强制转换函数指针Release（）的正确方法

Question

I have this error when try to cast function on code below :

error C2440: 'type cast' : cannot convert from 'overloaded-function' to 'ULONG (__stdcall *)(void)'

HRESULT hr = RealCoGetClassObject ( rclsid, dwClsContext, pServerInfo, riid, ppv );

if ( hr == S_OK )
{
    IUnknown * iukn = (IUnknown *)ppv;
    ULONG (WINAPI * RealRelease)() = (ULONG (WINAPI *)(void))(iukn->Release);
}

Answer 1

HRESULT hr = RealCoGetClassObject ( rclsid, dwClsContext, pServerInfo, riid, ppv )

ppv parameter here is supposed to be a pointer to your variable, which is going to be initialized with an interface pointer. Even though your fragment does not give a clear idea what you are trying to do, the post-call cast looks like you are not passing the proper pointer at the time of the call already. You are expected to cast the argument like this:

IUnknown * iukn;
HRESULT hr = RealCoGetClassObject ( rclsid, dwClsContext, pServerInfo, riid, 
    (VOID**) &iukn);
if(SUCCEEDED(hr))
{
    // NOTE: Hey, we got it into `iukn`
    // ...
    iukn->Release();

Or, otherwise, if you are sure that ppv is valid by the time of the call, and you, for instance, want to replace the returned poniter, then the cast is like this:

HRESULT hr = RealCoGetClassObject ( rclsid, dwClsContext, pServerInfo, riid, 
    ppv);
if(SUCCEEDED(hr))
{
    IUnknown*& iukn = *((IUnknown**) ppv);
    iukn->Release(); // Drop the returned thing
    iukn = ... // Put our stuff there

Answer 2

IUnknown::Release is a method, not a function. So you have to use a method pointer instead of function pointer:

typedef ULONG(STDMETHODCALLTYPE IUnknown::*ReleaseMethodPtr) (void);
ReleaseMethodPtr realRelease = &IUnknown::Release;

// for calling the method, you also need an instance pointer
(iukn->*realRelease)();

Having said that and not really knowing what your intent is, i don't think this is what you want. Because IUnknown::Release is a (pure) virtual method, calling the method pointer will always do exactly the same as just calling iukn->Release() .