当一次实现几个COM接口，我怎么upcast到IUnknown？

问题描述

假设我的COM对象实现了两个或多个COM接口：

  class CMyClass：public IPersistFile，public IPersistStream {
}; 
  

在实现QueryInterface（）时，我需要能够返回一个IUnknown *指针。因为两个基本接口都派生自IUnknown我不能隐式隐式 - 这样的上传将是暧昧的。明确地说，我需要使用以下两种方法之一：

  if（iid == __uuidof（IUnknown））{
 * ppv = static_cast< IPersistFile *>（this）; 
 static_cast< IPersistFile *>（this） - > AddRef（）; 
 return S_OK; 
} 
  

或

  if（iid == __uuidof（IUnknown））{
 * ppv = static_cast< IPersistStream *>（this）; 
 static_cast< IPersistStream *>（this） - > AddRef（）; 
 return S_OK; 
} 
  

看起来像唯一的要求是每当QI

我应该选择哪个向上转换，为什么？

Mark Ransom已经给出了正确的答案 - 任何事情都会做，只要它是一致的 - 但挑选第一个有一个小的优势。由于布局规则，第一个接口的 IUnknown * 将指向对象的开始。任何其他 IUnknown * 将指向对象中其他位置的后续vtable指针。为了调试的目的，知道ano对象在内存中开始的位置非常有用。

Suppose my COM object implements two or more COM interfaces:

class CMyClass : public IPersistFile, public IPersistStream {
};

when implementing QueryInterface() I need to be able to return an IUnknown* pointer. Since both base interfaces are derived from IUnknown I cannot upcast implicitly - such upcast would be umbiguous. To upcast explicitly I need to use either of the two ways:

 if( iid == __uuidof( IUnknown ) ) {
     *ppv = static_cast<IPersistFile*>( this );
     static_cast<IPersistFile*>( this )->AddRef();
     return S_OK;
 }

or

 if( iid == __uuidof( IUnknown ) ) {
     *ppv = static_cast<IPersistStream*>( this );
     static_cast<IPersistStream*>( this )->AddRef();
     return S_OK;
 }

Looks like the only requirement is that whenever QI() is called on an object it returns the same pointer each time and I meet that requirement if I choose any of the casts and just stick to it.

Which upcast should I choose and why?

解决方案

Mark Ransom already gave the correct answer - any will do, as long as it's consistent - but picking the first one has one minor advantage. Due to layout rules, the IUnknown* of the first interface will point to the start of the object. Any other IUnknown* will point to subsequent vtable pointers elsewhere in the object. For debugging purposes, it's very useful to know where ano object begins in memory.

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