C++ example of State Design Pattern

The State pattern allows an object to change its behavior when its internal state changes. This pattern can be observed in a vending machine. Vending machines have states based on the inventory, amount of currency deposited, the ability to make change, the item selected, etc. When currency is deposited and a selection is made, a vending machine will either deliver a product and no change, deliver a product and change, deliver no product due to insufficient currency on deposit, or deliver no product due to inventory depletion.

The frequency of use of State Pattern is Medium but is very useful and frequently used Telecoms Protocols implementation.

Example as follows:

//Program tested on Microsoft Visual Studio 2008 - Zahid Ghadialy
//State is part of Behavioral Patterns
//Behavioral Patterns deal with dynamic interactions among societies of classes and objects
//State allows an object to alter its behavior when its internal state changes.
//     The object will appear to change its class 

//We will take an example Bank card where depending on the deposit the customers status changes


#include<iostream>
#include<string>
#include "main.h"


Account* State::GetAccount(void)
{
 return account_;
}
void State::SetAccount(Account* account)
{
 account_ = account;
}

double State::GetBalance(void)
{
 return balance_;
}

void State::SetBalance(double balance)
{
 balance_ = balance;
}

string State::GetStateName(void)
{
 return stateName_;
}

RedState::RedState(State* state)
{
 this->balance_ = state->GetBalance();
 this->account_ = state->GetAccount();
 Initialise();
}

void RedState::Deposit(double amount)
{
 balance_ += amount;
 StateChangeCheck();
}

void RedState::Withdraw(double amount)
{
 double newAmount = amount + serviceFee_;
 if(balance_ - newAmount < lowerLimit_)
   cout<<"No funds available for withdrawal!"<<endl;
 else
   balance_ -= newAmount;
}

void RedState::PayInterest()
{
 //No interest is paid
}

void RedState::StateChangeCheck()
{
 if (balance_ > upperLimit_)
 {
   account_->SetState(reinterpret_cast<State*>(new SilverState(this)));
   delete this;
   return;
 }
}

void RedState::Initialise()
{
 stateName_ = "Red";
 //Should come from a data source
  interest_ = 0.0;
 lowerLimit_ = -100.0;
 upperLimit_ = 0.0;
 serviceFee_ = 15.0;
}

SilverState::SilverState(State* state)
{
 this->balance_ = state->GetBalance();
 this->account_ = state->GetAccount();
 Initialise();
}

SilverState::SilverState(double balance, Account* account)
{
 this->balance_ = balance;
 this->account_ = account;
 Initialise();
}

void SilverState::Deposit(double amount)
{
 balance_ += amount;
 StateChangeCheck();
}

void SilverState::Withdraw(double amount)
{
 balance_ -= amount;
 StateChangeCheck();
}

void SilverState::PayInterest()
{
 balance_ = balance_ * interest_;
 StateChangeCheck();
}

void SilverState::StateChangeCheck()
{
 if (balance_ < lowerLimit_)
 {
   account_->SetState(reinterpret_cast<State*>(new RedState(this)));
   delete this;
   return;
 }
 else if (balance_ > upperLimit_)
 {
   account_->SetState(reinterpret_cast<State*>(new GoldState(this)));
   delete this;
   return;
 }
}

void SilverState::Initialise()
{
 stateName_ = "Silver";
 //Should come from a data source
  interest_ = 1.0;
 lowerLimit_ = 0.0;
 upperLimit_ = 1000.0;
}

GoldState::GoldState(State* state)
{
 this->balance_ = state->GetBalance();
 this->account_ = state->GetAccount();
 Initialise();
}

void GoldState::Deposit(double amount)
{
 balance_ += amount;
 StateChangeCheck();
}

void GoldState::Withdraw(double amount)
{
 balance_ -= amount;
 StateChangeCheck();
}

void GoldState::PayInterest()
{
 balance_ = balance_ * interest_;
 StateChangeCheck();
}

void GoldState::StateChangeCheck()
{
 if (balance_ < 0.0)
 {
   account_->SetState(reinterpret_cast<State*>(new RedState(this)));
   delete this;
   return;
 }
 else if (balance_ < lowerLimit_)
 {
   account_->SetState(reinterpret_cast<State*>(new SilverState(this)));
   delete this;
   return;
 }
 else if (balance_ > upperLimit_)
 {
   cout<<"Your account is too big now. Please consider using Swiss banks"<<endl;
 }
}

void GoldState::Initialise()
{
 stateName_ = "Gold";
 //Should come from a data source
  interest_ = 5.0;
 lowerLimit_ = 1000.0;
 upperLimit_ = 10000000.0;
}

Account::Account(string owner):owner_(owner)
{
 state_ = reinterpret_cast<State*>(new SilverState(0.0, this)); //default
}

