C++ example for Prototype Design Pattern

The Protoype pattern is used when creating an instance of a class is very time-consuming or complex in some way. Then, rather than creating more instances, you make copies of the original instance, modifying them as appropriate.


Prototypes can also be used whenever you need classes that differ only in the type of processing they offer, for example in parsing of strings representing numbers in different radixes. This design pattern is not used very frequently.

The following is an example of Prototype Design Pattern:

//Program tested on Microsoft Visual Studio 2008 - Zahid Ghadialy
//Prototype is part of Creational Patterns
//Creational Patterns deal with initializing and configuring classes and objects
//Prototype creates a fully initialized instance to be copied or cloned

//We will take an example of creating Colour class.
//There are three parts to colour - Red, Green and Blue
//Simple colours like Red only contain the red component
//Complex colours like Angry and Peace contains all three components

#include <iostream>
#include <string>
#include <iomanip>
#include <map>

using namespace std;

//The abstract 'Protoype' class
class ColourPrototype
{
public:
 virtual ColourPrototype* Clone(void) = 0;
};

//The 'ConcretePrototype' class
class Colour : public ColourPrototype
{
public:
 Colour(int red, int green, int blue)
 {
   red_ = red, green_ = green, blue_ = blue;
 }
 ColourPrototype* Clone(void)
 {
   cout << "Cloning colour RGB: " << setw(3) << red_ << ", " << setw(3) << green_ <<", " << setw(3) << blue_ <<endl;
   ColourPrototype* colourPrototype = new Colour(red_, green_, blue_);
   return colourPrototype;
 }
 void SetRed(int red)
 {
   red_ = red;
 }
 void SetGreen(int green)
 {
   green_ = green;
 }
 void SetBlue(int blue)
 {
   blue_ = blue;
 }
 int GetRed(void)
 {
   return red_;
 }
 int GetGreen(void)
 {
   return green_;
 }
 int GetBlue(void)
 {
   return blue_;
 }

private:
 Colour(); //default constructor not allowed
  int red_, green_, blue_;
};

//Prototype manager
class ColourManager
{
public:
 virtual ~ColourManager()
 {
   while(!coloursMap_.empty())
   {
     map<string, ColourPrototype*>::iterator it = coloursMap_.begin();
     delete it->second;
     coloursMap_.erase(it);
   }
 }
 void AddColour(const string& colour, ColourPrototype* prototype)
 {
   coloursMap_[colour] = prototype;
 }
 ColourPrototype* GetColour(const string& colour)
 {
   map<string, ColourPrototype*>::const_iterator it = coloursMap_.find(colour);
   if(it != coloursMap_.end())
     return it->second;
   return NULL;
 }
 void PrintColours(void)
 {
   cout << "\nAvailable Colours and their values " << endl;
   map<string, ColourPrototype*>::const_iterator it = coloursMap_.begin();
   while(it != coloursMap_.end())
   {
     cout << setw(20) << it->first << " : ";
     cout << setw(3)  << dynamic_cast<Colour*>(it->second)->GetRed() << ", ";
     cout << setw(3)  << dynamic_cast<Colour*>(it->second)->GetGreen() << ", ";
     cout << setw(3)  << dynamic_cast<Colour*>(it->second)->GetBlue() << endl;
     ++it;
   }
 }
private:
 map<string, ColourPrototype*> coloursMap_;
};


//The Main method
int main()
{
 ColourManager* colourManager = new ColourManager();

 //Add simple colours
  colourManager->AddColour("Red", new Colour(255, 0, 0));
 colourManager->AddColour("Green", new Colour(0, 255, 0));
 colourManager->AddColour("Blue", new Colour(0, 0, 255));

 //Add complex colours
  colourManager->AddColour("Angry", new Colour(255, 54, 0));
 colourManager->AddColour("Peace", new Colour(128, 211, 128));
 colourManager->AddColour("Flame", new Colour(211, 34, 20));

 //Clone existing colours, modify and add them to the manager
  ColourPrototype* colour1 = (colourManager->GetColour("Red"))->Clone();
 (dynamic_cast<Colour*>(colour1))->SetRed(200);
 colourManager->AddColour("Light Red", colour1);

 ColourPrototype* colour2 = (colourManager->GetColour("Peace"))->Clone();
 (dynamic_cast<Colour*>(colour2))->SetRed(150);
 (dynamic_cast<Colour*>(colour2))->SetBlue(150);
 colourManager->AddColour("Extreme Peace", colour2);

 ColourPrototype* colour3 = (colourManager->GetColour("Flame"))->Clone();
 (dynamic_cast<Colour*>(colour3))->SetRed(255);
 colourManager->AddColour("Hot Flame", colour3);

 colourManager->PrintColours();

 //clean the memory
  delete colourManager;

 return 0;
}

The output is as follows:

For more details, please see: http://www.dofactory.com/Patterns/PatternPrototype.aspx

Other good example of Prototype is available here and here.