Factory Method を C++ で

Factory Method は、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。

Factory Method の詳細

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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}