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

c++ windows null handles
Coder_Dan picture Coder_Dan · Oct 11, 2010 · Viewed 10k times · Source

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?

Answer

MSalters picture MSalters · Oct 11, 2010

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).

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