Testing for an invalid windows handle: should I compare with 'NULL', '0' or even 'nullptr'?

Question

I'm coming from a background whereby pointers should generally be compared with 'NULL' and integers with '0'.

Since I didn't perceive Windows handles to be 'pointers' in the pure sense (being 'handles'), I'd got into the habit of comparing them with 0 rather than 'NULL'.

Clearly they're implemented internally as pointers nowadays, but I personally consider that to be merely for acquiring some type-safety rather than because they are intrinsically pointers.

Anyway, I just noticed that the help for CreateIC which returns an HDC states that if the function fails then it returns 'NULL'.

Now I'm confused - and am wondering what other people reckon - is it more correct to consider a Windows handle to be a pointer (and therefore check it against 'NULL' or 'nullptr' for modern compilers) or should it be considered to be an integer?

Nobe

Handles aren't pointers, but indexes to a table. Consult MSDN for return values. And anyway, why if (x){.. isn't good? – Maneuver

Thanks for this one - call me old-fashioned, but I normally reserve the 'if (x)' notation for expressions that evaluate to 'bool' results. I don't know whether this is good or bad practice, but it's what I'm used to. – Nobe

Answer 1

Compare it against the documented error return value. That means that you should compare it against INVALID_HANDLE, 0, -1, non-zero, or <=32 (I'm not kidding with the last one, see ShellExecute).

Pyramid

Thanks for that answer - I guess you're right, there's not a lot of consistency and I should just do what the MSDN tells me. – Nobe

Answer 2

To answer your question: the HANDLE type is declared in winnt.h as

typedef PVOID HANDLE;

Hence, technically it is a pointer.

However, I would just use whatever is documented; if the documentation states that NULL is returned, I use exactly that unless evidence shows that the documentation is incorrect.

I don't even think about pointers vs. integers. NULL is just an opaque value (in this situation) and HANDLE is an opaque type to me and I don't bother looking up what it is #define'd to.

Thrombocyte

Answer 3

I think INVALID_HANDLE_VALUE is usually the proper 'invalid' value for windows handles...and that evaluates to -1.

Molten

Thats not always true. It depends on what API you are calling ... which is a HUGE pain :( – Sulla

If you do: HWND hwnd = INVALID_HANDLE_VALUE you get a compile time error. Which seems odd to me. – Narvik