C++享元模式

意图

运用共享技术有效地支持大量细粒度的对象

适用性

• 一个应用程序使用了大量的对象
• 完全由于使用大量的对象，造成很大的存储开销
• 对象的大多数状态都可变为外部状态
• 如果删除对象的外部状态，那么可以用相对较少的共享对象取代很多组对象
• 应用程序不依赖于对象标识。由于 Flyweight 对象可以被共享，对于概念上明显有别的对象，标识测试将返回真值

示意性代码

#include <iostream>
#include <unordered_set>
using namespace std;

// 抽象共享对象
class BikeFlyWeight {
public:
    virtual void ride(string) = 0;

    virtual void back() = 0;

    virtual ~BikeFlyWeight() { };

    int getState() {
        return _state;
    }

protected:
    int _state = 0; // 0 是未使用中，1是使用中

};

// 具体共享对象
class MoBikeFlyWeight : public BikeFlyWeight {
public:
    MoBikeFlyWeight(string bikeId) : _bikeId(bikeId) { }

    void ride(string userName) override {
        _state = 1;
        cout << userName << "骑" << _bikeId << "号自行车出行！" << endl;
    }

    void back() override {
        _state = 0;
    }

private:
    string _bikeId;
};

// 共享对象的建造工厂
class BikeFlyWeightFactory {
public:

    ~BikeFlyWeightFactory() {
        delete instance;
        for (auto& bike : pool) {
            delete bike;
        }
    }

    static BikeFlyWeightFactory* getInstance() {
        return instance;
    }

    BikeFlyWeight* getBike() {
        for (const auto& bike : pool) {
            if (bike->getState() == 0) {
                return bike;
            }
        }
        return nullptr;
    }

private:
    BikeFlyWeightFactory() {
        for (int i = 0; i < 2; ++i) {
            pool.insert(new MoBikeFlyWeight(to_string(i)));
        }
    }
    BikeFlyWeightFactory(const BikeFlyWeightFactory&) = delete;
    BikeFlyWeightFactory(const BikeFlyWeightFactory&&) = delete;
    BikeFlyWeightFactory& operator=(const BikeFlyWeightFactory&) = delete;
    BikeFlyWeightFactory& operator=(const BikeFlyWeightFactory&&) = delete;

    static BikeFlyWeightFactory *instance;
    unordered_set<BikeFlyWeight*> pool; 
};

BikeFlyWeightFactory* BikeFlyWeightFactory::instance = new BikeFlyWeightFactory();

int main() {
    BikeFlyWeight* bike1 = BikeFlyWeightFactory::getInstance()->getBike();
    bike1->ride("ZhangSan");

    BikeFlyWeight* bike2 = BikeFlyWeightFactory::getInstance()->getBike();
    bike2->ride("ZhaoSi");
    bike2->back();

    BikeFlyWeight* bike3 = BikeFlyWeightFactory::getInstance()->getBike();
    bike3->ride("WangWu");

    if (bike1 == bike2) cout << "true" << endl;
    if (bike2 == bike3) cout << "true" << endl;
}

参考

1. 五分钟学设计模式.14.享元模式