A Reference Implementation of QueryInterface

The Implementation

NS_IMETHODIMP
nsMyImplementation::QueryInterface( REFNSIID aIID, void** aInstancePtr )
  {
    if ( !aInstancePtr )
      return NS_ERROR_NULL_POINTER;

    if ( aIID.Equals(nsIX::GetIID()) )
      *aInstancePtr = NS_STATIC_CAST(nsIX*, this);
    else if ( aIID.Equals(nsIY::GetIID()) )
      *aInstancePtr = NS_STATIC_CAST(nsIY*, this);

    // ...as many cases as needed...

    else if ( aIID.Equals(kISupportsIID) )
      *aInstancePtr = NS_STATIC_CAST(nsISupports*, NS_STATIC_CAST(nsIX*, this));
    else
      *aInstancePtr = 0;


    nsresult status;
    if ( !*aInstancePtr )
      status = NS_NOINTERFACE;
    else
      {
        NS_ADDREF( NS_REINTERPRET_CAST(nsISupports*, *aInstancePtr) );
        status = NS_OK;
      }

    return status;
  }

What's So Good About It?

It's clear and simple.

OK. It has more than one return, but the primary return is at the end of the function as expected; and the additional return is clear and alone at the top of the function.

It only has one AddRef

It AddRefs the resulting interface, not this, thus following the COM-correct way (particularly important in aggregation)

It uses T::GetIID() instead of kTIID thus saving a global declaration and global space

It uses C++'s static_cast, via NS_STATIC_CAST, which detects errors when you can't really get to the desired interface.

It clears the result, *aInstancePtr, (if supplied) when returning an error.