String Shift and Print

After my last blog post, someone pointed out a similar problem here. The problem goes:

Write a program that generates from the string "abcdefghijklmnopqrstuvwxyz{" the following:

a

bcb

cdedc

defgfed

efghihgfe

fghijkjihgf

ghijklmlkjihg

hijklmnonmlkjih

ijklmnopqponmlkji

jklmnopqrsrqponmlkj

klmnopqrstutsrqponmlk

lmnopqrstuvwvutsrqponml

mnopqrstuvwxyxwvutsrqponm

nopqrstuvwxyz{zyxwvutsrqpon

Well there is no time limit this time and we dont even need to use google. I think this is slightly trickier than the last one.

Here is my solution:

//Program tested on Microsoft Visual Studio 2008 - Zahid Ghadialy

#include<iostream>
#include<string>

using namespace std;

int main()
{
  string str = "abcdefghijklmnopqrstuvwxyz{";
  unsigned strlen = str.length();
  string tempStr;
  int loop = 0;
  while(tempStr.length() < strlen)
  {
    int start1 = loop;
    int count1 = loop + 1;
    tempStr = str.substr(start1,count1);

    count1--;
    for(; count1 > 0; count1--)
      tempStr.append(str.substr(start1+count1-1, 1));

    cout<<tempStr;
    cout<<endl;
    loop++;
  }
  return 0;
}

ASCII Print

Few weeks back, someone I know got asked the following question in an interview:

Write a C++ program in 15 minutes to print the following in the output:

a

bcb

cdedc

defgfed

efghihgfe

fghijkjihgf

ghijklmlkjihg

hijklmnonmlkjih

ijklmnopqponmlkji

jklmnopqrsrqponmlkj

klmnopqrstutsrqponmlk

lmnopqrstuvwvutsrqponml

mnopqrstuvwxyxwvutsrqponm

nopqrstuvwxyz{zyxwvutsrqpon

You can do search on the web for any information but all your searches would be logged and used to assess your answer.

Well my obvious guess would be to take help of ASCII table. My only search was for Ascii table in google. I did try and make it in less than 15 mins. My solution is below but do try it on your own before looking at my solution.

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

using namespace std;

int main()
{
  char c=(char)97; //'a' in ASCII
  char loop = 0;
  while(c <= 'n')
  {
    char x = 0;
    for(; x <=loop; x++)
    {
      cout<<char(c+x);
    }
    for(char y = 1; y < x; y++)
    {
      cout<<char(c+x-y-1);
    }
    cout<<endl;
    c++;
    loop++;
  }

  return 0;
}

Difference between Deep copy and Shallow copy in C++

So what is the difference between 'Deep copy' (sometimes referred to as 'Hard copy') and 'Shallow copy' in C++

Shallow copy: Copies the member values from one object into another.

Deep Copy: Copies the member values from one object into another. Any pointer objects are duplicated and Deep Copied.

There are some interesting discussions on Stack Overflow here and here. A related discussions here is interesting as well.

Program to demonstrate as follows:

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

using namespace std;

class MyString
{
public:
  MyString(){}
  MyString(string str)
  {
    size = str.size();
    data = new char(size+1); //+1 for '\0'
    memcpy(data, str.c_str(), size+1);
  }
  //MyString(const MyString& copy); - Use default for Shallow
  void DeepCopy(const MyString& copy) //Deep Copy
  {
    size = copy.size;
    data = new char(size+1); //+1 for '\0'
    memcpy(data, copy.data, size+1);
  }
  int   size;
  char* data;
};

int main()
{
  MyString m1("Zahid");
  cout<<"\nm1.size = "<<m1.size<<", &m1.data = "<<(void *)m1.data<<", *m1.data = "<<m1.data<<endl;

  MyString m2(m1); //Uses the default copy constructor - Shallow Copy
  cout<<"\nm2.size = "<<m2.size<<", &m2.data = "<<(void *)m2.data<<", *m2.data = "<<m2.data<<endl;

  MyString m3; //Another default constructor
  m3.DeepCopy(m1);
  cout<<"\nm3.size = "<<m3.size<<", &m3.data = "<<(void *)m3.data<<", *m3.data = "<<m3.data<<endl;

  return 0;
}

Output as follows:

Difference between 'new', 'new operator' and 'operator new' in C++

A question asked in a forum said:

Why does void* p = new (1024); gives me a compilation error while void* p = operator new (1024); works. What is the difference between new and "operator new"?

