Re: [boost] [fusion] for_each

Subject: Re: [boost] [fusion] for_each
From: Christopher Schmidt (mr.chr.schmidt_at_[hidden])
Date: 2010-10-06 06:22:07

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.

-Christopher