Wikibooks

C++ Programming/Code/Design Patterns/Behavioral Patterns

< C++ Programming | Code/Design Patterns
The latest reviewed version was checked on 30 August 2022. There are template/file changes awaiting review.

Behavioral Patterns

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Chain of Responsibility

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Chain of Responsibility pattern has the intent to avoid coupling the sender of a request to its receiver by giving more than one object a chance to handle the request. Chains the receiving objects and passes the requests along the chain until an object handles it. 

#include <iostream>

using namespace std;

class Handler {
    protected:
        Handler *next;

    public:
        Handler() { 
            next = NULL; 
        }

        virtual ~Handler() { }

        virtual void request(int value) = 0;

        void setNextHandler(Handler *nextInLine) {
            next = nextInLine;
        }
};

class SpecialHandler : public Handler {
    private:
        int myLimit;
        int myId;

    public:
        SpecialHandler(int limit, int id) {
            myLimit = limit;
            myId = id;
        }

        ~SpecialHandler() { }

        void request(int value) {
            if(value < myLimit) {
                cout << "Handler " << myId << " handled the request with a limit of " << myLimit << endl;
            } else if(next != NULL) {
                next->request(value);
            } else {
                cout << "Sorry, I am the last handler (" << myId << ") and I can't handle the request." << endl;
            }
        }
};

int main () {
    Handler *h1 = new SpecialHandler(10, 1);
    Handler *h2 = new SpecialHandler(20, 2);
    Handler *h3 = new SpecialHandler(30, 3);

    h1->setNextHandler(h2);
    h2->setNextHandler(h3);

    h1->request(18);

    h1->request(40);

    delete h1;
    delete h2;
    delete h3;

    return 0;
}

Command

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Command pattern is an Object behavioral pattern that decouples sender and receiver by encapsulating a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undo-able operations. It can also be thought as an object oriented equivalent of call back method.

Call Back: It is a function that is registered to be called at later point of time based on user actions. 

#include <iostream>

using namespace std;

/*the Command interface*/
class Command 
{
public:
	virtual void execute()=0;
};

/*Receiver class*/
class Light {

public:
	Light() {  }

	void turnOn() 
	{
		cout << "The light is on" << endl;
	}

	void turnOff() 
	{
		cout << "The light is off" << endl;
	}
};

/*the Command for turning on the light*/
class FlipUpCommand: public Command 
{
public:

	FlipUpCommand(Light& light):theLight(light)
	{

	}

	virtual void execute()
	{
		theLight.turnOn();
	}

private:
	Light& theLight;
};

/*the Command for turning off the light*/
class FlipDownCommand: public Command
{
public:   
	FlipDownCommand(Light& light) :theLight(light)
	{

	}
	virtual void execute() 
	{
		theLight.turnOff();
	}
private:
	Light& theLight;
};

class Switch {
public:
	Switch(Command& flipUpCmd, Command& flipDownCmd)
	:flipUpCommand(flipUpCmd),flipDownCommand(flipDownCmd)
	{

	}

	void flipUp()
	{
		flipUpCommand.execute();
	}

	void flipDown()
	{
		flipDownCommand.execute();
	}

private:
	Command& flipUpCommand;
	Command& flipDownCommand;
};

 
/*The test class or client*/
int main() 
{
	Light lamp;
	FlipUpCommand switchUp(lamp);
	FlipDownCommand switchDown(lamp);

	Switch s(switchUp, switchDown);
	s.flipUp();
	s.flipDown();
}

Interpreter

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Given a language, define a representation for its grammar along with an interpreter that uses the representation to interpret sentences in the language.


 

To do:
It would be good to refer the reader to lex and yacc, and/or its derivatives like flex and bison, for an alternate (time-tested?) approach to these problems. 



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

namespace wikibooks_design_patterns
{

//	based on the Java sample around here

typedef std::string String;
struct Expression;
typedef std::map<String,Expression*> Map;
typedef std::list<Expression*> Stack;

struct Expression {
    virtual int interpret(Map variables) = 0;
	virtual ~Expression() {}
};
 
class Number : public Expression {
private:
	int number;
public: 
	Number(int number)       { this->number = number; }
	int interpret(Map variables)  { return number; }
};
 
class Plus : public Expression {
    Expression* leftOperand;
    Expression* rightOperand;
public: 

    Plus(Expression* left, Expression* right) { 
        leftOperand = left; 
        rightOperand = right;
    }
    ~Plus(){ 
	delete leftOperand;
	delete rightOperand;
    }
 
    int interpret(Map variables)  { 
        return leftOperand->interpret(variables) + rightOperand->interpret(variables);
    }
};
 
class Minus : public Expression {
    Expression* leftOperand;
    Expression* rightOperand;
public: 
    Minus(Expression* left, Expression* right) { 
        leftOperand = left; 
        rightOperand = right;
    }
    ~Minus(){ 
	delete leftOperand;
	delete rightOperand;
    }
 
    int interpret(Map variables)  { 
        return leftOperand->interpret(variables) - rightOperand->interpret(variables);
    }
};
 
class Variable : public Expression {
    String name;
public: 
	Variable(String name)       { this->name = name; }
    int interpret(Map variables)  { 
        if(variables.end() == variables.find(name)) return 0;
        return variables[name]->interpret(variables); 
    }
};

//	While the interpreter pattern does not address parsing, a parser is provided for completeness.
 
