Factory Method

Saw this pattern actually being used at NVIDIA.

How is this different from the Visitor Pattern?

It seems to be basically the same thing, both built on basic Polymorphism. The main difference is the intent of the method. Factory is to instantiate many things of different types, let the child concrete classes actually implement it. While the visitor pattern is designed for a function / interface to work will multiple types out of the box.

Factory method is a creational design pattern which solves the problem of creating product objects without specifying their concrete classes.

/**
 * 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;
}