设计模式——工厂方法模式

引言

首先了解一下简单工厂模式：

假如有一个抽象的产品类，然后有很多具体的产品类，一个具体的工厂类根据传入的参数决定生产哪种产品。其优点是将产品的创建和使用分离，客户端不需要关心产品是如何创建的。

代码实例如下：

cpp 复制代码

///
//  Factory.cpp
//  Implementation of the Class Factory
//  Created on:      01-10-2014 18:41:33
//  Original author: colin
///

#include "Factory.h"
#include "ConcreteProductA.h"
#include "ConcreteProductB.h"
Product* Factory::createProduct(string proname){
	if ( "A" == proname )
	{
		return new ConcreteProductA();
	}
	else if("B" == proname)
	{
		return new ConcreteProductB();
	}
	return  NULL;
}

上述模式缺点是如果工厂需要管理的产品非常多，那么实现逻辑很复杂且每次增加新的产品要修改原有的工厂类。

基本思想

于是就有了工厂模式，核心是添加一层抽象工厂类，实现间接引用，符合开闭原则。

工厂模式将创建对象的工作交给了工厂的子类（延迟创建对象），自己只提供需要子类实现的方法。这样，在需要新增的时候就可以不破坏原来的结构。

e.g.富士康(提供场地人员制作方法比如生产、运输、组装) ，富士康里面有苹果的生产线、有华为的生产线。这些生产线来进行具体怎么生产，怎么组装，怎么运输。创建一个接口，然后再根据不同的需求实现这个接口。

代码实例：

cpp 复制代码

/**
 * 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;
}

输出如下：

复制代码

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}

工程方法模式很好理解，就是在简单工厂模式上面加了一层抽象，然后把针对具体产品的生成交给不同的工厂去做，客户端通过调用不同的工厂类方法生产不同的产品。

工厂方法模式优缺点

优点：

你可以避免创建者和具体产品之间的紧密耦合。
单一职责原则。你可以将产品创建代码放在程序的单一位置，从而使得代码更容易维护。
开闭原则 。无需更改现有客户端代码，你就可以在程序中引入新的产品类型。
缺点：
应用工厂方法模式需要引入许多新的子类，代码可能会因此变得更复杂。最好的情况是将该模式引入创建者类的现有层次结构中。