Re: stroustrup, void*, and reinterpret_cast

"Dilip" <>
6 Sep 2006 15:14:45 -0400
kanze wrote:

It sends the wrong message to the reader.

When I see a reinterpret_cast in code, I immediately assume that
there is something very delicate and implementation dependant
going on. When I see a static_cast, I suppose that while you
are subverting the type system somewhat, it is within more
reasonable bounds, and is portable---I don't have to pay
particular attention to it when porting to another system, for

There is a specific scenario that I have never been clear about in all
these years w.r.t reinterpret_casts.

In MSFT's COM-land, there is an API called CoCreateInstance -- its last
parameter is of type void**. This API is used to get a pointer to the
interface (abstract class to be precise) you are interested in. So
people always do something like this:

struct IUnknown
    virtual void QueryInterface(REFIID riid, void** ppv) = 0;
    virtual void AddRef() = 0;
    virtual void Release() = 0;

struct ISomeInterface : public IUnknown
     virtual void DoIt() = 0;

ISomeInterface* isi;
CoCreateInstance(......,...., reinterpret_cast<void**>(&isi));

Internally a lot of hocus-pocus happens and the API ends up calling an
implementation of a standard IUnknown method called QueryInterface that
returns a pointer to ISomeInterface like so:

class SomeInterfaceImpl : public ISomeInterface
     void QueryInterface(REFIID riid, void** ppv)
         *ppv = static_cast<ISomeInterface*>(this);


     // remaining implementations elided for clarity

Have the proper casts been used in the above code? Can you explain why
reinterpret_cast is needed in places where its been used?

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