Lets go back to the beginning. Once upon a time there was this C language that used 'malloc' to allocate memory. Then when C++ came, a new way was defined of allocating memory by the use of 'new'. Its supposed to be much safter and better way but some software gurus may differ. Memory for 'new' is allocated from 'Free Store' and memory by 'malloc' is generated from 'heap'. The 'heap' and 'Free Store' may be the same area and is a compiler implementation detail.

The syntax for 'operator new' is:

void* new (std::size_t size);

All it does is allocate a memory of size specified

On the other hand, 'new' does 2 things:

1. It calls 'operator new'

2. It calls constructor for the type of object

In the above case since 1024 is not a type, it will fail in calling its constructor.

'new' is also referred to as 'keyword new' or 'new operator' to cause more confusion :)

Here is a small example to play with 'operator new' after the example you will realise that regardless of what is passed, it always allocates a 4 byte memory.

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

using namespace std;

int main()
{
 //void* p = new (1024);

 void* p = operator new (1024);

 //Lets test if we can do anything with the memory
  cout<<"p = "<<p<<endl;
 cout<<"sizeof(p) = "<<sizeof(p)<<endl;
 int *x = static_cast<int *>(p);
 cout<<"*x before = "<<*x<<endl;
 *x = 23456;
 cout<<"*x after = "<<*x<<endl;

 void* q = operator new (0);
 cout<<"\nq = "<<q<<endl;
 cout<<"sizeof(q) = "<<sizeof(q)<<endl;
 x = static_cast<int *>(q);
 cout<<"*x before = "<<*x<<endl;
 *x = 65432;
 cout<<"*x after = "<<*x<<endl;

 void* z = operator new ('a');
 cout<<"\nz = "<<z<<endl;
 cout<<"sizeof(z) = "<<sizeof(z)<<endl;
 x = static_cast<int *>(z);
 cout<<"*x before = "<<*x<<endl;
 *x = 11111;
 cout<<"*x after = "<<*x<<endl;

 return 0;
}

The output is as follows:

My knowledge in this area is quite limited so please feel free to improve on my explanation or correct it.

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:

'new operator' throw and nothrow

I used to always think and while coding take care that to check if memory allocation is successful, we need to check if the returned pointer is not NULL. Well, in MSVC8 this may not necessarily be true. Depending on the library you include (see here) you may end up with either a throw or a NULL. The partial good news is that you can in future write your code to not throw memory alloc failure. See example:

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

using namespace std;

int main()
{
  try
  {
    char* p = new char[0x7fffffff];
    if (!p)
    {
      cout<<"Memory allocation for q failed. NULL returned."<<endl;
    }
  }
  catch(...)
  {
    cout<<"Exception caught by ..."<<endl;
  }

  char* q = new (std::nothrow) char[0x7fffffff];
  if (!q)
  {
    cout<<"Memory allocation for q failed. NULL returned."<<endl;
  }

  return 0;
}

The output is as follows:

Memory Allocation for char*

One common problem that I have noticed is that people copy char* pointers thinking that the pointers will be valid when used but this is rarely the case. See the example below.

