IID_PPV_ARGS and Dereferencing NULL

Question

A project I'm working on and some MSDN documentation has code such as this:

IFileOpenDialog *pFileOpen;
IID_PPV_ARGS(&pFileOpen)

Where IID_PPV_ARGS is:

#define IID_PPV_ARGS(ppType) __uuidof(**(ppType)), IID_PPV_ARGS_Helper(ppType)

My question is about the **(ppType) part. Wouldn't this end up dereferencing a null or unitialized pointer? Why does this work?

Answer 1

__uuidof() is a proprietary Microsoft extension, that the compiler knows how to deal with. It's evaluated at compile time, not at run time.

The compiler attempts to look up and substitute the UUID for the given interface. The interface declares its UUID using the __declspec(uuid("...")) extension. If the compiler can't find the UUID, the build will fail. No pointers are actually dereferenced at run time.

ppType is set to a IFileOpenDialog**, thus *(ppType) is a IFileOpenDialog* and **(ppType) is a IFileOpenDialog. As such, __uuidof(**(ppType)) is evaluated as __uuidof(IFileOpenDialog) at compile time.

It's the same as doing this:

char* ptr = nullptr;
size_t charSize = sizeof(*ptr);

sizeof(*ptr) will be evaluated as sizeof(char) at compile time, even though ptr is null.