc2 = sc->make_classifier();

This will work as long as make_classifer returns T*. If suddenly T is NCC and NCC::make_classifer returns SupervisedClassifer?*, it will not compile. Therefore make_classifer should return Derived*.