Design Pattern means that “we had some problem then we found the solution and gave name to this solution” and strategy is also one of the solution so we need to find the problem that strategy has solved and how ?
This design pattern comes under Behavioral Category (It tells that how objects are communicating with each other (objects have been created already)).
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C++ Strategy design pattern:
Standard Definition:
Strategy Design Pattern define a family of algorithms, encapsulate each one, and make then interchangeable. Strategy lets the algorithm vary independently from clients that use it.
Problem:
We want to separate out each algorithm and want to process the data in different way without disturbing existing code and possibility of switching behaviour during run time.
Solution:
Keep each algorithm in different object and process it separating, these algorithm can be interchangeable during run time on need basis(user wants to switch to different behaviour during run time), we can add new algorithm (new behaviour) by just adding new class, this solution called as “strategy design pattern“.
When we will use strategy pattern:
When user wants to do switch behaviour during run time. second there is probability of new behaviour can come in future.
Class Diagram:
Class Diagram of real example:
Source Code of real example in C++:
/*
* File:strategy.cpp
* This file is described the strategy Design Pattern with help of example
* Strategy pattern is structutal Pattern
* Author: Aplha Master
* Date: 31 Dec 2020
*/
//Header File
#include<iostream>
//TreadMill Class (Base class of different strategy)
class TreadMillStrategy
{
protected:
int m_speed;
public:
virtual void Start() = 0;
virtual void Stop() = 0;
};
class ChildStrategy:public TreadMillStrategy
{
public:
ChildStrategy(int speed){ m_speed = speed;}
~ChildStrategy(){}
void Start()
{
if(SpeedRestriction())
{
std::cout<<"Child TreadMill Mode Started, speed:"<<m_speed<<std::endl;
//Call the Driver api to start the Motor
}
else
{
std::cout<<"Child TreadMill Mode Cannot start, speed is more"<<std::endl;
}
}
void Stop() {std::cout<<"Child TreadMill Mode Stop"<<std::endl;}
private:
bool SpeedRestriction()
{ return (m_speed < 10) ? true : false;}
};
//Adult TreadMill Mode
class AdultStrategy:public TreadMillStrategy
{
public:
AdultStrategy(int speed){ m_speed = speed; }
~AdultStrategy(){}
void Start()
{
std::cout<<"Adult TreadMill Mode Started, speed:"<<m_speed<<std::endl;
IncreaseSpeed();
std::cout<<"Adult TreadMill Mode speed increased, speed:"<<m_speed<<std::endl;
}
void Stop() {std::cout<<"Adult TreadMill Mode Stop"<<std::endl;}
private:
void IncreaseSpeed()
{ m_speed = m_speed + 10;}
};
//Controller
class Controller
{
TreadMillStrategy* m_st;
public:
Controller(TreadMillStrategy* tm): m_st(tm){}
~Controller(){}
void SetStrategy(TreadMillStrategy* tr)
{
m_st = tr;
std::cout<<"*************Changing Strategy at run tim*************"<<std::endl;
}
void StartInterface(){m_st->Start();}
void StopInterface(){m_st->Stop();}
};
//CLient
int main()
{
// Strategy Pattern is Structutal Pattern so
// Inside Controller we will not create of TreaMillStrategy
// Below creation of different strategy can be done by Factory Pattern
// Creation of different strategy is depend upon Client(HOw is using strategy)
std::cout<<"Strategy Design Pattern"<<std::endl;
TreadMillStrategy* tm = new ChildStrategy(5);
Controller* con = new Controller(tm);
con->StartInterface();
con->StopInterface();
///////////////////////Change Strategy at run time;
delete tm;
tm = new AdultStrategy(10);
con->SetStrategy(tm);
con->StartInterface();
con->StopInterface();
delete tm;
delete con;
return 0;
}
Compilation Command:
g++ -std=c++11 strategy.cpp
Output:
Strategy Design Pattern
Child TreadMill Mode Started, speed:5
Child TreadMill Mode Stop
*************Changing Strategy at run tim*************
Adult TreadMill Mode Started, speed:10
Adult TreadMill Mode speed increased, speed:20
Adult TreadMill Mode Stop