享元模式其實類似於物件池。如果系統中有大量的細粒度物件，建立這些物件是不小的開銷。如果能共享和反覆使用這些物件，那麼就可以減少物件數量，提高系統性能。
下面實現了一個享元工廠，可以作為參考。

#include <map>
#include <boost/function.hpp>

namespace dp
{
	template<typename Param, class T>
	class flyweight_factory
	{
	public:
		template<typename Factory>
		flyweight_factory(Factory factory) : m_factory(factory) { }
		T* get(const Param& param)
		{
			flyweight_map::iterator it=m_flyweights.find(param);
			if(it==m_flyweights.end())
			{
				T* pobj=m_factory(param);
				if(pobj)
					it=m_flyweights.insert(std::make_pair(param, pobj)).first;
			}
			return (it==m_flyweights.end())?NULL:it->second;
		}
		void destroy(const Param& param)
		{
			flyweight_map::iterator it=m_flyweights.find(param);
			if(it!=m_flyweights.end())
			{
				delete it->second;
				m_flyweights.erase(it);
			}
		}

	private:
		typedef std::map<Param, T*> flyweight_map;
		boost::function<T*(const Param&)> m_factory;
		flyweight_map m_flyweights;
	};
}
 

class Char
{
public:
	void print() { putchar(c); }
protected:
	char c;
};

class SimpleChar : public Char
{
public:
	SimpleChar(char c) { this->c=c; }
};

void test()
{
	dp::flyweight_factory<char, Char> factory((boost::factory<SimpleChar*>()));
	Char* p=factory.get('a');
	p->print();
	p=NULL;
	factory.destroy('a');
}