Re: [boost] painless currying

Subject: Re: [boost] painless currying
From: Jeffrey Lee Hellrung, Jr. (jeffrey.hellrung_at_[hidden])
Date: 2011-09-03 16:06:43

On Sat, Sep 3, 2011 at 8:15 AM, Edward Diener <eldiener_at_[hidden]>wrote:

> On 9/1/2011 9:40 PM, Paul Mensonides wrote:
>
>> On Thu, 01 Sep 2011 18:23:16 -0400, Edward Diener wrote:
>>
>>  The remark refers in the resolution above refers to "my reading of the
>>> standard" but does not say which standard is being referred to. Is this
>>> part of C99 or C++11 ? I have asked about this on the C++ standard NG
>>> and am awaiting an answer.
>>>
>>> I am surprised by the evaluation of "#elif constant expression" when the
>>> corrwesponding #if statament is true because it means that #if - #elif
>>> is not equivalent to #if - #else - #if in this particular case, and I am
>>> sure many C++ programmers would have expected that the two were indeed
>>> equivalent.
>>>
>>
>> It doesn't "evaluate" it; it just parses it.  Even in normal code and, in
>> most cases, even with dynamically-typed code, the compiler or interpreter
>> still has to parse the else-if expressions.
>>
>
> So if I have:
>
> #if 1
> #else
> nonC++gobbledygook
> #endif
>
> The compiler still parses nonC++gobbledygook and issues an error if it is
> invalid C++ code ?
>

No, I believe here Paul is referring to a "regular" if/else if expressions,
not preprocessor directives.

  Neither C99 or C++98 (or C+
>> +11) are particularly clear on this.  To me that means that the solution
>> is on the code side rather than requiring a compiler to allow semi-
>> unspecified behavior in the short term and to alter the standard if it is
>> important enough in the long term.
>>
>
> I admit that I have always thought that an #if - #else - #endif path which
> is not taken can be anything and does not have to be valid C++. You seem to
> be saying otherwise.
>

I think Paul is saying that for

#if 1
#elif nonC++gobbledygook
#endif

the preprocessor may still parse the #elif expression, but it won't be
evaluated.  If you want to avoid it being parsed, do

#if 1
#else
#if nonC++gobbledygook
#endif
#endif

- Jeff