享元模式

存在大量细粒度对象的系统中，使用享元模式可以减少对相同对象的创建。

#include <iostream>
#include <memory>
#include <unordered_map>
#include <utility>
class Flyweight {
 public:
  virtual void Print() = 0;
  virtual ~Flyweight() = default;
};
class ConcreteFlyweight : public Flyweight {
 public:
  explicit ConcreteFlyweight(char key) : key_(key) {}
  void Print() override { std::cout << key_<<'\n'; }
 private:
  char key_;
};
class FlyweightFactory {
 public:
  std::unique_ptr<Flyweight>& FlyweightPtr(char c) {
    if (const auto it = m_.find(c); it != std::end(m_)) {
      std::cout << "existing key:";
      return it->second;
    }
    std::unique_ptr<Flyweight> p = std::make_unique<ConcreteFlyweight>(c);
    m_.emplace(c, std::move(p));
    return m_.at(c);
  }
 private:
  std::unordered_map<char, std::unique_ptr<Flyweight>> m_;
};
int main() {
  FlyweightFactory factory;
  factory.FlyweightPtr('a')->Print();  // a
  factory.FlyweightPtr('a')->Print();  // existing key: a
  factory.FlyweightPtr('b')->Print();  // b
  factory.FlyweightPtr('b')->Print();  // existing key: a
}