回顾享元模式，思考实作它的各种问题。
Prologue
略过
FlyWeight Pattern
实践
享元模式，是将简单对象的雷同的组成元素抽出并独自保护的一种结构型设计模式。这些雷同的组成元素被称为共享元件，它们在一个独自的容器中被唯一性地治理，而简单对象只需持有到该惟一实例的参考，而无需反复创立这样的雷同的元素，从而可能大幅度地削减内存占用。
以字处理器为例，每个字符都具备独立的、区别于其它字符的非凡属性：例如字体款式，背景、边框、对齐等等。如果一个文档中全副字符都独自存储一份它的所有属性的正本，那么这将会是宏大的内存需要。但思考到一大堆（例如1000个）字符可能都有雷同的“宋体，9pt”这样的属性，那么实际上咱们只须要独自存储一份“宋体，9pt”的字体款式属性，而一个字符只须要一个指向该字体款式属性的指针就能够了，这就比1000个字符的1000个字体款式属性拷贝要节约的多。
相似的案例还有相当多，例如例子零碎中的每个粒子（例如子弹、弹片，或者敌方飞机）都有一些雷同的属性（例如色彩，轮廓等等）占地不小，但值却雷同。
工厂模式
很容易想到，咱们能够在一个工厂中就地治理享元对象。当客户以具体值来申请一个享元对象时，工厂会从一个字典中检索享元是否存在，而后返回该元素的参考援用给客户。如果享元尚未存在，那么工厂会创立它，而后在返回援用。
不可变性
依照传统的说法，享元模式要求这些雷同的局部（享元，雷同的组成元素）是不可变的。但这并不是铁律。
一个办法是，以一个享元为整体，咱们能够整体批改对象持有的享元参考。
例如咱们正在批改字处理器中的一个单词的字体款式，从“宋体，9pt”改为“黑体，12pt”，那么咱们能够间接批改援用指向。也就是说，咱们提供 character.apply_font_style(font_style& style) 这样的整体批改接口。
另一个办法能够从更细的粒度登程进行批改，例如从“宋体，9pt”改为“宋体，10pt”，但在产生变更时，尝试从工厂中查证新值的参考。也就是说，咱们提供 character.set_font_size(float pt) 这样的接口，但在其实现过程中记得去查证享元工厂（管理器）以求更新外部援用。
C++ 实现
传统的享元模式的实现形式有这样的示例代码：
namespace hicc::dp::flyweight::basic { /** * flyweight Design Pattern * * Intent: Lets you fit more objects into the available amount of RAM by sharing * common parts of state between multiple objects, instead of keeping all of the * data in each object. */ struct shared_state { std::string brand_; std::string model_; std::string color_; shared_state(const std::string &brand, const std::string &model, const std::string &color) : brand_(brand) , model_(model) , color_(color) { } friend std::ostream &operator<<(std::ostream &os, const shared_state &ss) { return os << "[ " << ss.brand_ << " , " << ss.model_ << " , " << ss.color_ << " ]"; } }; struct unique_state { std::string owner_; std::string plates_; unique_state(const std::string &owner, const std::string &plates) : owner_(owner) , plates_(plates) { } friend std::ostream &operator<<(std::ostream &os, const unique_state &us) { return os << "[ " << us.owner_ << " , " << us.plates_ << " ]"; } }; /** * The flyweight stores a common portion of the state (also called intrinsic * state) that belongs to multiple real business entities. The flyweight accepts * the rest of the state (extrinsic state, unique for each entity) via its * method parameters. */ class flyweight { private: shared_state *shared_state_; public: flyweight(const shared_state *o) : shared_state_(new struct shared_state(*o)) { } flyweight(const flyweight &o) : shared_state_(new struct shared_state(*o.shared_state_)) { } ~flyweight() { delete shared_state_; } shared_state *state() const { return shared_state_; } void Operation(const unique_state &unique_state) const { std::cout << "flyweight: Displaying shared (" << *shared_state_ << ") and unique (" << unique_state << ") state.\n"; } }; /** * The flyweight Factory creates and manages the flyweight objects. It ensures * that flyweights are shared correctly. When the client requests a flyweight, * the factory either returns an existing instance or creates a new one, if it * doesn't exist yet. */ class flyweight_factory { std::unordered_map<std::string, flyweight> flyweights_; std::string key(const shared_state &ss) const { return ss.brand_ + "_" + ss.model_ + "_" + ss.color_; } public: flyweight_factory(std::initializer_list<shared_state> lists) { for (const shared_state &ss : lists) { this->flyweights_.insert(std::make_pair<std::string, flyweight>(this->key(ss), flyweight(&ss))); } } /** * Returns an existing flyweight with a given state or creates a new one. */ flyweight get(const shared_state &shared_state) { std::string key = this->key(shared_state); if (this->flyweights_.find(key) == this->flyweights_.end()) { std::cout << "flyweight_factory: Can't find a flyweight, creating new one.\n"; this->flyweights_.insert(std::make_pair(key, flyweight(&shared_state))); } else { std::cout << "flyweight_factory: Reusing existing flyweight.