/ Design Patterns / Creational patterns / Prototype

Prototype Design Pattern in C++

Prototype design pattern demo

Discussion. Image base class provides the mechanism for storing, finding, and cloning the prototype for all derived classes. Each derived class specifies a private static data member whose initialization "registers" a prototype of itself with the base class. When the client asks for a "clone" of a certain type, the base class finds the prototype and calls clone() on the correct derived class. 

#include <iostream>

enum imageType
{
  LSAT, SPOT
};

class Image
{
  public:
    virtual void draw() = 0;
    static Image *findAndClone(imageType);
  protected:
    virtual imageType returnType() = 0;
    virtual Image *clone() = 0;
    // As each subclass of Image is declared, it registers its prototype
    static void addPrototype(Image *image)
    {
        _prototypes[_nextSlot++] = image;
    }
  private:
    // addPrototype() saves each registered prototype here
    static Image *_prototypes[10];
    static int _nextSlot;
};

Image *Image::_prototypes[];
int Image::_nextSlot;

// Client calls this public static member function when it needs an instance
// of an Image subclass
Image *Image::findAndClone(imageType type)
{
  for (int i = 0; i < _nextSlot; i++)
    if (_prototypes[i]->returnType() == type)
      return _prototypes[i]->clone();
  return NULL;
}

class LandSatImage: public Image
{
  public:
    imageType returnType()
    {
        return LSAT;
    }
    void draw()
    {
        std::cout << "LandSatImage::draw " << _id << std::endl;
    }
    // When clone() is called, call the one-argument ctor with a dummy arg
    Image *clone()
    {
        return new LandSatImage(1);
    }
  protected:
    // This is only called from clone()
    LandSatImage(int dummy)
    {
        _id = _count++;
    }
  private:
    // Mechanism for initializing an Image subclass - this causes the
    // default ctor to be called, which registers the subclass's prototype
    static LandSatImage _landSatImage;
    // This is only called when the private static data member is initiated
    LandSatImage()
    {
        addPrototype(this);
    }
    // Nominal "state" per instance mechanism
    int _id;
    static int _count;
};

// Register the subclass's prototype
LandSatImage LandSatImage::_landSatImage;
// Initialize the "state" per instance mechanism
int LandSatImage::_count = 1;

class SpotImage: public Image
{
  public:
    imageType returnType()
    {
        return SPOT;
    }
    void draw()
    {
        std::cout << "SpotImage::draw " << _id << std::endl;
    }
    Image *clone()
    {
        return new SpotImage(1);
    }
  protected:
    SpotImage(int dummy)
    {
        _id = _count++;
    }
  private:
    SpotImage()
    {
        addPrototype(this);
    }
    static SpotImage _spotImage;
    int _id;
    static int _count;
};

SpotImage SpotImage::_spotImage;
int SpotImage::_count = 1;

// Simulated stream of creation requests
const int NUM_IMAGES = 8;
imageType input[NUM_IMAGES] =
{
  LSAT, LSAT, LSAT, SPOT, LSAT, SPOT, SPOT, LSAT
};

int main()
{
  Image *images[NUM_IMAGES];

  // Given an image type, find the right prototype, and return a clone
  for (int i = 0; i < NUM_IMAGES; i++)
    images[i] = Image::findAndClone(input[i]);

  // Demonstrate that correct image objects have been cloned
  for (int i = 0; i < NUM_IMAGES; i++)
    images[i]->draw();

  // Free the dynamic memory
  for (int i = 0; i < NUM_IMAGES; i++)
    delete images[i];
}

Output

LandSatImage::draw 1
LandSatImage::draw 2
LandSatImage::draw 3
SpotImage::draw 1
LandSatImage::draw 4
SpotImage::draw 2
SpotImage::draw 3
LandSatImage::draw 5

Code examples

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