Account::~Account()
{
 delete state_;
}

double Account::GetBalance(void)
{
 return state_->GetBalance();
}

void Account::Deposit(double amount)
{
 state_->Deposit(amount);
 cout<<"Deposited $"<<amount<<endl;
 cout<<"Balance   $"<<GetBalance()<<endl;
 cout<<"Status     "<<state_->GetStateName()<<endl;
 cout<<"\n";
}

void Account::Withdraw(double amount)
{
 state_->Withdraw(amount);
 cout<<"Withdrew  $"<<amount<<endl;
 cout<<"Balance   $"<<GetBalance()<<endl;
 cout<<"Status     "<<state_->GetStateName()<<endl;
 cout<<"\n";
}

void Account::PayInterest()
{
 state_->PayInterest();
 cout<<"Interest Paid --------"<<endl;
 cout<<"Balance   $"<<GetBalance()<<endl;
 cout<<"Status     "<<state_->GetStateName()<<endl;
 cout<<"\n";
}

void Account::SetState(State* state)
{
 state_ = state;
}

State* Account::GetState(void)
{
 return state_;
}


//The Main method
int main()
{
 Account* account = new Account("Dr. Who");
 account->Withdraw(10.00);
 account->Withdraw(30.00);
 account->Withdraw(70.00);
 account->Deposit(234.00);
 account->Deposit(5000.00);
 account->Withdraw(5200.00);
 account->Deposit(1500.00);
 account->Deposit(1.00);
 account->Withdraw(1200.00);

 delete account;

 return 0;
}

EDIT: main.h (added at a later date than the post)

 
using namespace std;
 
//Forward Declaration
class Account;
 
// The 'State' abstract class
class State
{
public:
  Account* GetAccount(void);
  void SetAccount(Account* account);
  double GetBalance(void);
  void SetBalance(double balance);
  string GetStateName(void);
  virtual void Deposit(double amount)=0;
  virtual void Withdraw(double amount)=0;
  virtual void PayInterest(void) = 0;
protected:
  Account* account_;
  double balance_;
  double interest_;
  double lowerLimit_;
  double upperLimit_;
  string stateName_;;
};
 
// A 'ConcreteState' class
// Red indicates that account is overdrawn
class RedState : State
{
public:
  RedState(State* state);
  void Deposit(double amount);
  void Withdraw(double amount);
  void PayInterest();
  void StateChangeCheck();
 
private:
 
  RedState(); //Not allowed
  void Initialise();
  double serviceFee_;
};
 
// A 'ConcreteState' class
// Silver indicates less interest bearing state
class SilverState : State
{
public:
  SilverState(State* state);
  SilverState(double balance, Account* account);
  void Deposit(double amount);
  void Withdraw(double amount);
  void PayInterest();
  void StateChangeCheck();
 
private:
  SilverState(); //Not allowed
  void Initialise();
};
 
// A 'ConcreteState' class
// Gold indicates high interest bearing state
class GoldState : State
{
public:
  GoldState(State* state);
  void Deposit(double amount);
  void Withdraw(double amount);
  void PayInterest();
  void StateChangeCheck();
 
private:
  GoldState(); //Not allowed
  void Initialise();
};
 
 
// The 'Context' class - defined here as its used for forward declaration
class Account
{
public:
  Account(string owner);
  ~Account();
  double GetBalance(void);
  void Deposit(double amount);
  void Withdraw(double amount);
  void PayInterest();
  void SetState(State* state);
  State* GetState(void);
private:
  State* state_;
  string owner_;
  Account();
};
 

The output is as follows:

Further reading:

http://www.dofactory.com/Patterns/PatternState.aspx

http://www.patterndepot.com/put/8/state.pdf

http://sourcemaking.com/design_patterns/state

C++ example of Observer Design Pattern

Definition: Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.

Observers register themselves with the Subject as they are created. Whenever the Subject changes, it broadcasts to all registered Observers that it has changed.

The Observer defines a one-to-many relationship so that when one object changes state, the others are notified and updated automatically. Some auctions demonstrate this pattern. Each bidder possesses a numbered paddle that is used to indicate a bid. The auctioneer starts the bidding, and “observes” when a paddle is raised to accept the bid. The acceptance of the bid changes the bid price which is broadcast to all of the bidders in the form of a new bid.

Observer is a very popular pattern and its frequency of use is very high.

The following is an example of Observer Design Pattern:

//Program tested on Microsoft Visual Studio 2008 - Zahid Ghadialy
//Observer is part of Behavioral Patterns 
//Behavioral Patterns deal with dynamic interactions among societies of classes and objects
//An Observer is a way of notifying change to a number of classes.  

//We will take an example of Stock Price where, Observers can register to be told about
//the stock price change of a company

#include<iostream>
#include<string>
#include<list>

using namespace std;


//Forward Declaration
class Stock;

// The 'Observer' interface
class IInvestor
{
public:
  virtual void Update(Stock* stock){};
};

// The 'Subject' abstract class
class Stock
{
public:
  Stock(string symbol, double price) : symbol_(symbol), price_(price) { }
  void Attach(IInvestor* investor)
  {
    investors_.push_back(investor);
  }
  void Detach(IInvestor* investor)
  {
    investors_.remove(investor);
  }
  void Notify()
  {
    list<IInvestor*>::iterator it = investors_.begin();
    while(it != investors_.end())
    {
      (*it)->Update(this);
      ++it;
    }
  }
  double GetPrice(void)
  {
    return price_;
  }
  void SetPrice(double price)
  {
    price_ = price;
    Notify();
  }
  string GetSymbol(void)
  {
    return symbol_;
  }

private:
  string symbol_;
  double price_;
  list<IInvestor*> investors_;

