实现设计模式：工厂模式

工厂模式将创建对象的代码从客户代码中分离，提供了一种在大系统中管理创建众多对象的有效方法。当从配置文件或流中创建对象时，经常采用这种模式。

这里提供一种通用实现，可以作为参考。

#include <map>#include <boost/function.hpp>namespace factory{template<class T, class Allocator=std::allocator<T> >class simple_factory{public:>template<typename Key>T* operator()(Key&){T* pT=static_cast<T*>(m_allocate.allocate(1));m_allocate.construct(pT, T());return pT;}private:Allocator m_allocate;};template<typename Key, class T, class Allocator=std::allocator<T> >class factory{public:typedef Allocator allocator_type;typedef typename Allocator::value_type value_type;typedef typename Allocator::pointer pointer;typedef typename Allocator::reference reference;typedef Key key_type;template<class Factory>void insert(const key_type& key, Factory& factory){m_factories.insert(std::make_pair(key, factory));}template<class Derive>void insert_class(const key_type& key){insert(key, simple_factory<Derive, allocator_type>());}pointer construct(const key_type& key) const{factories_type::const_iterator it=m_factories.find(key);if(it!=m_factories.end())return (it->second)(key);return NULL;}void destroy(pointer p){allocator_type a;a.destroy(p);a.deallocate(p, 1);}private:        typedef boost::function<pointer (key_type)> factory_type;        typedef std::map<key_type, factory_type, std::less<key_type>, typename allocator_type::rebind<std::pair<const typename key_type, typename factory_type> >::other > factories_type;factories_type m_factories;};}

为了保证创建和销毁对象的一致性，工厂类提供了销毁对象的方法。
以下是个测试例子：

struct Base{int x;};struct A : public Base{A() { x=1; }};struct B : public Base{B() { x=2; }};void test(){factory::factory<int, Base> fact;fact.insert_class<A>(1);fact.insert_class<B>(2);Base* pBase=fact.construct(1);cout<<pBase->x<<endl;fact.destroy(pBase);}