class Evaluator : public Expression {
    Expression* syntaxTree;
 
public:
	Evaluator(String expression){
        Stack expressionStack;

	size_t last = 0;
	for (size_t next = 0; String::npos != last; last = (String::npos == next) ? next : (1+next)) {
	    next = expression.find(' ', last);
	    String token( expression.substr(last, (String::npos == next) ? (expression.length()-last) : (next-last)));

            if  (token == "+") {
		Expression* right = expressionStack.back(); expressionStack.pop_back();
                Expression* left = expressionStack.back(); expressionStack.pop_back();
                Expression* subExpression = new Plus(right, left);
                expressionStack.push_back( subExpression );
            }
            else if (token == "-") {
                // it's necessary remove first the right operand from the stack
                Expression* right = expressionStack.back(); expressionStack.pop_back();
                // ..and after the left one
                Expression* left = expressionStack.back(); expressionStack.pop_back();
                Expression* subExpression = new Minus(left, right);
                expressionStack.push_back( subExpression );
            }
            else                        
                expressionStack.push_back( new Variable(token) );
        }

        syntaxTree = expressionStack.back(); expressionStack.pop_back();
    }

     ~Evaluator() {
	delete syntaxTree;
     }
 
    int interpret(Map context) {
        return syntaxTree->interpret(context);
    }
};

}

void main()
{
	using namespace wikibooks_design_patterns;

	Evaluator sentence("w x z - +");

	static
	const int sequences[][3] = {
		{5, 10, 42}, {1, 3, 2}, {7, 9, -5},
	};
	for (size_t i = 0; sizeof(sequences)/sizeof(sequences[0]) > i; ++i) {
		Map variables;
		variables["w"] = new Number(sequences[i][0]);
		variables["x"] = new Number(sequences[i][1]);
		variables["z"] = new Number(sequences[i][2]);
		int result = sentence.interpret(variables);
		for (Map::iterator it = variables.begin(); variables.end() != it; ++it) delete it->second;
    
		std::cout<<"Interpreter result: "<<result<<std::endl;
	}
}

Iterator

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The 'iterator' design pattern is used liberally within the STL for traversal of various containers. The full understanding of this will liberate a developer to create highly reusable and easily understandable[citation needed] data containers.

The basic idea of the iterator is that it permits the traversal of a container (like a pointer moving across an array). However, to get to the next element of a container, you need not know anything about how the container is constructed. This is the iterators job. By simply using the member functions provided by the iterator, you can move, in the intended order of the container, from the first element to the last element.

Let us start by considering a traditional single dimensional array with a pointer moving from the start to the end. This example assumes knowledge of pointer arithmetic. Note that the use of "it" or "itr," henceforth, is a short version of "iterator." 

 const int ARRAY_LEN = 42;
 int *myArray = new int[ARRAY_LEN];
 // Set the iterator to point to the first memory location of the array
 int *arrayItr = myArray;
 // Move through each element of the array, setting it equal to its position in the array
 for(int i = 0; i < ARRAY_LEN; ++i)
 {
    // set the value of the current location in the array
    *arrayItr = i;
    // by incrementing the pointer, we move it to the next position in the array.
    // This is easy for a contiguous memory container, since pointer arithmetic 
    // handles the traversal.
    ++arrayItr;
 }
 // Do not be messy, clean up after yourself
 delete[] myArray;

This code works very quickly for arrays, but how would we traverse a linked list, when the memory is not contiguous? Consider the implementation of a rudimentary linked list as follows: 

 class IteratorCannotMoveToNext{}; // Error class
 class MyIntLList
 {
 public:
     // The Node class represents a single element in the linked list. 
     // The node has a next node and a previous node, so that the user 
     // may move from one position to the next, or step back a single 
     // position. Notice that the traversal of a linked list is O(N), 
     // as is searching, since the list is not ordered.
     class Node
     {
     public:
         Node():mNextNode(0),mPrevNode(0),mValue(0){}
         Node *mNextNode;
         Node *mPrevNode;
         int mValue;
     };
     MyIntLList():mSize(0) 
     {}
     ~MyIntLList()
     {
         while(!Empty())
             pop_front();
     } // See expansion for further implementation;
     int Size() const {return mSize;}
     // Add this value to the end of the list
     void push_back(int value)
     {
         Node *newNode = new Node;
         newNode->mValue = value;
         newNode->mPrevNode = mTail;
         mTail->mNextNode = newNode;
         mTail = newNode;
         ++mSize;
     }
     // Remove the value from the beginning of the list
     void pop_front()
     {
         if(Empty())
             return;
         Node *tmpnode = mHead;
         mHead = mHead->mNextNode;
         delete tmpnode;
         --mSize;
     }
     bool Empty()
     {return mSize == 0;}
 
     // This is where the iterator definition will go, 
     // but lets finish the definition of the list, first
 
 private:
     Node *mHead;
     Node *mTail;
     int mSize;
 };

This linked list has non-contiguous memory, and is therefore not a candidate for pointer arithmetic. And we do not want to expose the internals of the list to other developers, forcing them to learn them, and keeping us from changing it.

This is where the iterator comes in. The common interface makes learning the usage of the container easier, and hides the traversal logic from other developers.

Let us examine the code for the iterator, itself. 

     /*
      *  The iterator class knows the internals of the linked list, so that it 
      *  may move from one element to the next. In this implementation, I have 
      *  chosen the classic traversal method of overloading the increment 
      *  operators. More thorough implementations of a bi-directional linked 
      *  list would include decrement operators so that the iterator may move 
      *  in the opposite direction.
      */
     class Iterator
     {
     public:
         Iterator(Node *position):mCurrNode(position){}
         // Prefix increment
         const Iterator &operator++()
         {
             if(mCurrNode == 0 || mCurrNode->mNextNode == 0)
                 throw IteratorCannotMoveToNext();e
             mCurrNode = mCurrNode->mNextNode;
             return *this;
         }
         // Postfix increment
         Iterator operator++(int)
         {
             Iterator tempItr = *this;
             ++(*this);
             return tempItr;
         }
         // Dereferencing operator returns the current node, which should then 
         // be dereferenced for the int. TODO: Check syntax for overloading 
         // dereferencing operator
         Node * operator*()
         {return mCurrNode;}
         // TODO: implement arrow operator and clean up example usage following
     private:
         Node *mCurrNode;
     };
     // The following two functions make it possible to create 
     // iterators for an instance of this class.
     // First position for iterators should be the first element in the container.
     Iterator Begin(){return Iterator(mHead);}
     // Final position for iterators should be one past the last element in the container.
     Iterator End(){return Iterator(0);}

With this implementation, it is now possible, without knowledge of the size of the container or how its data is organized, to move through each element in order, manipulating or simply accessing the data. This is done through the accessors in the MyIntLList class, Begin() and End(). 

