design-patterns-cpp/flyweight/Flyweight.cpp

/*
 * C++ Design Patterns: Flyweight
 * Author: Jakub Vojvoda [github.com/JakubVojvoda]
 * 2016
 *
 * Source code is licensed under MIT licence
 * (for more details see LICENCE)
 *
 */

#include <iostream>
#include <map>

/*
 * Flyweight
 * declares an interface through which flyweights can receive
 * and act on extrinsic state
 */
class Flyweight {
public:
  virtual ~Flyweight() { /* ... */ }
  virtual void operation() = 0;
  // ...
};

/*
 * UnsharedConcreteFlyweight
 * not all subclasses need to be shared
 */
class UnsharedConcreteFlyweight : public Flyweight {
public:
  UnsharedConcreteFlyweight(int intrinsic_state) :
    state(intrinsic_state) {}

  void operation() {
    std::cout << "Unshared Flyweight with state " << state << std::endl;
  }
  // ...

private:
  int state;
  // ...
};

/*
 * ConcreteFlyweight
 * implements the Flyweight interface and adds storage
 * for intrinsic state
 */
class ConcreteFlyweight : public Flyweight {
public:
  ConcreteFlyweight(int all_state) :
    state(all_state) {}

  void operation() {
    std::cout << "Concrete Flyweight with state " << state << std::endl;
  }
  // ...

private:
  int state;
  // ...
};

/*
 * FlyweightFactory
 * creates and manages flyweight objects and ensures
 * that flyweights are shared properly
 */
class FlyweightFactory {
public:
  virtual ~FlyweightFactory() {
    for (auto it = flies.begin(); it != flies.end(); it++) {
        delete it->second;
    }
    flies.clear();
  }

  Flyweight *getFlyweight(int key) {
    if (flies.find(key) != flies.end()) {
      return flies[key];
    }
    Flyweight *fly = new ConcreteFlyweight(key);
    flies.insert(std::pair<int, Flyweight *>(key, fly));
    return fly;
  }
  // ...

private:
  std::map<int, Flyweight *> flies;
  // ...
};


int main()
{
  FlyweightFactory *factory = new FlyweightFactory;
  factory->getFlyweight(1)->operation();
  factory->getFlyweight(2)->operation();

  return 0;
}