c++ - 使用派生类的数据调用基类的方法 : is there an elegant way to do this?

Question

我正在研究 C++ 库。这个库将包含一堆简单的对象，它们彼此非常相似。

我已经为我的对象定义了一个简单的接口(interface):

struct ICommand_v1
{
  virtual void GetCommandName(wchar_t* nameBuffer, size_t* nameBufferSize) = 0;
  virtual void GetCommandGUID(wchar_t* guidBuffer, size_t* guidBufferSize) = 0;
  virtual bool Execute(int argc, wchar_t* argv[]) = 0;
};

我的难点在于 GetCommandName 和 GetCommandGUID 函数。每个子类的它们应该几乎相同；有一点完整性检查，然后返回命令的内置名称或 GUID。一个 GetCommandName 与另一个，或一个 GetCommandGUID 与另一个的唯一区别是返回值。我不想在我的库将使用的大约两打对象中重复这些方法，所以我尝试为我的对象创建一个基类:

struct ICommandImplementation_v1: ICommand_v1
{
public:
  virtual void GetCommandName(wchar_t* nameBuffer, size_t* nameBufferSize)
  {
    size_t workingBufferSize = *nameBufferSize;

    if ((nameBuffer == nullptr) || (wcslen(commandName) > workingBufferSize))
    {
      *nameBufferSize = wcslen(commandName);
      return;
    }

    wcsncpy(nameBuffer, commandName, workingBufferSize);
    *nameBufferSize = wcslen(commandName);
  }

  virtual void GetCommandGUID(wchar_t* guidBuffer, size_t* guidBufferSize)
  {
    size_t workingBufferSize = *guidBufferSize;

    if ((guidBuffer == nullptr) || (wcslen(commandGUID) > workingBufferSize))
    {
      *guidBufferSize = wcslen(commandGUID);
      return;
    }

    wcsncpy(guidBuffer, commandGUID, workingBufferSize);
    *guidBufferSize = wcslen(commandGUID);
  }

  virtual bool Execute(int argc, wchar_t* argv[])
  {
    return true;
  }

private:
  const wchar_t* commandName = TEXT("Default");
  const wchar_t* commandGUID = TEXT("Default");
};

然后，我尝试让我的对象覆盖基类的 commandName 和 commandGUID 属性:

struct AboutCommand: ICommandImplementation_v1
{
public:
  bool Execute(int UNUSED(argc), wchar_t* UNUSED(argv[]))
  {
    return true;
  }

private:
  const wchar_t* commandName = TEXT("AboutCommand");
  const wchar_t* commandGUID = TEXT("01eba0e6-81b9-4fa7-a9f3-407d330da9b3");
};

当然，这实际上并没有奏效。创建 AboutCommand 对象并调用其 GetCommandName 返回“Default”。这是有道理的 - 我们实际上是在调用 ICommandImplementation_v1 的 GetCommandName，而 ICommandImplementation_v1 不知道 AboutCommand或其阴影 commandName。

在这一点上，我想我可能必须在 ICommandImplementation_v1 中创建一个简单的 protected 方法，该方法采用命令名称以及缓冲区/缓冲区大小(ICommandImplementation_v1: :GetRealCommandName(const wchar_t* commandName = TEXT("Default"), wchar_t* nameBuffer, size_t* nameBufferSize)，然后 AboutCommand 的 GetCommandName 只会用它自己的 commandName 调用这个函数。不过，这对我来说似乎很笨拙；有没有更简洁、更优雅的方法来做到这一点？

Answer 1

最简单的方法是将两个字符串作为参数传递给基类:

struct ICommandImplementation_v1: ICommand_v1
{
public:
  ICommandImplementation_v1(wchar_t const* name, wchar_t const* guid):
     commandName(name),
     commandGUID(guid)
  {
  }
  void GetCommandName(wchar_t* nameBuffer, size_t* nameBufferSize)
  {
    size_t workingBufferSize = *nameBufferSize;

    if ((nameBuffer == nullptr) || (wcslen(commandName) > workingBufferSize))
    {
      *nameBufferSize = wcslen(commandName);
      return;
    }

    wcsncpy(nameBuffer, commandName, workingBufferSize);
    *nameBufferSize = wcslen(commandName);
  }

  void GetCommandGUID(wchar_t* guidBuffer, size_t* guidBufferSize)
  {
    size_t workingBufferSize = *guidBufferSize;

    if ((guidBuffer == nullptr) || (wcslen(commandGUID) > workingBufferSize))
    {
      *guidBufferSize = wcslen(commandGUID);
      return;
    }

    wcsncpy(guidBuffer, commandGUID, workingBufferSize);
    *guidBufferSize = wcslen(commandGUID);
  }

  virtual bool Execute(int argc, wchar_t* argv[])
  {
    return true;
  }

private:
  const wchar_t* commandName = TEXT("Default");
  const wchar_t* commandGUID = TEXT("Default");
};

struct AboutCommand: ICommandImplementation_v1
{
public:
  AboutCommand(): 
      ICommandImplementation_v1(TEXT("AboutCommand"), 
                                TEXT("01eba0e6-81b9-4fa7-a9f3-407d330da9b3")) 
  {}

  bool Execute(int UNUSED(argc), wchar_t* UNUSED(argv[]))
  {
    return true;
  }
};

一般来说，你想要的是"template method" .不是在基类中定义两个私有(private)字段，而是定义两个私有(private)纯虚方法，它们被称为 GetCommandGUID 和 GetCommandName 并返回特定于子类的值:

struct ICommandImplementation_v1: ICommand_v1
{
public:
  void GetCommandName(wchar_t* nameBuffer, size_t* nameBufferSize)
  {
    size_t workingBufferSize = *nameBufferSize;

    if ((nameBuffer == nullptr) || (wcslen(commandName()) > workingBufferSize))
    {
      *nameBufferSize = wcslen(commandName());
      return;
    }

    wcsncpy(nameBuffer, commandName(), workingBufferSize);
    *nameBufferSize = wcslen(commandName());
  }

  void GetCommandGUID(wchar_t* guidBuffer, size_t* guidBufferSize)
  {
    size_t workingBufferSize = *guidBufferSize;

    if ((guidBuffer == nullptr) || (wcslen(commandGUID()) > workingBufferSize))
    {
      *guidBufferSize = wcslen(commandGUID());
      return;
    }

    wcsncpy(guidBuffer, commandGUID, workingBufferSize);
    *guidBufferSize = wcslen(commandGUID);
  }

  virtual bool Execute(int argc, wchar_t* argv[])
  {
    return true;
  }

private:
  virtual const wchar_t* commandName() = 0;
  virtual const wchat_t* commandGUID() = 0;
};

struct AboutCommand: ICommandImplementation_v1
{
public:
  bool Execute(int UNUSED(argc), wchar_t* UNUSED(argv[]))
  {
    return true;
  }

private:
  const wchar_t* commandName() {return TEXT("AboutCommand")}
  const wchar_t* commandGUID() {TEXT("01eba0e6-81b9-4fa7-a9f3-407d330da9b3")}
};

请帮自己一个忙，不要使用 wchar_t* 而是使用 std::wstring 作为字符串。