Question

Is there an alternative to using the following?

class IGraphBuilder;

public ref class Device
{
private:
    IGraphBuilder* pGraphBuilder;

public:
    void Configure()
    {
        pin_ptr<IGraphBuilder*> ppGraphBuilder = &pGraphBuilder;

        HRESULT hr = CoCreateInstance(CLSID_FilterGraph,
            NULL,
            CLSCTX_INPROC,
            IID_IGraphBuilder, (void**)ppGraphBuilder);

reinterpret_cast(ppGraphBuilder) compiles but I’m a bit confused if this is correct for this case.

If this wasn’t C++/CLI (where &NativeMember actually means interior_ptr<Type>(NativeMember)) I would simply use static_cast<void**>(&pGraphBuilder) but even after correctly casting to pin_ptr the following doesn’t compile

pin_ptr<IGraphBuilder*> ppGraphBuilder = &pGraphBuilder;
static_cast<void**>(ppGraphBuilder)

Is there any solution or am I forced to use (void**) because pin_ptr is weird?

Answer 1

reinterpret_cast (and thus C cast) is potentially not ok, although it may work due to the allegedly trivial layout of pin_ptr. Indeed you have to call the conversion operator from cli::pin_ptr<IGraphBuilder*> to IGraphBuilder** first (hence the complain from the compiler).

reinterpret_cast<void**>(static_cast<IGraphBuilder**>(ppGraphBuilder))

is correct. You may want to introduce a intermediary variable of type IGraphBuilder** first:

pin_ptr<IGraphBuilder*> p = &pGraphBuilder;
IGraphBuilder** ppGraphBuilder = p;

HRESULT hr = CoCreateInstance(CLSID_FilterGraph,
        NULL,
        CLSCTX_INPROC,
        IID_IGraphBuilder, reinterpret_cast<void**>(p));