 // Create a list
 MyIntLList myList;
 // Add some items to the list
 for(int i = 0; i < 10; ++i)
     myList.push_back(i);
 // Move through the list, adding 42 to each item.
 for(MyIntLList::Iterator it = myList.Begin(); it != myList.End(); ++it)
     (*it)->mValue += 42;


 

To do:

  • Discussion of iterators in the STL, and the usefulness of iterators within the algorithms library.
  • Iterators best practices
  • Warnings on creation of and usage of
  • When use of operator[] is better and simplifies understanding
  • Cautions about the impact of templates on generated code size (this could make for a good student research paper)


The following program gives the implementation of an iterator design pattern with a generic template: 

/************************************************************************/
/* Iterator.h                                                           */
/************************************************************************/
#ifndef MY_ITERATOR_HEADER
#define MY_ITERATOR_HEADER

#include <iterator>
#include <vector>
#include <set>

//////////////////////////////////////////////////////////////////////////
template<class T, class U>
class Iterator
{
public:
	typedef typename std::vector<T>::iterator iter_type;
	Iterator(U *pData):m_pData(pData){
		m_it = m_pData->m_data.begin();
	}

	void first()
	{
		m_it = m_pData->m_data.begin();
	}

	void next()
	{
		m_it++;
	}

	bool isDone()
	{
		return (m_it == m_pData->m_data.end());
	}

	iter_type current()
	{
		return m_it;
	}
private:
	U *m_pData;
	iter_type m_it;
};

template<class T, class U, class A>
class setIterator
{
public:
	typedef typename std::set<T,U>::iterator iter_type;
	
	setIterator(A *pData):m_pData(pData)
	{
		m_it = m_pData->m_data.begin();
	}

	void first()
	{
		m_it = m_pData->m_data.begin();
	}

	void next()
	{
		m_it++;
	}

	bool isDone()
	{
		return (m_it == m_pData->m_data.end());
	}

	iter_type current()
	{
		return m_it;
	}

private:
	A			*m_pData;		
	iter_type		m_it;
};
#endif

/************************************************************************/
/* Aggregate.h                                                          */
/************************************************************************/
#ifndef MY_DATACOLLECTION_HEADER
#define MY_DATACOLLECTION_HEADER
#include "Iterator.h"

template <class T>
class aggregate
{
	friend class Iterator<T, aggregate>;
public:
	void add(T a)
	{
		m_data.push_back(a);
	}

	Iterator<T, aggregate> *create_iterator()
	{
		return new Iterator<T, aggregate>(this);
	}
	

private:
	std::vector<T> m_data;
};
template <class T, class U>
class aggregateSet
{
	friend class setIterator<T, U, aggregateSet>;
public:
	void add(T a)
	{
		m_data.insert(a);
	}

	setIterator<T, U, aggregateSet> *create_iterator()
	{
		return new setIterator<T,U,aggregateSet>(this);
	}

	void Print()
	{
		copy(m_data.begin(), m_data.end(), std::ostream_iterator<T>(std::cout, "\n"));
	}

private:
	std::set<T,U> m_data;
};

#endif

/************************************************************************/
/* Iterator Test.cpp                                                    */
/************************************************************************/
#include <iostream>
#include <string>
#include "Aggregate.h"
using namespace std;

class Money
{
public:
	Money(int a = 0): m_data(a) {}

	void SetMoney(int a)
	{
		m_data = a;
	}

	int GetMoney()
	{
		return m_data;
	}
	
private:
	int m_data;
};

class Name
{
public:
	Name(string name): m_name(name) {}

	const string &GetName() const
	{
		return m_name;
	}

	friend ostream &operator<<(ostream& out, Name name)
	{
		out << name.GetName();
		return out;
	}

private:
	string m_name;
};

struct NameLess
{
	bool operator()(const Name &lhs, const Name &rhs) const
	{
		return (lhs.GetName() < rhs.GetName());
	}
};

int main()
{
	//sample 1
	cout << "________________Iterator with int______________________________________" << endl;
	aggregate<int> agg;
	
	for (int i = 0; i < 10; i++)
		agg.add(i);

	Iterator< int,aggregate<int> > *it = agg.create_iterator();
	for(it->first(); !it->isDone(); it->next())
		cout << *it->current() << endl;	

	//sample 2
	aggregate<Money> agg2;
	Money a(100), b(1000), c(10000);
	agg2.add(a);
	agg2.add(b);
	agg2.add(c);

	cout << "________________Iterator with Class Money______________________________" << endl;
	Iterator<Money, aggregate<Money> > *it2 = agg2.create_iterator();
	for (it2->first(); !it2->isDone(); it2->next())
		cout << it2->current()->GetMoney() << endl;

	//sample 3
	cout << "________________Set Iterator with Class Name______________________________" << endl;
	
	aggregateSet<Name, NameLess> aset;
	aset.add(Name("Qmt"));
	aset.add(Name("Bmt"));
	aset.add(Name("Cmt"));
	aset.add(Name("Amt"));

	setIterator<Name, NameLess, aggregateSet<Name, NameLess> > *it3 = aset.create_iterator();
	for (it3->first(); !it3->isDone(); it3->next())
		cout << (*it3->current()) << endl;
}

Console output: 

________________Iterator with int______________________________________
0
1
2
3
4
5
6
7
8
9
________________Iterator with Class Money______________________________
100
1000
10000
________________Set Iterator with Class Name___________________________
Amt
Bmt
Cmt
Qmt

Mediator

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Define an object that encapsulates how a set of objects interact. Mediator promotes loose coupling by keeping objects from referring to each other explicitly, and it lets you vary their interaction independently. 

