Improved Implementation

Consider:

class NearestNeighbour
{
private:
    DistanceBase* _d;
public:
    NearestNeighbour(DistanceBase& d) : _d(&d) {}
...

• This uses a pointer to the external DistanceBase object
• The external object must be in scope at all times
• Nasty.
• There is an elegant solution: clone()