Comment by minipci1321 | Hacker News Reader

Comment by minipci1321 | Hacker News Reader

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I certainly agree with what you said as you said it. However, as usual, it is the implementation that kills it.

a) I believe sizeof (double) * 8 == 64 is not enforced by the standard either. One could argue that it is hard to find anything else these days, but (for a contrived example) I believe nothing prevents you from making your own port of GCC for some purpose, and make 'double' be 'float'. (From my memories, it is even far enough from rocket science.)

b) It already happened to me personally, to work with code where people have implemented their own uintXX types (because of the lack of the standard ones), and then over time these became narrower/vider.

So if we would like to have it portable, I think, nothing beats:

union { double d; char cc [sizeof (double)]; };

and there we are, back to what is in the standard.

I mostly agree, but with b) IMO the problem was that you worked in an environment that was already not concerned with the standard. I'm not sure if it's OK by the standard for a user to define types named e.g. int32_t that don't comply to the standard restrictions.

sizeof (double) can be guaranteed to be 8 if __STDC_IEC_559__ is defined, so checking that is an additional hurdle if you want to use memcpy for punning. That said, I think it can be assumed that you're writing non-portable code for a specific set of platforms already if you are punning floats to integers, because the representation is implementation dependent in the first place. Probably you also know the environments in which it will be compiled as well, so the example is a bit contrived.

Very much agree that unions are the easiest and standard way to do type punning. There's so many misconceptions about this somehow being unclear in the standard. Sure, it does reflect poorly on the quality of the standard that Linux contributors don't understand it and that Linus Torvalds actively ignores it, but if you know what you're looking for it's often not that hard to find a definite answer.

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