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

class MediatorInterface;

class ColleagueInterface {
		std::string name;
	public:
		ColleagueInterface (const std::string& newName) : name (newName) {}
		std::string getName() const {return name;}
		virtual void sendMessage (const MediatorInterface&, const std::string&) const = 0;
		virtual void receiveMessage (const ColleagueInterface*, const std::string&) const = 0;
};

class Colleague : public ColleagueInterface {
	public:
		using ColleagueInterface::ColleagueInterface;
		virtual void sendMessage (const MediatorInterface&, const std::string&) const override;
	private:
		virtual void receiveMessage (const ColleagueInterface*, const std::string&) const override;
};

class MediatorInterface {
    private:
		std::list<ColleagueInterface*> colleagueList;
    public:
    	const std::list<ColleagueInterface*>& getColleagueList() const {return colleagueList;}
		virtual void distributeMessage (const ColleagueInterface*, const std::string&) const = 0;
		virtual void registerColleague (ColleagueInterface* colleague) {colleagueList.emplace_back (colleague);}
};

class Mediator : public MediatorInterface {
    virtual void distributeMessage (const ColleagueInterface*, const std::string&) const override;
};

void Colleague::sendMessage (const MediatorInterface& mediator, const std::string& message) const {
	mediator.distributeMessage (this, message);
}

void Colleague::receiveMessage (const ColleagueInterface* sender, const std::string& message) const {
 	std::cout << getName() << " received the message from " << sender->getName() << ": " << message << std::endl;			
}

void Mediator::distributeMessage (const ColleagueInterface* sender, const std::string& message) const {
	for (const ColleagueInterface* x : getColleagueList())
		if (x != sender)  // Do not send the message back to the sender
			x->receiveMessage (sender, message);
}
 
int main() {
	Colleague *bob = new Colleague ("Bob"),  *sam = new Colleague ("Sam"),  *frank = new Colleague ("Frank"),  *tom = new Colleague ("Tom");
	Colleague* staff[] = {bob, sam, frank, tom};
	Mediator mediatorStaff, mediatorSamsBuddies;
	for (Colleague* x : staff)
		mediatorStaff.registerColleague(x);
	bob->sendMessage (mediatorStaff, "I'm quitting this job!");
	mediatorSamsBuddies.registerColleague (frank);  mediatorSamsBuddies.registerColleague (tom);  // Sam's buddies only
	sam->sendMessage (mediatorSamsBuddies, "Hooray!  He's gone!  Let's go for a drink, guys!");	
	return 0;
}

Memento

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Without violating encapsulation the Memento Pattern will capture and externalize an object’s internal state so that the object can be restored to this state later. Though the Gang of Four uses friend as a way to implement this pattern it is not the best design[citation needed]. It can also be implemented using PIMPL (pointer to implementation or opaque pointer). Best Use case is 'Undo-Redo' in an editor.

The Originator (the object to be saved) creates a snap-shot of itself as a Memento object, and passes that reference to the Caretaker object. The Caretaker object keeps the Memento until such a time as the Originator may want to revert to a previous state as recorded in the Memento object.

See memoize for an old-school example of this pattern. 

#include <iostream>
#include <string>
#include <sstream>
#include <vector>

const std::string NAME = "Object";

template <typename T>
std::string toString (const T& t) {
	std::stringstream ss;
	ss << t;
	return ss.str();
}

class Memento;

class Object {
  	private:
    	    int value;
    	    std::string name;
    	    double decimal;  // and suppose there are loads of other data members
  	public:
	    Object (int newValue): value (newValue), name (NAME + toString (value)), decimal ((float)value / 100) {}
	    void doubleValue() {value = 2 * value;  name = NAME + toString (value);  decimal = (float)value / 100;}
	    void increaseByOne() {value++;  name = NAME + toString (value);  decimal = (float)value / 100;}
	    int getValue() const {return value;}
	    std::string getName() const {return name;}
	    double getDecimal() const {return decimal;}
	    Memento* createMemento() const;
	    void reinstateMemento (Memento* mem);
};

class Memento {
  	private:
 	    Object object;
  	public:
    	    Memento (const Object& obj):  object (obj) {}
    	    Object snapshot() const {return object;}  // want a snapshot of Object itself because of its many data members
};

Memento* Object::createMemento() const {
	return new Memento (*this);
}

void Object::reinstateMemento (Memento* mem) {
	*this = mem->snapshot();
}

class Command {
  	private:
	    typedef void (Object::*Action)();
	    Object* receiver;
	    Action action;
	    static std::vector<Command*> commandList;
	    static std::vector<Memento*> mementoList;
	    static int numCommands;
	    static int maxCommands;
  	public:
	    Command (Object *newReceiver, Action newAction): receiver (newReceiver), action (newAction) {}
	    virtual void execute() {
	    	if (mementoList.size() < numCommands + 1)
	    		mementoList.resize (numCommands + 1);
	        mementoList[numCommands] = receiver->createMemento();  // saves the last value
	    	if (commandList.size() < numCommands + 1)
	    		commandList.resize (numCommands + 1);
	        commandList[numCommands] = this;  // saves the last command
	        if (numCommands > maxCommands)
	          	maxCommands = numCommands;
	        numCommands++;
	        (receiver->*action)();
	    }
	    static void undo() {
	        if (numCommands == 0)
	        {
	            std::cout << "There is nothing to undo at this point." << std::endl;
	            return;
	        }
	        commandList[numCommands - 1]->receiver->reinstateMemento (mementoList[numCommands - 1]);
	        numCommands--;
	    }
	    void static redo() {
	        if (numCommands > maxCommands)
	        {
	            std::cout << "There is nothing to redo at this point." << std::endl;
	            return ;
	        }
	        Command* commandRedo = commandList[numCommands];
	        (commandRedo->receiver->*(commandRedo->action))();
	        numCommands++;
	    }
};

std::vector<Command*> Command::commandList;
std::vector<Memento*> Command::mementoList;
int Command::numCommands = 0;
int Command::maxCommands = 0;

int main()
{
	int i;
	std::cout << "Please enter an integer: ";
	std::cin >> i;
	Object *object = new Object(i);
	
