Wednesday 24 November 2010

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




Wednesday 17 November 2010

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:




Wednesday 10 November 2010

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:

Wednesday 3 November 2010

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: