C++ 工厂方法模式讲解和代码示例
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、 工厂方法和抽象工厂模式之间的区别， 请参阅工厂模式比较。
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}