	Command *commands[3];
	commands[1] = new Command(object, &Object::doubleValue);
	commands[2] = new Command(object, &Object::increaseByOne);
	
	std::cout << "0.Exit,  1.Double,  2.Increase by one,  3.Undo,  4.Redo: ";
	std::cin >> i;
	
	while (i != 0)
	{
		if (i == 3)
		  	Command::undo();
		else if (i == 4)
		  	Command::redo();
		else if (i > 0 && i <= 2)
		  	commands[i]->execute();
		else
		{
			std::cout << "Enter a proper choice: ";
			std::cin >> i;
			continue;
		}
		std::cout << "   " << object->getValue() << "  " << object->getName() << "  " << object->getDecimal() << std::endl;
		std::cout << "0.Exit,  1.Double,  2.Increase by one,  3.Undo,  4.Redo: ";
		std::cin >> i;
	}
}

Observer

edit

The Observer Pattern defines a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.

Problem
In one place or many places in the application we need to be aware about a system event or an application state change. We'd like to have a standard way of subscribing to listening for system events and a standard way of notifying the interested parties. The notification should be automated after an interested party subscribed to the system event or application state change. There also should be a way to unsubscribe.
Forces
Observers and observables probably should be represented by objects. The observer objects will be notified by the observable objects.
Solution
After subscribing the listening objects will be notified by a way of method call.
#include <list>
#include <algorithm>
#include <iostream>
using namespace std;

// The Abstract Observer
class ObserverBoardInterface
{
public:
    virtual void update(float a,float b,float c) = 0;
};

// Abstract Interface for Displays
class DisplayBoardInterface
{
public:
    virtual void show() = 0;
};

// The Abstract Subject
class WeatherDataInterface
{
public:
    virtual void registerOb(ObserverBoardInterface* ob) = 0;
    virtual void removeOb(ObserverBoardInterface* ob) = 0;
    virtual void notifyOb() = 0;
};

// The Concrete Subject
class ParaWeatherData: public WeatherDataInterface
{
public:
    void SensorDataChange(float a,float b,float c)
    {
        m_humidity = a;
        m_temperature = b;
        m_pressure = c;
        notifyOb();
    }

    void registerOb(ObserverBoardInterface* ob)
    {
        m_obs.push_back(ob);
    }

    void removeOb(ObserverBoardInterface* ob)
    {
        m_obs.remove(ob);
    }
protected:
    void notifyOb()
    {
        list<ObserverBoardInterface*>::iterator pos = m_obs.begin();
        while (pos != m_obs.end())
        {
            ((ObserverBoardInterface* )(*pos))->update(m_humidity,m_temperature,m_pressure);
            (dynamic_cast<DisplayBoardInterface*>(*pos))->show();
            ++pos;
        }
    }

private:
    float        m_humidity;
    float        m_temperature;
    float        m_pressure;
    list<ObserverBoardInterface* > m_obs;
};

// A Concrete Observer
class CurrentConditionBoard : public ObserverBoardInterface, public DisplayBoardInterface
{
public:
    CurrentConditionBoard(ParaWeatherData& a):m_data(a)
    {
        m_data.registerOb(this);
    }
    void show()
    {
        cout<<"_____CurrentConditionBoard_____"<<endl;
        cout<<"humidity: "<<m_h<<endl;
        cout<<"temperature: "<<m_t<<endl;
        cout<<"pressure: "<<m_p<<endl;
        cout<<"_______________________________"<<endl;
    }

    void update(float h, float t, float p)
    {
        m_h = h;
        m_t = t;
        m_p = p;
    }

private:
    float m_h;
    float m_t;
    float m_p;
    ParaWeatherData& m_data;
};

// A Concrete Observer
class StatisticBoard : public ObserverBoardInterface, public DisplayBoardInterface
{
public:
    StatisticBoard(ParaWeatherData& a):m_maxt(-1000),m_mint(1000),m_avet(0),m_count(0),m_data(a)
    {
        m_data.registerOb(this);
    }

    void show()
    {
        cout<<"________StatisticBoard_________"<<endl;
        cout<<"lowest  temperature: "<<m_mint<<endl;
        cout<<"highest temperature: "<<m_maxt<<endl;
        cout<<"average temperature: "<<m_avet<<endl;
        cout<<"_______________________________"<<endl;
    }

    void update(float h, float t, float p)
    {
        ++m_count;
        if (t>m_maxt)
        {
            m_maxt = t;
        }
        if (t<m_mint)
        {
            m_mint = t;
        }
        m_avet = (m_avet * (m_count-1) + t)/m_count;
    }

private:
    float m_maxt;
    float  m_mint;
    float m_avet;
    int m_count;
    ParaWeatherData& m_data;
};


int main(int argc, char *argv[])
{
   
    ParaWeatherData * wdata = new ParaWeatherData;
    CurrentConditionBoard* currentB = new CurrentConditionBoard(*wdata);
    StatisticBoard* statisticB = new StatisticBoard(*wdata);

    wdata->SensorDataChange(10.2, 28.2, 1001);
    wdata->SensorDataChange(12, 30.12, 1003);
    wdata->SensorDataChange(10.2, 26, 806);
    wdata->SensorDataChange(10.3, 35.9, 900);

    wdata->removeOb(currentB);

    wdata->SensorDataChange(100, 40, 1900);  
    
    delete statisticB;
    delete currentB;
    delete wdata;

    return 0;
}

State

edit

The State Pattern allows an object to alter its behavior when its internal state changes. The object will appear as having changed its class. 

