Re: [PATCH 1/5] add SWAP macro

From: Johannes Schindelin <Johannes.Schindelin@gmx.de>
To: "René Scharfe" <l.s.r@web.de>
Cc: 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: Tue, 31 Jan 2017 13:03:55 +0100 (CET)	[thread overview]
Message-ID: <alpine.DEB.2.20.1701311302310.3469@virtualbox> (raw)
In-Reply-To: <77098ac8-1084-a5c4-a5e6-fb382e3dc3a0@web.de>

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Hi René,

On Mon, 30 Jan 2017, 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.

Thank you for clarifying.

> > 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".

Okay, now we know what the tool says.

I would like to take a step back and state that it does not make much
sense to support SWAP(a++, b), as it is confusing at best.

Ciao,
Dscho