建造者模式

构造函数可能非常复杂，例如：一个蛋糕，可能有各种形状、口味、尺寸、奶油、水果……构造函数需要接收大量参数，执行复杂的初始化过程（例如，需要大量条件判断这些参数是否为空）。为了解决这个问题，引入建造者模式，简化构造函数。

建造者模式需要三个关键组件

• 目标类：比如class Car
• 建造者：比如class CarBuilder，包含了构造车的各种方法
• 监督者

构造者模式用于建造非常复杂的对象，因此，这个目标对象可能不遵循相同的接口

class Car {
  public:
    std::vector<std::string> parts_; // 下划线用于区分成员变量和参数/局部变量
    // std::unordered_map<std::string, std::string> parts_;
    void listParts() const {
      std::cout << "Car parts: ";
      for (size_t i = 0; i < parts_.size(); i++) {
        if (i == parts_.size() - 1) {
          std::cout << parts_[i];
        } else {
          std::cout << parts_[i] << ", ";
        }
      }
      std::cout << "\n\n";
    }
};

class Builder {
  public:
    virtual void reset() = 0;
    virtual void setSeats(...) = 0;
    virtual void setEngine(...) = 0;
    virtual void setTripComputer(...) = 0;
    virtual void setGPS(...) = 0;
};

class CarBuilder : public Builder {
  private:
    Car* car_;
  public:
    CarBuilder(): car_(nullptr) {
      reset();
    }
    void reset() {
      if (car_) delete car_;
      car_ = new Car();
    }
    void setSeats(...) {
      // set num of seats in the car
    }
    void setEngine(...) {
      // set engine type
    }
    void setTripComputer(...) {
      // set Trip computer type
    }
    void setGPS(...) {
      // install GPS
    }
    Car* getProduct() {
      Car* car = car_;
      reset();
      return car;
    }
};

class Director {
  public:
    Car* constructSportsCar(Builder& builder) {
      builder.reset();
      builder.setSeats(2);
      builder.setEngine("someEngine");
      builder.setTripComputer(true);
      builder.setGPS(true);
      return builder.getProduct();
    }
    Car* constructSUV(Builder& builder) {
      // steps to build an SUV
    }
};

int main(int argc, char* argv[]) {
  CarBuilder builder = CarBuilder();
  Director director = Director();
  Car* newCar = director.constructSportsCar(builder);
  newCar->listParts();
  delete newCar;
  return 0;
}