From: René Scharfe <l.s.r@web.de>
Date: Tue, 31 Jan 2017 21:03:01 GMT
Subject: Re: [PATCH 1/5] add SWAP macro
Message-ID: <8e94756a-c3a5-9b81-268d-d0f36876f710@web.de>
In-Reply-To: <20170130222157.GC35626@google.com>

Am 30.01.2017 um 23:21 schrieb Brandon Williams:
> On 01/30, René Scharfe wrote:
>> 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
>
> Perhaps we could disallow a side-effect operator in the macro.  By
> disallow I mean place a comment at the definition to the macro and
> hopefully catch something like that in code-review.  We have the same
> issue with the `ALLOC_GROW()` macro.

SWAP(a++, ...) is caught by the compiler, SWAP(*a++, ...) works fine. 
Technically that should be enough. :)  A comment wouldn't hurt, of course.

René