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

I agree with the answers from Konrad and jalf. Unless there is a significant benefit from having "getSelectedDocs" then it clutters the interface of DocContainer.

Adding this member triggers my smelly code sensor. DocContainer is obviously a container so why not use iterators to scan over individual documents?

class DocContainer
{
public:
  iterator begin ();
  iterator end ();

  // ...
  bool isDocSelected (Doc *) const;
};

Then, use a functor that creates the vector of documents as it needs to:

typedef std::vector  DocVector;
class IsDocSelected {
public:
  IsDocSelected (DocContainer const & docs, DocVector & results)
  : docs (docs)
  , results (results)
  {}

  void operator()(Doc & doc) const
  {
    if (docs.isDocSelected (&doc))
    {
      results.push_back (&doc);
    }
  }
private:
  DocContainer const & docs;
  DocVector & results;
};


void foo (DocContainer & docs)
{
  DocVector results;
  std :: for_each (docs.begin ()
    , docs.end ()
    , IsDocSelected (docs, results));
}

This is a bit more verbose (at least until we have lambdas), but an advantage to this kind of approach is that the specific type of filtering is not coupled with the DocContainer class. In the future, if you need a new list of documents that are "NotSelected" there is no need to change the interface to DocContainer, you just write a new "IsDocNotSelected" class.