ATL Internals 2ed复习.chapter 6.interface map tricks-优快云博客

本文探讨了在实现多个接口时遇到的命名冲突问题，并提供了几种解决方案，包括微软特定的方法和通用的forwardingshims技巧。

Handling Name Conflicts

接口定义有时会遇到命名冲突，例如：

interface ICowboy : IUnknown {
    HRESULT Draw();
};

interface IArtist : IUnknown {
   HRESULT Draw();
};

// Ace Powell was a cowboy/artist who lived in the western US
// from 1912 to his death in 1978. I'd like to thank Tim Ewald
// for this fabulous example, which I have used to death
// for years.
class CAcePowell :
    public CComObjectRootEx<CComSingleThreadModel>,
    public ICowboy,
    public IArtist
{
public:
BEGIN_COM_MAP(CAcePowell)
  COM_INTERFACE_ENTRY(ICowboy)
  COM_INTERFACE_ENTRY(IArtist)
END_COM_MAP()
...
  STDMETHODIMP Draw() { /* Act as a cowboy or an artist? */ }
};

上述代码中，Draw()不同的接口表现完全不同

微软解决方案

class CAcePowell :
    public CComObjectRootEx<CComSingleThreadModel>,
    public ICowboy,
    public IArtist {
public:
BEGIN_COM_MAP(CAcePowell)
  COM_INTERFACE_ENTRY(ICowboy)
  COM_INTERFACE_ENTRY(IArtist)
END_COM_MAP()
...
  STDMETHODIMP IArtist::Draw() {
    /* Draw like an artist */
    return S_OK;
  }

  STDMETHODIMP ICowboy::Draw() {
    /* Draw like a cowboy */
    return S_OK;
  }
};

上述代码只能用在头文件中，而且并非标准cpp，只能用在微软编译器

forwarding shims解决方案

struct _IArtist : public IArtist {
  STDMETHODIMP Draw() { return ArtistDraw(); }
  STDMETHOD(ArtistDraw)() =0;
};

struct _ICowboy : public ICowboy {
  STDMETHODIMP Draw() { return CowboyDraw(); }
  STDMETHOD(CowboyDraw)() =0;
};

_IArtist和_ICowboy被称为shim class

class CAcePowell :
    public CComObjectRootEx<CComSingleThreadModel>,
    public _ICowboy,
    public _IArtist {
public:
BEGIN_COM_MAP(CAcePowell)
  COM_INTERFACE_ENTRY(ICowboy)
  COM_INTERFACE_ENTRY(IArtist)
END_COM_MAP()
...
  STDMETHODIMP ArtistDraw();
  STDMETHODIMP CowboyDraw();
};

上述代码使用额外的vtable来解决问题，如果不想要这个负担，例子：

template <typename Deriving>
struct ATL_NO_VTABLE _IArtist : public IArtist {
  STDMETHODIMP Draw() {
    return static_cast<Deriving*>(this)->ArtistDraw();
  }
};

template <typename Deriving>
struct ATL_NO_VTABLE _ICowboy : public ICowboy {
  STDMETHODIMP Draw() {
    return static_cast<Deriving*>(this)->CowboyDraw();
  }
};

class ATL_NO_VTABLE CAcePowell :
    public CComObjectRootEx<CComSingleThreadModel>,
    public _ICowboy<CAcePowell>,
    public _IArtist<CAcePowell> {
public:
BEGIN_COM_MAP(CAcePowell)
  COM_INTERFACE_ENTRY(ICowboy)
  COM_INTERFACE_ENTRY(IArtist)
END_COM_MAP()
...
  HRESULT ArtistDraw();
  HRESULT CowboyDraw();
};

下面的代码是错误的：

template <typename Deriving>
struct ATL_NO_VTABLE _ICowboy : public ICowboy {
  STDMETHODIMP Draw() {
    return static_cast<Deriving*>(this)->CowboyDraw();
  }
};

class ATL_NO_VTABLE CAcePowell :
    public CComObjectRootEx<CComSingleThreadModel>,
    public _ICowboy<CAcePowell>,
    public IArtist {
public:
BEGIN_COM_MAP(CAcePowell)
  COM_INTERFACE_ENTRY(ICowboy)
  COM_INTERFACE_ENTRY(IArtist)
END_COM_MAP()
...
  HRESULT Draw();       // Use for both IArtist::Draw and
                        // ICowboy::Draw
  HRESULT CowboyDraw(); // Never called!
};