\n"; } return this->flyweights_.at(key); } void list() const { size_t count = this->flyweights_.size(); std::cout << "\nflyweight_factory: I have " << count << " flyweights:\n"; for (std::pair<std::string, flyweight> pair : this->flyweights_) { std::cout << pair.first << "\n"; } } }; // ... void AddCarToPoliceDatabase( flyweight_factory &ff, const std::string &plates, const std::string &owner, const std::string &brand, const std::string &model, const std::string &color) { std::cout << "\nClient: Adding a car to database.\n"; const flyweight &flyweight = ff.get({brand, model, color}); // The client code either stores or calculates extrinsic state and passes it // to the flyweight's methods. flyweight.Operation({owner, plates}); }} // namespace hicc::dp::flyweight::basicvoid test_flyweight_basic() { using namespace hicc::dp::flyweight::basic; flyweight_factory *factory = new flyweight_factory({ {"Chevrolet", "Camaro2018", "pink"}, {"Mercedes Benz", "C300", "black"}, {"Mercedes Benz", "C500", "red"}, {"BMW", "M5", "red"}, {"BMW", "X6", "white"} }); factory->list(); AddCarToPoliceDatabase(*factory, "CL234IR", "James Doe", "BMW", "M5", "red"); AddCarToPoliceDatabase(*factory, "CL234IR", "James Doe", "BMW", "X1", "red"); factory->list(); delete factory;}其输入后果如同这样：
--- BEGIN OF test_flyweight_basic ----------------------flyweight_factory: I have 5 flyweights:BMW_X6_whiteMercedes Benz_C500_redMercedes Benz_C300_blackBMW_M5_redChevrolet_Camaro2018_pinkClient: Adding a car to database.flyweight_factory: Reusing existing flyweight.flyweight: Displaying shared ([ BMW , M5 , red ]) and unique ([ James Doe , CL234IR ]) state.Client: Adding a car to database.flyweight_factory: Can't find a flyweight, creating new one.flyweight: Displaying shared ([ BMW , X1 , red ]) and unique ([ James Doe , CL234IR ]) state.flyweight_factory: I have 6 flyweights:BMW_X1_redMercedes Benz_C300_blackBMW_X6_whiteMercedes Benz_C500_redBMW_M5_redChevrolet_Camaro2018_pink--- END OF test_flyweight_basic ----------------------能够看到，像 [ BMW , X1 , red ] 这样的一个享元，单个实例较大（数十、数百乃至数十K 字节），而援用参考不过是一个指针的大小（通常是 64 bytes on 64-bit OS），那么最终节俭的内存是十分可观的。
元编程中的 FlyWeight Pattern
下面的示例，曾经是旧格调了，C++11 当前咱们须要大量地应用智能指针、以及模板语法，而在 C++17 之后更好的原位结构能力容许咱们的代码可能更加 meaningful。
flyweight_factory
一个想法是，咱们认为一个尽可能通用的享元工厂可能是有利于代码书写的。所以咱们尝试这样一个享元工厂模板：
namespace hicc::dp::flyweight::meta { template<typename shared_t = shared_state_impl, typename unique_t = unique_state_impl> class flyweight { std::shared_ptr<shared_t> shared_state_; public: flyweight(flyweight const &o) : shared_state_(std::move(o.shared_state_)) { } flyweight(shared_t const &o) : shared_state_(std::make_shared<shared_t>(o)) { } ~flyweight() {} auto state() const { return shared_state_; } auto &state() { return shared_state_; } void Operation(const unique_t &unique_state) const { std::cout << "flyweight: Displaying shared (" << *shared_state_ << ") and unique (" << unique_state << ") state.\n"; } friend std::ostream &operator<<(std::ostream &os, const flyweight &o) { return os << *o.shared_state_; } }; template<typename shared_t = shared_state_impl, typename unique_t = unique_state_impl, typename flyweight_t = flyweight<shared_t, unique_t>, typename hasher_t = std::hash<shared_t>> class flyweight_factory { public: flyweight_factory() {} explicit flyweight_factory(std::initializer_list<shared_t> args) { for (auto const &ss : args) { flyweights_.emplace(_hasher(ss), flyweight_t(ss)); } } flyweight_t get(shared_t const &shared_state) { auto key = _hasher(shared_state); if (this->flyweights_.find(key) == this->flyweights_.end()) { std::cout << "flyweight_factory: Can't find a flyweight, creating new one.\n"; this->flyweights_.emplace(key, flyweight_t(shared_state)); } else { std::cout << "flyweight_factory: Reusing existing flyweight.\n"; } return this->flyweights_.at(key); } void list() const { size_t count = this->flyweights_.size(); std::cout << "\nflyweight_factory: I have " << count << " flyweights:\n"; for (auto const &pair : this->flyweights_) { std::cout << pair.first << " => " << pair.second << "\n"; } } private: std::unordered_map<std::size_t, flyweight_t> flyweights_; hasher_t _hasher{}; };} // namespace hicc::dp::flyweight::meta而后咱们就能够以派生类的形式间接应用这个享元工厂了：