#include <iostream>
#include <string>
#include <cstdlib>
#include <ctime>
#include <memory>

enum Input {DUCK_DOWN, STAND_UP, JUMP, DIVE};

class Fighter;
class StandingState;  class JumpingState;  class DivingState;

class FighterState {
	public:
		static std::shared_ptr<StandingState> standing;
		static std::shared_ptr<DivingState> diving;
		virtual ~FighterState() = default;
		virtual void handleInput (Fighter&, Input) = 0;
		virtual void update (Fighter&) = 0;
};

class DuckingState : public FighterState {
	private:
		int chargingTime;
		static const int FullRestTime = 5;
	public:
		DuckingState() : chargingTime(0) {}
		virtual void handleInput (Fighter&, Input) override;
		virtual void update (Fighter&) override;
};

class StandingState : public FighterState {
	public:
		virtual void handleInput (Fighter&, Input) override;
		virtual void update (Fighter&) override;
};

class JumpingState : public FighterState {
	private:
		int jumpingHeight;
	public:
		JumpingState() {jumpingHeight = std::rand() % 5 + 1;}
		virtual void handleInput (Fighter&, Input) override;
		virtual void update (Fighter&) override;
};

class DivingState : public FighterState {
	public:
		virtual void handleInput (Fighter&, Input) override;
		virtual void update (Fighter&) override;
};

std::shared_ptr<StandingState> FighterState::standing (new StandingState);
std::shared_ptr<DivingState> FighterState::diving (new DivingState);

class Fighter {
	private:
		std::string name;
		std::shared_ptr<FighterState> state;
		int fatigueLevel = std::rand() % 10;
	public:
		Fighter (const std::string& newName) : name (newName), state (FighterState::standing) {}
		std::string getName() const {return name;}
		int getFatigueLevel() const {return fatigueLevel;}
		virtual void handleInput (Input input) {state->handleInput (*this, input);}  // delegate input handling to 'state'.
		void changeState (std::shared_ptr<FighterState> newState) {state = newState;  updateWithNewState();}
		void standsUp() {std::cout << getName() << " stands up." << std::endl;}
		void ducksDown() {std::cout << getName() << " ducks down." << std::endl;}
		void jumps() {std::cout << getName() << " jumps into the air." << std::endl;}
		void dives() {std::cout << getName() << " makes a dive attack in the middle of the jump!" << std::endl;}
		void feelsStrong() {std::cout << getName() << " feels strong!" << std::endl;}
		void changeFatigueLevelBy (int change) {fatigueLevel += change;  std::cout << "fatigueLevel = " << fatigueLevel << std::endl;}
	private:
		virtual void updateWithNewState() {state->update(*this);}  // delegate updating to 'state'
};

void StandingState::handleInput (Fighter& fighter, Input input)  {
	switch (input) {
		case STAND_UP:  std::cout << fighter.getName() << " remains standing." << std::endl;  return;
		case DUCK_DOWN:  fighter.changeState (std::shared_ptr<DuckingState> (new DuckingState));  return fighter.ducksDown();
		case JUMP:  fighter.jumps();  return fighter.changeState (std::shared_ptr<JumpingState> (new JumpingState));
		default:  std::cout << "One cannot do that while standing.  " << fighter.getName() << " remains standing by default." << std::endl;
	}
}

void StandingState::update (Fighter& fighter) {
	if (fighter.getFatigueLevel() > 0)
		fighter.changeFatigueLevelBy(-1);
}

void DuckingState::handleInput (Fighter& fighter, Input input)  {
	switch (input) {
		case STAND_UP:  fighter.changeState (FighterState::standing);  return fighter.standsUp();
		case DUCK_DOWN:
			std::cout << fighter.getName() << " remains in ducking position, ";
			if (chargingTime < FullRestTime) std::cout << "recovering in the meantime." << std::endl;
			else std::cout << "fully recovered." << std::endl;
			return update (fighter);
		default:
			std::cout << "One cannot do that while ducking.  " << fighter.getName() << " remains in ducking position by default." << std::endl;
			update (fighter);
	}
}

void DuckingState::update (Fighter& fighter) {
	chargingTime++;
	std::cout << "Charging time = " << chargingTime << "." << std::endl;
	if (fighter.getFatigueLevel() > 0)
		fighter.changeFatigueLevelBy(-1);
	if (chargingTime >= FullRestTime && fighter.getFatigueLevel() <= 3)
		fighter.feelsStrong();
}

void JumpingState::handleInput (Fighter& fighter, Input input)  {
	switch (input) {
		case DIVE:  fighter.changeState (FighterState::diving);  return fighter.dives();
		default:
			std::cout << "One cannot do that in the middle of a jump.  " << fighter.getName() << " lands from his jump and is now standing again." << std::endl;
			fighter.changeState (FighterState::standing);
	}
}

void JumpingState::update (Fighter& fighter) {
	std::cout << fighter.getName() << " has jumped " << jumpingHeight << " feet into the air." << std::endl;
	if (jumpingHeight >= 3)
		fighter.changeFatigueLevelBy(1);
}

void DivingState::handleInput (Fighter& fighter, Input)  {
	std::cout << "Regardless of what the user input is, " << fighter.getName() << " lands from his dive and is now standing again." << std::endl;
	fighter.changeState (FighterState::standing);
}

void DivingState::update (Fighter& fighter) {
	fighter.changeFatigueLevelBy(2);
}

int main() {
	std::srand(std::time(nullptr));
	Fighter rex ("Rex the Fighter"), borg ("Borg the Fighter");
	std::cout << rex.getName() << " and " << borg.getName() << " are currently standing." << std::endl;
	int choice;
	auto chooseAction = [&choice](Fighter& fighter) {
		std::cout << std::endl << DUCK_DOWN + 1 << ") Duck down  " << STAND_UP + 1 << ") Stand up  " << JUMP + 1
			<< ") Jump  " << DIVE + 1 << ") Dive in the middle of a jump" << std::endl;
		std::cout << "Choice for " << fighter.getName() << "? ";
		std::cin >> choice;
		const Input input1 = static_cast<Input>(choice - 1);
		fighter.handleInput (input1);	
	};
	while (true) {
		chooseAction (rex);
		chooseAction (borg);
	}
}

Strategy

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Defines a family of algorithms, encapsulates each one, and make them interchangeable. Strategy lets the algorithm vary independently from clients who use it. 

