Re: [Boost-users] Metaprogramming Question

Subject: Re: [Boost-users] Metaprogramming Question
From: Jeremiah Willcock (jewillco_at_[hidden])
Date: 2012-02-09 18:04:23

On Thu, 9 Feb 2012, Kim Barrett wrote:

> On Feb 9, 2012, at 1:47 AM, John M. Dlugosz wrote:
>> On 2/9/2012 12:21 AM, Jeremiah Willcock wrote:
>>> Would it be reasonable for your use case to have MY_FANCY_ASSERT be:
>>>
>>> #define MY_FANCY_ASSERT(cond, str) \
>>> if (cond) {} else format(str) % __FILE__ % __LINE__
>>>
>>> or similar? It would restrict where you could use it, but would be
>>> simple and guarantee the properties you want.
>>
>> Actually, that's not at all too unreasonable.  I had shied away from
>> making a macro that "eats" the following arguments in a non-expression
>> way, on general principles.  But I see that what you have doesn't
>> leave a dangling-else problem, and I suppose people won't notice that
>> it's at all funny.
>
> I have a similar macro and just yesterday received a bug report from
> one of my users because code like this
>
>    if (cond1) MY_FANCY_ASSERT(cond2, ...) ... ;
>
> was resulting in compiler warnings from gcc: -Wparentheses (enabled by
> -Wall) complains about potential confusion over which "if" an "else"
> belongs to.  I was unable to find a way to suppress the warning
> locally to the macro, even with the local diagnostic control pragmas
> available in gcc4.6 (though it might be argued that some of what I was
> seeing should be considered gcc bugs (I intend to file a bug report),
> so it might be that some future version of gcc will provide sufficient
> warning control for this situation).  The workarounds are, of course,
> to move cond1 into the macro's conditional or to put braces around the
> whole outer "then" statement.

Here's a variant that might work better for that:

#define MY_FANCY_ASSERT(cond, str) \
   for (bool BOOST_PP_CAT(assertcond, __LINE__) = (cond); \
        BOOST_PP_CAT(assertcond, __LINE__); \
        BOOST_PP_CAT(assertcond, __LINE__) = false) \
     format(str) % __FILE__ % __LINE__

-- Jeremiah Willcock