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外观

C++ 外观模式讲解和代码示例

外观是一种结构型设计模式 能为复杂系统 程序库或框架提供一个简单 但有限 的接口

尽管外观模式降低了程序的整体复杂度 但它同时也有助于将不需要的依赖移动到同一个位置

在 C++ 中使用模式

复杂度

流行度

使用示例 使用 C++ 开发的程序中会经常使用外观模式 它在与复杂程序库和 API 协作时特别有用

识别方法 外观可以通过使用简单接口 但将绝大部分工作委派给其他类的类来识别 通常情况下 外观管理其所使用的对象的完整生命周期

概念示例

本例说明了外观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 Subsystem can accept requests either from the facade or client directly.
 * In any case, to the Subsystem, the Facade is yet another client, and it's not
 * a part of the Subsystem.
 */
class Subsystem1 {
 public:
  std::string Operation1() const {
    return "Subsystem1: Ready!\n";
  }
  // ...
  std::string OperationN() const {
    return "Subsystem1: Go!\n";
  }
};
/**
 * Some facades can work with multiple subsystems at the same time.
 */
class Subsystem2 {
 public:
  std::string Operation1() const {
    return "Subsystem2: Get ready!\n";
  }
  // ...
  std::string OperationZ() const {
    return "Subsystem2: Fire!\n";
  }
};

/**
 * The Facade class provides a simple interface to the complex logic of one or
 * several subsystems. The Facade delegates the client requests to the
 * appropriate objects within the subsystem. The Facade is also responsible for
 * managing their lifecycle. All of this shields the client from the undesired
 * complexity of the subsystem.
 */
class Facade {
 protected:
  Subsystem1 *subsystem1_;
  Subsystem2 *subsystem2_;
  /**
   * Depending on your application's needs, you can provide the Facade with
   * existing subsystem objects or force the Facade to create them on its own.
   */
 public:
  /**
   * In this case we will delegate the memory ownership to Facade Class
   */
  Facade(
      Subsystem1 *subsystem1 = nullptr,
      Subsystem2 *subsystem2 = nullptr) {
    this->subsystem1_ = subsystem1 ?: new Subsystem1;
    this->subsystem2_ = subsystem2 ?: new Subsystem2;
  }
  ~Facade() {
    delete subsystem1_;
    delete subsystem2_;
  }
  /**
   * The Facade's methods are convenient shortcuts to the sophisticated
   * functionality of the subsystems. However, clients get only to a fraction of
   * a subsystem's capabilities.
   */
  std::string Operation() {
    std::string result = "Facade initializes subsystems:\n";
    result += this->subsystem1_->Operation1();
    result += this->subsystem2_->Operation1();
    result += "Facade orders subsystems to perform the action:\n";
    result += this->subsystem1_->OperationN();
    result += this->subsystem2_->OperationZ();
    return result;
  }
};

/**
 * The client code works with complex subsystems through a simple interface
 * provided by the Facade. When a facade manages the lifecycle of the subsystem,
 * the client might not even know about the existence of the subsystem. This
 * approach lets you keep the complexity under control.
 */
void ClientCode(Facade *facade) {
  // ...
  std::cout << facade->Operation();
  // ...
}
/**
 * The client code may have some of the subsystem's objects already created. In
 * this case, it might be worthwhile to initialize the Facade with these objects
 * instead of letting the Facade create new instances.
 */

int main() {
  Subsystem1 *subsystem1 = new Subsystem1;
  Subsystem2 *subsystem2 = new Subsystem2;
  Facade *facade = new Facade(subsystem1, subsystem2);
  ClientCode(facade);

  delete facade;

  return 0;
}

Output.txt: 执行结果

Facade initializes subsystems:
Subsystem1: Ready!
Subsystem2: Get ready!
Facade orders subsystems to perform the action:
Subsystem1: Go!
Subsystem2: Fire!

外观在其他编程语言中的实现

Java 外观模式讲解和代码示例 C# 外观模式讲解和代码示例 PHP 外观模式讲解和代码示例 Python 外观模式讲解和代码示例 Ruby 外观模式讲解和代码示例 Swift 外观模式讲解和代码示例 TypeScript 外观模式讲解和代码示例 Go 外观模式讲解和代码示例