#include <iostream>
using namespace std;

class StrategyInterface
{
    public:
        virtual void execute() const = 0;
};

class ConcreteStrategyA: public StrategyInterface
{
    public:
        void execute() const override
        {
            cout << "Called ConcreteStrategyA execute method" << endl;
        }
};

class ConcreteStrategyB: public StrategyInterface
{
    public:
        void execute() const override
        {
            cout << "Called ConcreteStrategyB execute method" << endl;
        }
};

class ConcreteStrategyC: public StrategyInterface
{
    public:
        void execute() const override
        {
            cout << "Called ConcreteStrategyC execute method" << endl;
        }
};

class Context
{
    private:
        StrategyInterface * strategy_;

    public:
        explicit Context(StrategyInterface *strategy):strategy_(strategy)
        {
        }

        void set_strategy(StrategyInterface *strategy)
        {
            strategy_ = strategy;
        }

        void execute() const
        {
            strategy_->execute();
        }
};

int main(int argc, char *argv[])
{
    ConcreteStrategyA concreteStrategyA;
    ConcreteStrategyB concreteStrategyB;
    ConcreteStrategyC concreteStrategyC;

    Context contextA(&concreteStrategyA);
    Context contextB(&concreteStrategyB);
    Context contextC(&concreteStrategyC);

    contextA.execute(); // output: "Called ConcreteStrategyA execute method"
    contextB.execute(); // output: "Called ConcreteStrategyB execute method"
    contextC.execute(); // output: "Called ConcreteStrategyC execute method"
    
    contextA.set_strategy(&concreteStrategyB);
    contextA.execute(); // output: "Called ConcreteStrategyB execute method"
    contextA.set_strategy(&concreteStrategyC);
    contextA.execute(); // output: "Called ConcreteStrategyC execute method"

    return 0;
}

Template Method

edit

By defining a skeleton of an algorithm in an operation, deferring some steps to subclasses, the Template Method lets subclasses redefine certain steps of that algorithm without changing the algorithm's structure. 

#include <ctime>
#include <assert.h>
#include <iostream>

namespace wikibooks_design_patterns
{
/**
 * An abstract class that is common to several games in
 * which players play against the others, but only one is
 * playing at a given time.
 */

class Game
{
public:
    Game(): playersCount(0), movesCount(0), playerWon(-1)
    {
        srand( (unsigned)time( NULL));
    }

    /* A template method : */
    void playOneGame(const int playersCount = 0)
    {
        if (playersCount)
        {
            this->playersCount = playersCount;
        }

        InitializeGame();
        assert(this->playersCount);

        int j = 0;
        while (!endOfGame())
        {
            makePlay(j);
            j = (j + 1) % this->playersCount;
            if (!j)
            {
                ++movesCount;
            }
        }
        printWinner();
    }

protected:
    virtual void initializeGame() = 0;
    virtual void makePlay(int player) = 0;
    virtual bool endOfGame() = 0;
    virtual void printWinner() = 0;

private:
    void InitializeGame()
    {
        movesCount = 0;
        playerWon = -1;

        initializeGame();
    }

protected:
    int playersCount;
    int movesCount;
    int playerWon;
};
 
//Now we can extend this class in order 
//to implement actual games:
 
class Monopoly: public Game {
 
    /* Implementation of necessary concrete methods */
    void initializeGame() {
        // Initialize players
	playersCount = rand() * 7 / RAND_MAX + 2;
        // Initialize money
    }
    void makePlay(int player) {
        // Process one turn of player

	//	Decide winner
	if (movesCount < 20)
	    return;
	const int chances = (movesCount > 199) ? 199 : movesCount;
	const int random = MOVES_WIN_CORRECTION * rand() * 200 / RAND_MAX;
	if (random < chances)
	    playerWon = player;
    }
    bool endOfGame() {
        // Return true if game is over 
        // according to Monopoly rules
	return (-1 != playerWon);
    }
    void printWinner() {
	assert(playerWon >= 0);
	assert(playerWon < playersCount);

        // Display who won
	std::cout<<"Monopoly, player "<<playerWon<<" won in "<<movesCount<<" moves."<<std::endl;
    }

private:
    enum
    {
        MOVES_WIN_CORRECTION = 20,
    };
};
 
class Chess: public Game {
 
    /* Implementation of necessary concrete methods */
    void initializeGame() {
        // Initialize players
	playersCount = 2;
        // Put the pieces on the board
    }
    void makePlay(int player) {
	assert(player < playersCount);

        // Process a turn for the player

	//	decide winner
	if (movesCount < 2)
	    return;
	const int chances = (movesCount > 99) ? 99 : movesCount;
	const int random = MOVES_WIN_CORRECTION * rand() * 100 / RAND_MAX;
	//std::cout<<random<<" : "<<chances<<std::endl;
	if (random < chances)
	    playerWon = player;
    }
    bool endOfGame() {
        // Return true if in Checkmate or 
        // Stalemate has been reached
	return (-1 != playerWon);
    }
    void printWinner() {
	assert(playerWon >= 0);
	assert(playerWon < playersCount);

        // Display the winning player
	std::cout<<"Player "<<playerWon<<" won in "<<movesCount<<" moves."<<std::endl;
    }

private:
    enum
    {
	MOVES_WIN_CORRECTION = 7,
    };
};

}

int main()
{
    using namespace wikibooks_design_patterns;

    Game* game = NULL;

    Chess chess;
    game = &chess;
    for (unsigned i = 0; i < 100; ++i)
	 game->playOneGame();

    Monopoly monopoly;
    game = &monopoly;
    for (unsigned i = 0; i < 100; ++i)
	game->playOneGame();

    return 0;
}

Visitor

edit

The Visitor Pattern will represent an operation to be performed on the elements of an object structure by letting you define a new operation without changing the classes of the elements on which it operates. 

