Default constructors

• We often need to be able to construct objects with no arguments, then set them up later.
• One must be able to check that they've been set up correctly when used.
• Solution: Access objects through functions
• eg: DistanceBase& distance()

class NearestNeighbour
{
private:
    DistanceBase* _d;
public:
    ...
    NearestNeighbour() : _d(NULL) {}
    void distance(const DistanceBase& d) { _d = d.clone(); }

    DistanceBase& distance();  // Checks _d is valid

    int nearest(const RD_Array<RD_Vector>& data, 
                const RD_Vector& v);
        // Uses distance().distance(dv)
};

Implementation: Return error if object not set up.

DistanceBase& NearestNeighbour::distance()
{  
   if (_d) return *_d;
   else 
   {
       cerr<<"NearestNeighbour::distance() : ";
       cerr<<"No object defined."<<endl;
       abort();
   }
}