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工厂方法

工厂方法在 C++ 中的实现

工厂方法是一种创建型设计模式 解决了在不指定具体类的情况下创建产品对象的问题

工厂方法定义了一个方法 且必须使用该方法代替通过直接调用构造函数来创建对象 new 操作符 的方式 子类可重写该方法来更改将被创建的对象所属类

如果你不清楚工厂 工厂方法抽象工厂模式之间的区别 请参阅工厂模式比较

在 C++ 中使用模式

复杂度

流行度

使用示例 工厂方法模式在 C++ 代码中得到了广泛使用 当你需要在代码中提供高层次的灵活性时 该模式会非常实用

识别方法 工厂方法可通过构建方法来识别 它会创建具体类的对象 但以抽象类型或接口的形式返回这些对象

概念示例

本例说明了工厂方法设计模式的结构并重点回答了下面的问题

  • 它由哪些类组成
  • 这些类扮演了哪些角色
  • 模式中的各个元素会以何种方式相互关联

main.cc: 概念示例

/**
 * 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: 执行结果

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}

工厂方法在其他编程语言中的实现

工厂方法在 C# 中的实现 工厂方法在 Java 中的实现 工厂方法在 PHP 中的实现 工厂方法在 Python 中的实现 工厂方法在 Ruby 中的实现 工厂方法在 Swift 中的实现 工厂方法在 TypeScript 中的实现