The Strategy Pattern

• Encapsulates an algorithm from a family
• Allows algorithm to vary independently of the client
• Example:

class DistanceBase
{
public:
    virtual ~DistanceBase() {}
    virtual double distance(const RD_Vector&) = 0;
};

class L1Distance : public DistanceBase
{
    virtual double distance(const RD_Vector& v)
    { 
        double sum=0.0;
        for (int i=1;i<=v.nelems();++i) sum+=fabs(v);
        return sum;
    }
};

class L2Distance : public DistanceBase
{
    virtual double distance(const RD_Vector& v)
    { return sqrt(dot(v,v)); }
};

Client:

class NearestNeighbour
{
private:
    DistanceBase* _d;
public:
    NearestNeighbour(DistanceBase& d) : _d(&d) {}
    int nearest(const RD_Array<RD_Vector>& data,
                const RD_Vector& v);
        // Uses _d->distance(dv)
};

We can change the functionality without changing the implementation:

L1Distance distance1;
L2Distance distance2;
NearestNeighbour nearest1(distance1);  // Uses L1 metric
NearestNeighbour nearest2(distance2);  // Uses L2 metric