工厂方法

以图书馆管理系统为例，需要API提供查询不同专业分类的书目列表，一般实现，

class LibraryManager {
public:LibraryManager() {}//按专业分类查询BookList Query(Type type) {if(type == TA) {return ...;} else if (type == TB) {} else if (...) {//...} else {return type_null;}        }
};

可以把构建返回对象，提取到工厂类，

class BookFactory {
public:Booklist CreateBooklist(Type type) {if(type == TA) {return ...;} else if (type == TB) {} else if (...) {//...} else {return type_null;} }
};
class LibraryManager {
public:LibraryManager(BookFactory factory) {this.factory_ = factory;}BookList Query(Type type) {return this.factory.CreateBooklist(type);}
private:this.factory_;
};

现在增加一个需求，增加文学书籍类，专业类、文学类都要支持按语种检索，工厂模式便于对新增需求的扩展，

class BookFactory {
public: virtual Booklist CreateBooklist(Type type) = 0;
}
class LanguageBookFactory: public BookFactory {
public: Booklist CreateBooklist(Type type) {//}
};
class ProfessionalBookFactory: public BookFactory {
public: Booklist CreateBooklist(Type type) {//}
};

抽象工厂方法

另一个场景，读者查、借还书都需要管理系统从数据库查询书籍、读者账户系统，同时考虑系统未来用不同的第三方数据库，如何设计呢？

class LibraryManager {
public:void QueryDB() {if(config_.db_type == MYSQL) {//具体实现} else if(config_.db_type == SQLLITE) {...} else {...}        }
private:
Config config_;
};

用统一的工厂接口提供获取数据的功能，

class DataFactory {
public:virtual ReaderInfo RetriveReaderInfo() = 0;virtual BookInfo RetriveBookInfo() = 0;
};
class DataFactoryMysql: public DataFactory {
//具体实现
}
class DataFactorySqllite: public DataFactory {
//具体实现
}
class LibraryManager {
public:void QueryDB() {if(config_.db_type == MYSQL) {auto mysql = std::make_shared<DataFactoryMysql>();mysql->RetriveReaderInfo();} else if(config_.db_type == SQLLITE) {...} else {...}        }