Re: [PATCH 1/5] add SWAP macro

From: René Scharfe <l.s.r@web.de>
Date: Jan 30, 2017, 22:09 UTC
Message-ID: <77098ac8-1084-a5c4-a5e6-fb382e3dc3a0@web.de>
In-Reply-To: <alpine.DEB.2.20.1701302158110.3469@virtualbox>

Am 30.01.2017 um 22:03 schrieb Johannes Schindelin:

> It is curious, though, that an
> expression like "sizeof(a++)" would not be rejected.

Clang normally warns about something like this ("warning: expression with side effects has no effect in an unevaluated context [-Wunevaluated-expression]"), but not if the code is part of a macro. I don't know if that's intended, but it sure is helpful in the case of SWAP.

> Further, what would SWAP(a++, b) do? Swap a and b, and *then* increment a?

That might be a valid expectation, but GCC says "error: lvalue required as unary '&' operand" and clang puts it "error: cannot take the address of an rvalue of type".

René

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