[C++][设计模式][抽象工厂]详细讲解

目录

• 1.动机
• 2.模式定义
• 3.要点总结
• 4.代码感受
• • 1.代码一
  • 2.代码二 -- 工厂方法
  • 3.代码三 -- 抽象工厂

    ---

    1.动机

    • 在软件系统中，经常面临着“一系列相互依赖的对象”的创建工作；同时，由于需求的变化，往往存在更多系列对象的创建工作
    • 如何应对这种变化？如何绕过常规的对象创建方法(new)，提供一 种“封装机制”来避免客户程序和这种“多系列具体对象创建工作”的紧耦合？

      ---

      2.模式定义

      • 定义：提供一个接口，让该接口负责创建一系列“相关或者相互依赖的对象”，无需指定它们具体的类
        • 个人感觉抽象工厂叫家族工厂可能更形象:P
        • 结构

          

          ---

          3.要点总结

          • 如果没有应对“多系列对象构建”的需求变化，则没有必要使用Abstract Factory模式，这时候使用简单的工厂完全可以
          • “系列对象”指的是在某一特定系列下的对象之间有相互依赖、 或作用的关系。不同系列的对象之间不能相互依赖
          • Abstract Factory模式主要在于应对“新系列”的需求变动
            • 缺点：难以应对“新对象”的需求变动

              ---

              4.代码感受

              1.代码一

              class EmployeeDAO
              {
              public:
                  vector GetEmployees()
                  {
                      SqlConnection* connection = new SqlConnection();
                      connection->ConnectionString = "...";
                      SqlCommand* command = new SqlCommand();
                      command->CommandText = "...";
                      command->SetConnection(connection);
                      SqlDataReader* reader = command->ExecuteReader();
                      while (reader->Read())
                      {}
                  }
              };
              

              2.代码二 – 工厂方法

              // 数据库访问有关的基类
              class IDBConnection
              {};
              class IDBConnectionFactory
              {
              public:
                  virtual IDBConnection* CreateDBConnection() = 0;
              };
              class IDBCommand
              {};
              class IDBCommandFactory
              {
              public:
                  virtual IDBCommand* CreateDBCommand() = 0;
              };
              class IDataReader
              {};
              class IDataReaderFactory
              {
              public:
                  virtual IDataReader* CreateDataReader() = 0;
              };
              //支持SQL Server
              class SqlConnection: public IDBConnection
              {};
              class SqlConnectionFactory:public IDBConnectionFactory
              {};
              class SqlCommand: public IDBCommand
              {};
              class SqlCommandFactory:public IDBCommandFactory
              {};
              class SqlDataReader: public IDataReader
              {};
              class SqlDataReaderFactory:public IDataReaderFactory
              {};
              //支持Oracle
              class OracleConnection: public IDBConnection
              {};
              class OracleCommand: public IDBCommand
              {};
              class OracleDataReader: public IDataReader
              {};
              class EmployeeDAO
              {
                  IDBConnectionFactory* dbConnectionFactory;
                  IDBCommandFactory* dbCommandFactory;
                  IDataReaderFactory* dataReaderFactory;
              public:
                  vector GetEmployees()
                  {
                      IDBConnection* connection =
                          dbConnectionFactory->CreateDBConnection();
                      connection->ConnectionString("...");
                      IDBCommand* command =
                          dbCommandFactory->CreateDBCommand();
                      command->CommandText("...");
                      command->SetConnection(connection); // 关联性
                      IDBDataReader* reader = command->ExecuteReader(); // 关联性
                      while (reader->Read())
                      {}
                  }
              };
              

              3.代码三 – 抽象工厂

              //数据库访问有关的基类
              class IDBConnection
              {};
              class IDBCommand
              {};
              class IDataReader
              {};
              class IDBFactory
              {
              public:
                  virtual IDBConnection* CreateDBConnection() = 0;
                  virtual IDBCommand* CreateDBCommand() = 0;
                  virtual IDataReader* CreateDataReader() = 0;
              };
              //支持SQL Server
              class SqlConnection: public IDBConnection
              {};
              class SqlCommand: public IDBCommand
              {};
              class SqlDataReader: public IDataReader
              {};
              class SqlDBFactory:public IDBFactory
              {
              public:
                  virtual IDBConnection* CreateDBConnection()
                  {}
                  virtual IDBCommand* CreateDBCommand()
                  {}
                  virtual IDataReader* CreateDataReader()
                  {}
               
              };
              //支持Oracle
              class OracleConnection: public IDBConnection
              {};
              class OracleCommand: public IDBCommand
              {};
              class OracleDataReader: public IDataReader
              {};
              class EmployeeDAO
              {
                  IDBFactory* dbFactory;
              public:
                  vector GetEmployees()
                  {
                      IDBConnection* connection =
                          dbFactory->CreateDBConnection();
                      connection->ConnectionString("...");
                      IDBCommand* command =
                          dbFactory->CreateDBCommand();
                      command->CommandText("...");
                      command->SetConnection(connection);
                      IDBDataReader* reader = command->ExecuteReader();
                      while (reader->Read())
                      {}
                  }
              };