Cast DirectX interfaces to IUnknown pointers

Question

Having quite a few interfaces in my code, I wanted to encapsulate repeating Release code in another method and not in a macro because this is C++ and I hate using macros. My initial attempt was to write a method such as

void SafeRelease(IUnknown **ppInterface) {
    if(*ppInterface) {
        (*ppInterface)->Release();
        (*ppInterface) = nullptr;
    }
}

however applying this method to a IDirect3DSurface9 * e.g. like SafeRelease(&mySurface) yields the error IDirect3DSurface9 ** is incompatible with IUnknown **.

1. What am I doing here wrong?
2. Is there a better approach (hopefully not using macros) to implement such a function?

Answer 1

Here's my method:

template <typename T> void SafeRelease(T*& ptr)
{
    if(ptr)
    {
        ptr->Release();
        ptr = nullptr;
    }
}

Example usage:

IDirect3DDevice9 *pD3DDevice = NULL;
d3d->CreateDevice(..., &pD3DDevice);
SafeRelease(pD3DDevice);

You may inline this function if you want.