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::basic void 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;}
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其输出结果如同这样：
--- BEGIN OF test_flyweight_basic ---------------------- flyweight_factory: I have 5 flyweights:BMW_X6_whiteMercedes Benz_C500_redMercedes Benz_C300_blackBMW_M5_redChevrolet_Camaro2018_pink Client: 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 ----------------------
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可以看到，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），而引用参考不过是一个指针的大小（通常是 64 bytes on 64-bit OS），那么最终节省的内存是非常可观的。
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