class vehicle : public flyweight_factory<shared_state_impl, unique_state_impl> { public: using flyweight_factory<shared_state_impl, unique_state_impl>::flyweight_factory; void AddCarToPoliceDatabase( const std::string &plates, const std::string &owner, const std::string &brand, const std::string &model, const std::string &color) { std::cout << "\nClient: Adding a car to database.\n"; auto const &flyweight = this->get({brand, model, color}); flyweight.Operation({owner, plates}); }};其中 using flyweight_factory<shared_state_impl, unique_state_impl>::flyweight_factory; 是 C++17 当前的新语法，它将父类的所有构造函数原样复制给派生类，从而让你不用拷贝粘贴代码而后批改类名。
在 vehicle 模板类中咱们应用默认的 flyweight<shared_t, unique_t>，但你能够在 flyweight_factory 的模板参数中批改它以便提供你本人的享元类具体实现。
测试代码
void test_flyweight_meta() { using namespace hicc::dp::flyweight::meta; auto factory = std::make_unique<vehicle>( std::initializer_list<shared_state_impl>{ {"Chevrolet", "Camaro2018", "pink"}, {"Mercedes Benz", "C300", "black"}, {"Mercedes Benz", "C500", "red"}, {"BMW", "M5", "red"}, {"BMW", "X6", "white"}}); factory->list(); factory->AddCarToPoliceDatabase("CL234IR", "James Doe", "BMW", "M5", "red"); factory->AddCarToPoliceDatabase("CL234IR", "James Doe", "BMW", "X1", "red"); factory->list();}附加
咱们应用了稍稍不同的根底类 shared_state_impl 以及 unique_state_impl：
namespace hicc::dp::flyweight::meta { struct shared_state_impl { std::string brand_; std::string model_; std::string color_; shared_state_impl(const std::string &brand, const std::string &model, const std::string &color) : brand_(brand) , model_(model) , color_(color) { } shared_state_impl(shared_state_impl const &o) : brand_(o.brand_) , model_(o.model_) , color_(o.color_) { } friend std::ostream &operator<<(std::ostream &os, const shared_state_impl &ss) { return os << "[ " << ss.brand_ << " , " << ss.model_ << " , " << ss.color_ << " ]"; } }; struct unique_state_impl { std::string owner_; std::string plates_; unique_state_impl(const std::string &owner, const std::string &plates) : owner_(owner) , plates_(plates) { } friend std::ostream &operator<<(std::ostream &os, const unique_state_impl &us) { return os << "[ " << us.owner_ << " , " << us.plates_ << " ]"; } };} // namespace hicc::dp::flyweight::metanamespace std { template<> struct hash<hicc::dp::flyweight::meta::shared_state_impl> { typedef hicc::dp::flyweight::meta::shared_state_impl argument_type; typedef std::size_t result_type; result_type operator()(argument_type const &s) const { result_type h1(std::hash<std::string>{}(s.brand_)); hash_combine(h1, s.model_, s.color_); return h1; } };} // namespace std这是因为咱们在 flyweight_factory 中应用了 std::hash 技术来治理一个享元的键值，所以咱们必须明确地实现 shared_state_impl 的 std::hash 特化版本。
而在这个特化版本中咱们又利用了一个特地的 hash_combine 函数。
hash_combine
这是一个技术性很强的概念，因为它波及到了一个神奇的幻数（magicnum）0x9e3779b9。
咱们在 hicc-cxx/cmdr-cxx 中提供了一个源于 boost::hash_combine 的同名扩大：
namespace std { template<typename T, typename... Rest> inline void hash_combine(std::size_t &seed, T const &t, Rest &&...rest) { std::hash<T> hasher; seed ^= 0x9e3779b9 + (seed << 6) + (seed >> 2) + hasher(t); int i[] = {0, (hash_combine(seed, std::forward<Rest>(rest)), 0)...}; (void) (i); } template<typename T> inline void hash_combine(std::size_t &seed, T const &v) { std::hash<T> hasher; seed ^= 0x9e3779b9 + (seed << 6) + (seed >> 2) + hasher(v); }} // namespace std它的作用在于计算一系列的对象的 hash 值并组合它们。
对于如何正确地组合一堆 hash 值，较为简单地办法是：
std::size_t h1 = std::hash<std::string>("hello");std::size_t h2 = std::hash<std::string>("world");std::size_t h = h1 | (h2 << 1);但依然有更多的探讨，其中失去了公认的最佳伎俩（C++中）是源自于 boost::hash_combine 实现代码的一个办法：
seed ^= hasher(v) + 0x9e3779b9 + (seed<<6) + (seed>>2);就目前已知的学术钻研中，这是最佳的。
那么谁制作了这么奇幻的一个神经质数(golden ratio)呢，大体可考的原作应该是： A Hash Function for Hash Table Lookup 或 Hash Functions for Hash Table Lookup 。原作者 Bob Jenkins，原发于 DDJ 刊物 1997 年，代码大概是成形于 1996 年。而这个数嘛，源于这个表达式：$\frac{2^{32}}{\frac{1+\sqrt{5}}{2}}$ （)。
Epilogue
好，尽管不算太尽人意，但我的确实现了一个 C++17 的勉强比拟通用的 flyweight_factory 模板类，就将就了吧。