好吧，在将此标记为重复之前，让我澄清一下自己。
我正在阅读有关访客模式及其适用用途的信息。

我偶然发现了这篇文章:
When should I use the Visitor Design Pattern?

写下第一个答案的用户说:

Now we want to add a new operation to the hierarchy, namely we want each animal to make its sound. As far as the hierarchy is this simple, you can do it with straight polymorphism:
...
But proceeding in this way, each time you want to add an operation you must modify the interface to every single class of the hierarchy.

最佳答案

访问者模式适用于需要双重分发且编程语言不支持它的情况。

我是C#开发人员，C#不直接支持它，而且我认为C++也不支持。 (尽管在C#的较新版本中，在C#4.0之后，仍有dynamic关键字可以解决问题)。

我将以一个示例来说明一种情况，在这种情况下，我们需要双重调度以及访问者如何帮助我们做到这一点。
(由于我是C#开发人员，所以我的代码库是C#，请耐心等待，但我保证，我已尽力使它尽可能保持语言中立)

示例:

可以说我有3种类型的移动设备-iPhone，Android，Windows Mobile。

所有这三个设备都安装了蓝牙 radio 。

假设蓝牙 radio 可以来自两个独立的OEM:英特尔和Broadcom。

只是为了使示例与我们的讨论相关，我们还假设英特尔 radio 公开的API与Broadcom radio 公开的API不同。

这是我类(class)的样子–




现在，我要介绍一个操作–在移动设备上打开蓝牙。

它的功能签名应该像这样–

 void SwitchOnBlueTooth(IMobileDevice mobileDevice, IBlueToothRadio blueToothRadio)

 class Client
  {
      public void SwitchOnBlueTooth(IMobileDevice mobileDevice, IBlueToothVisitor blueToothRadio) 
      {
          mobileDevice.TurnOn(blueToothRadio);        
      }
  }


 [TestClass]
public class VisitorPattern
{

    Client mClient = new Client();

    [TestMethod]
    public void AndroidOverBroadCom()
    {
        IMobileDevice device = new Android();
        IBlueToothVisitor btVisitor = new BroadComBlueToothVisitor();

        mClient.SwitchOnBlueTooth(device, btVisitor);
    }

    [TestMethod]
    public void AndroidOverIntel()
    {
        IMobileDevice device = new Android();
        IBlueToothVisitor btVisitor = new IntelBlueToothVisitor();

        mClient.SwitchOnBlueTooth(device, btVisitor);
    }

    [TestMethod]
    public void iPhoneOverBroadCom()
    {
        IMobileDevice device = new iPhone();
        IBlueToothVisitor btVisitor = new BroadComBlueToothVisitor();

        mClient.SwitchOnBlueTooth(device, btVisitor);
    }

    [TestMethod]
    public void iPhoneOverIntel()
    {
        IMobileDevice device = new iPhone();
        IBlueToothVisitor btVisitor = new IntelBlueToothVisitor();

        mClient.SwitchOnBlueTooth(device, btVisitor);
    }
}

     /// <summary>
        /// Visitable class interface 
        /// </summary>
       interface IMobileDevice
        {
           /// <summary>
           /// It is the 'Accept' method of visitable class
           /// </summary>
            /// <param name="blueToothVisitor">Visitor Visiting the class</param>
           void TurnOn(IBlueToothVisitor blueToothVisitor);
        }

       class iPhone : IMobileDevice
       {
           public void TurnOn(IBlueToothVisitor blueToothVisitor)
           {
               blueToothVisitor.SwitchOn(this);
           }
       }

       class Android : IMobileDevice
       {
           public void TurnOn(IBlueToothVisitor blueToothVisitor)
           {
               blueToothVisitor.SwitchOn(this);
           }
       }

       class WindowsMobile : IMobileDevice
       {
           public void TurnOn(IBlueToothVisitor blueToothVisitor)
           {
               blueToothVisitor.SwitchOn(this);
           }
       }

        interface IBlueToothRadio
        {

        }

        class BroadComBlueToothRadio : IBlueToothRadio
        {

        }

        class IntelBlueToothRadio : IBlueToothRadio
        {

        }

/// <summary>
/// Wiki Page - The Visitor pattern encodes a logical operation on the whole hierarchy into a single class containing one method per type. 
/// </summary>
interface IBlueToothVisitor
{
    void SwitchOn(iPhone device);
    void SwitchOn(WindowsMobile device);
    void SwitchOn(Android device);
}


class IntelBlueToothVisitor : IBlueToothVisitor
{
    IBlueToothRadio intelRadio = new IntelBlueToothRadio();

    public void SwitchOn(iPhone device)
    {
        Console.WriteLine("Swithing On intel radio on iPhone");
    }

    public void SwitchOn(WindowsMobile device)
    {
        Console.WriteLine("Swithing On intel radio on Windows Mobile");
    }

    public void SwitchOn(Android device)
    {
        Console.WriteLine("Swithing On intel radio on Android");
    }
}

class BroadComBlueToothVisitor : IBlueToothVisitor
{
    IBlueToothRadio broadCom = new BroadComBlueToothRadio();

    public void SwitchOn(iPhone device)
    {
        Console.WriteLine("Swithing On BroadCom radio on iPhone");
    }

    public void SwitchOn(WindowsMobile device)
    {
        Console.WriteLine("Swithing On BroadCom radio on Windows Mobile");
    }

    public void SwitchOn(Android device)
    {
        Console.WriteLine("Swithing On BroadCom radio on Android");
    }
}

  public void SwitchOnBlueTooth(IMobileDevice mobileDevice, IBlueToothRadio blueToothRadio)

关于c++ - 何时真正使用访客模式，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/33456948/