Member functions for derived information in a class

Question

While designing an interface for a class I normally get caught in two minds whether should I provide member functions which can be calculated / derived by using combinations of

Answer 1

The STL has basically aimed for small interfaces, so in your case, if and only if getSelectedDocs can be implemented more efficiently than a combination of isDocSelected and getDoc it would be implemented as a member function.

This technique may not be applicable anywhere but it's a good rule of thumbs to prevent clutter in interfaces.