Prototype in C++
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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.h>
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();
}
class LandSatImage: public Image
{
public:
imageType returnType()
{
return LSAT;
}
void draw()
{
cout << "LandSatImage::draw " << _id << 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 inited
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()
{
cout << "SpotImage::draw " << _id << 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 (i = 0; i < NUM_IMAGES; i++)
images[i]->draw();
// Free the dynamic memory
for (i = 0; i < NUM_IMAGES; i++)
delete images[i];
}
LandSatImage::draw 1
LandSatImage::draw 2
LandSatImage::draw 3
SpotImage::draw 1
LandSatImage::draw 4
SpotImage::draw 2
SpotImage::draw 3
LandSatImage::draw 5
