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Factory Method

Factory Method を C++ で

Factory Method 生成に関するデザインパターンの一つで 具象クラスを指定することなく プロダクト 訳注 本パターンでは 生成されるモノのことを一般にプロダクトと呼びます のオブジェクトを生成することを可能とします

Factory Method では オブジェクトの生成において 直接のコンストラクター呼び出し new 演算子 代わりに使用すべきメソッドを定義します サブクラスにおいてこのメソッドを上書きすることにより 生成されるオブジェクトのクラスを変更します

もし各種ファクトリー系のパターンやコンセプトの違いで迷った場合は ファクトリーの比較 をご覧ください

複雑度

人気度

使用例 Factory Method パターンは C++ コードでは広く使われます コードに高度の柔軟性を持たせたい時にとても役に立ちます

見つけ方 具象クラスで具象オブジェクトを作成し それを抽象型またはインターフェースのオブジェクトとして返すような生成メソッドの存在により Factory Method を識別できます

概念的な例

この例は Factory Method デザインパターンの構造を説明するためのものです 以下の質問に答えることを目的としています

  • どういうクラスからできているか
  • それぞれのクラスの役割は
  • パターンの要素同士はどう関係しているのか

main.cc: 概念的な例

/**
 * The Product interface declares the operations that all concrete products must
 * implement.
 */

class Product {
 public:
  virtual ~Product() {}
  virtual std::string Operation() const = 0;
};

/**
 * Concrete Products provide various implementations of the Product interface.
 */
class ConcreteProduct1 : public Product {
 public:
  std::string Operation() const override {
    return "{Result of the ConcreteProduct1}";
  }
};
class ConcreteProduct2 : public Product {
 public:
  std::string Operation() const override {
    return "{Result of the ConcreteProduct2}";
  }
};

/**
 * The Creator class declares the factory method that is supposed to return an
 * object of a Product class. The Creator's subclasses usually provide the
 * implementation of this method.
 */

class Creator {
  /**
   * Note that the Creator may also provide some default implementation of the
   * factory method.
   */
 public:
  virtual ~Creator(){};
  virtual Product* FactoryMethod() const = 0;
  /**
   * Also note that, despite its name, the Creator's primary responsibility is
   * not creating products. Usually, it contains some core business logic that
   * relies on Product objects, returned by the factory method. Subclasses can
   * indirectly change that business logic by overriding the factory method and
   * returning a different type of product from it.
   */

  std::string SomeOperation() const {
    // Call the factory method to create a Product object.
    Product* product = this->FactoryMethod();
    // Now, use the product.
    std::string result = "Creator: The same creator's code has just worked with " + product->Operation();
    delete product;
    return result;
  }
};

/**
 * Concrete Creators override the factory method in order to change the
 * resulting product's type.
 */
class ConcreteCreator1 : public Creator {
  /**
   * Note that the signature of the method still uses the abstract product type,
   * even though the concrete product is actually returned from the method. This
   * way the Creator can stay independent of concrete product classes.
   */
 public:
  Product* FactoryMethod() const override {
    return new ConcreteProduct1();
  }
};

class ConcreteCreator2 : public Creator {
 public:
  Product* FactoryMethod() const override {
    return new ConcreteProduct2();
  }
};

/**
 * The client code works with an instance of a concrete creator, albeit through
 * its base interface. As long as the client keeps working with the creator via
 * the base interface, you can pass it any creator's subclass.
 */
void ClientCode(const Creator& creator) {
  // ...
  std::cout << "Client: I'm not aware of the creator's class, but it still works.\n"
            << creator.SomeOperation() << std::endl;
  // ...
}

/**
 * The Application picks a creator's type depending on the configuration or
 * environment.
 */

int main() {
  std::cout << "App: Launched with the ConcreteCreator1.\n";
  Creator* creator = new ConcreteCreator1();
  ClientCode(*creator);
  std::cout << std::endl;
  std::cout << "App: Launched with the ConcreteCreator2.\n";
  Creator* creator2 = new ConcreteCreator2();
  ClientCode(*creator2);

  delete creator;
  delete creator2;
  return 0;
}

Output.txt: 実行結果

App: Launched with the ConcreteCreator1.
Client: I'm not aware of the creator's class, but it still works.
Creator: The same creator's code has just worked with {Result of the ConcreteProduct1}

App: Launched with the ConcreteCreator2.
Client: I'm not aware of the creator's class, but it still works.
Creator: The same creator's code has just worked with {Result of the ConcreteProduct2}

他言語での Factory Method

Factory Method を C# で Factory Method を Go で Factory Method を Java で Factory Method を PHP で Factory Method を Python で Factory Method を Ruby で Factory Method を Rust で Factory Method を Swift で Factory Method を TypeScript で