  Stock();
};

// The 'ConcreteSubject' class
class Company : public Stock
{
public:
  Company(string name, string symbol, double price) : name_(name), Stock(symbol, price) {}
  string GetName(void)
  {
    return name_;
  }
private:
  string name_;
};

// The 'ConcreteObserver' class
class Investor : public IInvestor
{
public:
  Investor(string name) : name_(name){}
  void Update(Stock* stock)
  {
    cout<<"Notified "<<name_<<" about "<<(reinterpret_cast<Company*>(stock))->GetName() \
              <<" change to "<<stock->GetSymbol()<<stock->GetPrice()<<endl;
  }
private:
  string name_;
  Investor();
};

//The Main method
int main()
{
  Company* c1 = new Company("Google", "$", 123.0);
  cout<<"Created company Google with Stock Price 123.0\n"<<endl;

  Investor* i1 = new Investor("Billy");
  c1->Attach(i1);
  cout<<"Created investor Billy following Google\n"<<endl;

  c1->SetPrice(125.0);

  Investor* i2 = new Investor("Timmy");
  c1->Attach(i2);
  Investor* i3 = new Investor("Lenny");
  c1->Attach(i3);
  cout<<"\nCreated investor Timmy and Lenny following Google\n"<<endl;

  c1->SetPrice(145.0);

  c1->Detach(i1);
  c1->Detach(i3);
  cout<<"\nInvestor Billy and Lenny not interested in Google anymore\n"<<endl;

  c1->SetPrice(165.0);

  delete i1;
  delete i2;
  delete i3;
  delete c1;

  return 0;
}

The output is as follows:
Further Reading:

http://www.dofactory.com/Patterns/PatternObserver.aspx#_self2

http://sourcemaking.com/design_patterns/observer

http://www.patterndepot.com/put/8/observer.pdf

Number of bits to represent an arbitrary positive number X

Simple program to find the number of bits required to represent a positive number.

//This program is to find out the number of bits required to represent a positive integer
//Program tested on Microsoft Visual Studio 2008 - Zahid Ghadialy

#include<iostream>
#include <cmath>

using namespace std;

unsigned int TraditionalApproach(const unsigned int& num)
{
  if(num)
  {
    return (int)floor(log((double)num)/log(2.0) + 1.0);
  }
  return 1;
}

unsigned int SimplifiedApproach(const unsigned int& num)
{
  if(num)
  {
    unsigned int tempNum = num;
    unsigned int numOfBits = 0;
    while(tempNum)
    {
      numOfBits++;
      tempNum >>= 1;
    }
    return numOfBits;
  }
  return 1;
}


int main()
{
  //Finding number of bits the traditional way
  cout<<"\n** Traditional Approach **\n";
  cout<<"The number of bits required to represent 0 = "<<TraditionalApproach(0)<<endl;
  cout<<"The number of bits required to represent 1 = "<<TraditionalApproach(1)<<endl;
  cout<<"The number of bits required to represent 15 = "<<TraditionalApproach(15)<<endl;
  cout<<"The number of bits required to represent 16 = "<<TraditionalApproach(16)<<endl;
  cout<<"The number of bits required to represent 75 = "<<TraditionalApproach(75)<<endl;
  cout<<"The number of bits required to represent 125 = "<<TraditionalApproach(125)<<endl;
  cout<<"The number of bits required to represent 130 = "<<TraditionalApproach(130)<<endl;

  cout<<"\n** Simplified Approach **\n";
  cout<<"The number of bits required to represent 0 = "<<SimplifiedApproach(0)<<endl;
  cout<<"The number of bits required to represent 1 = "<<SimplifiedApproach(1)<<endl;
  cout<<"The number of bits required to represent 15 = "<<SimplifiedApproach(15)<<endl;
  cout<<"The number of bits required to represent 16 = "<<SimplifiedApproach(16)<<endl;
  cout<<"The number of bits required to represent 75 = "<<SimplifiedApproach(75)<<endl;
  cout<<"The number of bits required to represent 125 = "<<SimplifiedApproach(125)<<endl;
  cout<<"The number of bits required to represent 130 = "<<SimplifiedApproach(130)<<endl;

  return 0;
}

The output is as follows:

Memory Management with 'new'

Sometimes it is useful to take memory management in our control to make sure we have the right amount of memory already reserved in advance. This could be to speed up memory allocation/deallocation or for debugging purpose where contiguous memory allocation can speed up debugging or for variety of reasons.

The following example shows one way in which memory can be reserved in a chunk for the program.

//Program tested on Microsoft Visual Studio 2008 - Zahid Ghadialy
#include<iostream>

using namespace std;

class Class
{
public:
  int x;
  char y;
  bool z;
};

int main()
{
  unsigned char tempBuf[100];
  cout<<"Pointer for tempBuf = " << &tempBuf << endl;

  Class* c = new (tempBuf) Class;
  cout<<"Pointer for c = " << c << endl;

  return 0;
}

The output is as follows: