Strategy Design Pattern

This is one of behavioural design pattern.
The strategy pattern is a design pattern that allows a set of similar algorithms to be defined and encapsulated in their own classes. The algorithm to be used for a particular purpose may then be selected at run-time according to your requirements.

The strategy pattern is used to create an interchangeable family of algorithms from which the required process is chosen at run-time. This allows the behavior of a program to change dynamically according to configuration details or user preferences. It also increases flexibility by allowing new algorithms to be easily incorporated in the future.

Checklist

1. Create a abstract base class which contains behaviors.
2. Implements the behaviors in derived classes.
3. Define a strategy class which aggregates with  behavior class.


Example: While traveling, we can choose any of transport service; bus, cab, train, bike. Each travel service have their estimate time & cost of travel. Traveler can run timely choose which service he/she can use while traveling. 

#include<iostream>
using namespace std;

class Vehicle
{
    protected:
        int costperkm;
    public:
        virtual void Travel(int distance)=0;
};

class Cab : public Vehicle
{
    public:
        Cab()
        {
            costperkm = 10;
        }
        void Travel(int distance)
        {
            cout<<"Trave by Cab : ";
            cout<<"Total cost "<<distance*costperkm<<endl;
        }
};

class Bus : public Vehicle
{
    public:
        Bus()
        {
            costperkm = 5;
        }
        void Travel(int distance)
        {
            cout<<"Travle by Bus : ";
            cout<<"Total cost "<<distance*costperkm<<endl;
        }
};

class Train : public Vehicle
{
    public:
        Train()
        {
            costperkm = 2;
        }
        void Travel(int distance)
        {
            cout<<"Travel by Train :";
            cout<<"Total cost "<<distance*costperkm<<endl;
        }
};

class Travel
{
    Vehicle *trans;

    public:
        void SetVehicle(Vehicle *v)
        {
            trans = v;
        }
        void TravelUser(int distance)
        {
            trans->Travel(distance);
        }
};

int main()
{
    Vehicle *cab = new Cab;
    Travel t;
    t.SetVehicle(cab);
    t.TravelUser(20);
}