Re: [PATCH 1/5] add SWAP macro

From: Brandon Williams <bmwill@google.com>
Date: Jan 30, 2017, 22:21 UTC
Message-ID: <20170130222157.GC35626@google.com>
In-Reply-To: <77098ac8-1084-a5c4-a5e6-fb382e3dc3a0@web.de>

On 01/30, René Scharfe wrote:

Show 17 quoted lines

> 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