Re: [PATCH 1/5] add SWAP macro

From: Brandon Williams <bmwill@google.com>
To: "René Scharfe" <l.s.r@web.de>
Cc: Johannes Schindelin <Johannes.Schindelin@gmx.de>,
	Johannes Sixt <j6t@kdbg.org>, Git List <git@vger.kernel.org>,
	Junio C Hamano <gitster@pobox.com>
Subject: Re: [PATCH 1/5] add SWAP macro
Date: Mon, 30 Jan 2017 14:21:57 -0800	[thread overview]
Message-ID: <20170130222157.GC35626@google.com> (raw)
In-Reply-To: <77098ac8-1084-a5c4-a5e6-fb382e3dc3a0@web.de>

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.

-- 
Brandon Williams