Re: [boost] [fusion] for_each

Subject: Re: [boost] [fusion] for_each
From: Joel de Guzman (joel_at_[hidden])
Date: 2010-10-06 18:56:40

On 10/6/2010 6:22 PM, Christopher Schmidt wrote:
> Joel de Guzman schrieb:
>> On 10/6/10 8:01 AM, Jeffrey Lee Hellrung, Jr. wrote:
>>> On 10/05/2010 04:41 PM, Joel de Guzman wrote:
>>>> On 10/6/10 5:32 AM, Jeffrey Lee Hellrung, Jr. wrote:
>>> [...]
>>>>> That said, it does look the documentation for boost::fusion::for_each
>>>>> (indicating that F is received by value) contradicts the source code
>>>>> (where F is received by reference-to-const)...
>>>>
>>>> Jeffrey, I don't see that here (F is received by value):
>>>> http://tinyurl.com/2vdao6r. Do you?
>>>
>>> Hmmm...that looks difference from
>>>
>>> http://www.boost.org/doc/libs/1_44_0/libs/fusion/doc/html/fusion/algorithm/iteration/functions/for_each.html
>>>
>>
>> Something must've changed. Christopher?
>
> It wasn't me. In fact, the file hasn't been changed for three years:
>
> https://svn.boost.org/trac/boost/log/trunk/boost/fusion/algorithm/iteration/for_each.hpp
>
> Anyway, the functor should be passed by value. Let me quote Tobias
> (Schwinger) here, from an email from the 29th of October 2009 - in which
> he talks about passing by reference vs. passing by value in the
> functional subset of Fusion:
>
>> The target function is accepted by value - as the default. The functions
>> are designed in a way
>> that allows reference types to be used by explicitly specifying the
>> template parameter 'Function'.
>>
>> The reasons for the default were:
>>
>> 1. C++98-References and built-in function types do not mix well. There
>> is a clause in the
>> Standard rendering the program ill-formed as soon as a const qualifier
>> gets added. There's
>> a defect report about the issue and compiler-specific behavior may vary.
>>
>> Example:
>>
>>     void a_func();
>>     template<  typename F>  void take_func(F const&);
>>     take_func(a_func); // might or might not work
>>
>> 2. Consistency with existing practice, just look at the standard lib's
>> <algorithm>  header, for
>> instance.

That makes perfect sense. In that case, the code should reflect the
docs.

Regards,
-- 
Joel de Guzman
http://www.boostpro.com
http://spirit.sf.net