//Program tested on Microsoft Visual Studio 2008 - Zahid Ghadialy
//Example demonstrates allocation of memory for char*
#include<iostream>

using namespace std;

typedef struct {
  char const * name;
} TypeInfo;

TypeInfo* MakeType(const char *typeName)
{
  TypeInfo* newNamedTypeInfo = new TypeInfo;

  char* newTypeName = new char[strlen(typeName) + 1]; 
  strcpy(newTypeName, typeName);
  newNamedTypeInfo->name = newTypeName;

  return newNamedTypeInfo;
}

int main()
{
  TypeInfo* testType = MakeType("Apple Banana");
  cout << "testType->name = " << testType->name << endl;

  return 0;
}

Please suggest any improvements.

The output is as follows:

Note that there is a memory leak in the program if you can spot it and fix it then you are already half way through :)

C++ example of Proxy Design Pattern

The Proxy pattern is used when you need to represent a complex object by a simpler one. If creating an object is expensive in time or computer resources, Proxy allows you to postpone this creation until you need the actual object. A Proxy usually has the same methods as the object it represents, and once the object is loaded, it passes on the method calls from the Proxy to the actual object.

There are several cases where a Proxy can be useful:

1. If an object, such as a large image, takes a long time to load.
2. If the object is on a remote machine and loading it over the network may be slow, especially during peak network load periods.
3. If the object has limited access rights, the proxy can validate the access permissions for that user.

Proxies can also be used to distinguish between requesting an instance of an object and the actual need to access it. For example, program initialization may set up a number of objects which may not all be used right away. In that case, the proxy can load the real object only when it is needed.


The Proxy provides a surrogate or place holder to provide access to an object. A check or bank draft is a proxy for funds in an account. A check can be used in place of cash for making purchases and ultimately controls access to cash in the issuer’s account.



The frequency of use of Proxy is medium high

Lets look at the example below. Note that the actual Math class is created in the MathProxy when math operations need to be performed.

//Program tested on Microsoft Visual Studio 2008 - Zahid Ghadialy
//Proxy is part of Structural Patterns 
//Structural Patterns deal with decoupling the interface and implementation of classes and objects
//A Proxy provides a surrogate or placeholder for another object to control access to it.  

//The example we consider here a math class
//The proxy provides an interface but the real class is only initiated when it is used

#include<iostream>
#include<string>

using namespace std;


// The 'Subject interface
class IMath
{
public:
  virtual double Add(double x, double y) = 0;
  virtual double Sub(double x, double y) = 0;
  virtual double Mul(double x, double y) = 0;
  virtual double Div(double x, double y) = 0;
};

// The 'RealSubject' class
class Math : public IMath
{
public:
  double Add(double x, double y)
  {
    return x + y;
  }
  double Sub(double x, double y)
  {
    return x - y;
  }
  double Mul(double x, double y)
  {
    return x * y;
  }
  double Div(double x, double y)
  {
    return x / y;
  }
};

// The 'Proxy Object' class
class MathProxy : public IMath
{
public:
  MathProxy()
  {
    math_ = NULL;
  }
  virtual ~MathProxy()
  {
    if(math_)
      delete math_;
  }
  double Add(double x, double y)
  {
    return getMathInstance()->Add(x, y);
  }
  double Sub(double x, double y)
  {
    return getMathInstance()->Sub(x, y);
  }
  double Mul(double x, double y)
  {
    return getMathInstance()->Mul(x, y);
  }
  double Div(double x, double y)
  {
    return getMathInstance()->Div(x, y);
  }
private:
  Math* math_;
  Math* getMathInstance(void)
  {
    if(!math_)
      math_ = new Math();
    return math_;
  }
};

//The Main method
int main()
{
  // Create math proxy
  MathProxy proxy;

  //Do the math
  cout<<"4 + 2 = "<<proxy.Add(4, 2)<<endl;
  cout<<"4 - 2 = "<<proxy.Sub(4, 2)<<endl;
  cout<<"4 * 2 = "<<proxy.Mul(4, 2)<<endl;
  cout<<"4 / 2 = "<<proxy.Div(4, 2)<<endl;

  return 0;
}

The output is as follows:



For more information see:

http://sourcemaking.com/design_patterns/proxy

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

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