#include <string>
#include <iostream>
#include <vector>
#include <memory>
using namespace std;
 
class Wheel;
class Engine;
class Body;
class Car;
 
// interface to all car 'parts'
struct CarElementVisitor 
{
	virtual void visit(Wheel& wheel) const = 0;
	virtual void visit(Engine& engine) const = 0;
	virtual void visit(Body& body) const = 0;
	
	virtual void visitCar(Car& car) const = 0;
};
 
// interface to one part
struct CarElement 
{
	virtual void accept(const CarElementVisitor& visitor) = 0;	
};
 
// wheel element, there are four wheels with unique names
class Wheel : public CarElement
{
	public:
		explicit Wheel(const string& name) : name_(name){}

		const string& getName() const 
		{
			return name_;
		}

		void accept(const CarElementVisitor& visitor)  
		{
			visitor.visit(*this);
		}

	private:
	    string name_;
};
 
class Engine : public CarElement
{
	public:
		void accept(const CarElementVisitor& visitor) 
		{
			visitor.visit(*this);
		}
};
 
class Body : public CarElement
{
	public:
		void accept(const CarElementVisitor& visitor) 
		{
			visitor.visit(*this);
		}
};
 
class Car 
{
	public:
		vector<unique_ptr<CarElement>>& getElements()
		{
			return elements_;
		}
		
		Car() {
			// assume that neither push_back nor Wheel(const string&) may throw
			elements_.push_back( make_unique<Wheel>("front left") );
			elements_.push_back( make_unique<Wheel>("front right") );
			elements_.push_back( make_unique<Wheel>("back left") );
			elements_.push_back( make_unique<Wheel>("back right") );
			elements_.push_back( make_unique<Body>() );
			elements_.push_back( make_unique<Engine>() );
		}

	private:
		vector<unique_ptr<CarElement>> elements_;
};


// PrintVisitor and DoVisitor show by using a different implementation the Car class is unchanged even though the algorithm is different in PrintVisitor and DoVisitor.

class CarElementPrintVisitor : public CarElementVisitor 
{
	public:
		void visit(Wheel& wheel) const
		{ 
			cout << "Visiting " << wheel.getName() << " wheel" << endl;
		}
		void visit(Engine& engine) const
		{
			cout << "Visiting engine" << endl;
		}

		void visit(Body& body) const
		{
			cout << "Visiting body" << endl;
		}

		void visitCar(Car& car) const
		{
			cout << endl << "Visiting car" << endl;
			vector<unique_ptr<CarElement>>& elems = car.getElements();
			
			for(auto &it : elems)
			{
				// this issues the callback i.e. to this from the element
				it->accept(*this);	  
			}
			cout << "Visited car" << endl;
		}
};

class CarElementDoVisitor : public CarElementVisitor 
{
	public:
		// these are specific implementations added to the original object without modifying the original struct
		void visit(Wheel& wheel) const
		{
			cout << "Kicking my " << wheel.getName() << " wheel" << endl;
		}

		void visit(Engine& engine) const
		{
			cout << "Starting my engine" << endl;
		}

		void visit(Body& body) const
		{
			cout << "Moving my body" << endl;
		}

		void visitCar(Car& car) const
		{
			cout << endl << "Starting my car" << endl;
			vector<unique_ptr<CarElement>>& elems = car.getElements();

			for(auto& it : elems)
			{
				it->accept(*this);	// this issues the callback i.e. to this from the element  
			}
			cout << "Stopped car" << endl;
		}
};

int main()
{
	Car car;
	CarElementPrintVisitor printVisitor;
	CarElementDoVisitor doVisitor;
	
	printVisitor.visitCar(car);
	doVisitor.visitCar(car);

	return 0;
}

Model-View-Controller (MVC)

edit

A pattern often used by applications that need the ability to maintain multiple views of the same data. The model-view-controller pattern was until recently[citation needed] a very common pattern especially for graphic user interlace programming, it splits the code in 3 pieces. The model, the view, and the controller.

The Model is the actual data representation (for example, Array vs Linked List) or other objects representing a database. The View is an interface to reading the model or a fat client GUI. The Controller provides the interface of changing or modifying the data, and then selecting the "Next Best View" (NBV).

Newcomers will probably see this "MVC" model as wasteful, mainly because you are working with many extra objects at runtime, when it seems like one giant object will do. But the secret to the MVC pattern is not writing the code, but in maintaining it, and allowing people to modify the code without changing much else. Also, keep in mind, that different developers have different strengths and weaknesses, so team building around MVC is easier. Imagine a View Team that is responsible for great views, a Model Team that knows a lot about data, and a Controller Team that see the big picture of application flow, handing requests, working with the model, and selecting the most appropriate next view for that client.


 

To do:
Erm, someone please come up with a better example than the following... I can not think of any

  • Perhaps a banking program for customer access to their accounts: A web UI on a traditional browser vs mobile app? And then handling checking/savings accounts vs loan vs line-of-credit accounts? fwiw


For example: A naive central database can be organized using only a "model", for example, a straight array. However, later on, it may be more applicable to use a linked list. All array accesses will have to be remade into their respective Linked List form (for example, you would change myarray[5] into mylist.at(5) or whatever is equivalent in the language you use).

Well, if we followed the MVC pattern, the central database would be accessed using some sort of a function, for example, myarray.at(5). If we change the model from an array to a linked list, all we have to do is change the view with the model, and the whole program is changed. Keep the interface the same but change the underpinnings of it. This would allow us to make optimizations more freely and quickly than before.

One of the great advantages of the Model-View-Controller Pattern is obviously the ability to reuse the application's logic (which is implemented in the model) when implementing a different view. A good example is found in web development, where a common task is to implement an external API inside of an existing piece of software. If the MVC pattern has cleanly been followed, this only requires modification to the controller, which can have the ability to render different types of views dependent on the content type requested by the user agent.

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