Re: [boost] [iterator] Using zip_iterator with std::pair

Subject: Re: [boost] [iterator] Using zip_iterator with std::pair
From: Claas H. Köhler (claas.koehler_at_[hidden])
Date: 2012-10-18 06:11:19

On 17/10/12 13:21, Jeffrey Lee Hellrung, Jr. wrote:
> On Wed, Oct 17, 2012 at 12:08 AM, "Claas H. Köhler" <claas.koehler_at_[hidden]>wrote:
> 
>>
>>
>> On 17/10/12 04:28, Jeffrey Lee Hellrung, Jr. wrote:
>>> On Tue, Oct 16, 2012 at 10:06 AM, "Claas H. Köhler" <
>> claas.koehler_at_[hidden]>wrote:
>>>
>>>>
>>>>
>>>> On 16/10/12 14:27, Jeffrey Lee Hellrung, Jr. wrote:
>>>>> On Tue, Oct 16, 2012 at 4:21 AM, "Claas H. Köhler" <
>> claas.koehler_at_[hidden]
>>>>> wrote:
>>>>>
>>>>>>
>>>>>>
>>>>>> On 16/10/12 12:01, Giovanni Piero Deretta wrote:
>>>>>>> On Tue, Oct 16, 2012 at 9:06 AM, "Claas H. Köhler" <
>>>> claas.koehler_at_[hidden]>
>>>>>> wrote:
>>>>>>>> Hi!
>>>>>>>>
>>>>>>>> I would like to use the boost::zip_iterator with std::pair.
>>>>>> Unfortunately this does not work out of
>>>>>>>> the box, but there exists a namespace
>>>>>>  boost::detail::tuple_impl_specific, which apparently needs to
>>>>>>>> be specialised for each tuple type.
>>>>>>
>>>>>
>>>>> Hmmm...I guess it makes sense you might have to specialize something if
>>>>> zip_iterator doesn't specifically know about your desired tuple type.
>>>>>
>>>>>
>>>>>>>> I managed to re-implement tuple_meta_transform and
>>>>>> tuple_meta_accumulate for std::tuple, which seems
>>>>>>>> to be straightforward. However, there are functions of the form
>>>>>>>>
>>>>>>>> template<typename Tuple, typename Fun>
>>>>>>>> complex_result_type tuple_transform(const Tuple&, Fun)
>>>>>>>>
>>>>>>>> which have to be overloaded as well, if I am not mistaken. However,
>>>>>> overloading this with something like
>>>>>>>> template<typename... Args, typename Fun>
>>>>>>>> complex_result_type tuple_transform(const std::tuple<Args...>&, Fun)
>>>>>>>>
>>>>>>>> is probably not going to work, since this would be a partial
>> template
>>>>>> specialisation of a function.
>>>>>>>
>>>>>>> That's an overload actually and should work fine. Did you try it?
>>>>>>>
>>>>>>> HTH,
>>>>>>
>>>>>> Thanks for the hint. I have to admit I did not try it yet. I have just
>>>>>> created a similar simple test
>>>>>> case, which indeed compiles/works fine. Shame on me for not trying it
>>>> out.
>>>>>> For me this was always a
>>>>>> classical example of template function specialisation. I guess I
>> should
>>>>>> take a look at template
>>>>>> basics again :-)
>>>>>>
>>>>>> Sorry for that and thanks for the quick response.
>>>>>>
>>>>>
>>>>> I'm glad you got it working, but ideally, I think, zip_iterator should
>>>> work
>>>>> out of the box for std::pair and std::tuple. Could I trouble you to
>>>> provide
>>>>> patches based on what you needed to get this to work?
>>>>>
>>>>> - Jeff
>>>>
>>>> Unfortunately things are not working yet. As far as I can judge, the
>>>> problem is that some of the
>>>> functions are called from within the zip_iterator.hpp header file, which
>>>> is before I can actually
>>>> define the overloaded functions. As a result the overloaded functions
>> are
>>>> not called.
>>>>
>>>> Should I get this working, however, I am glad to share the wisdom.
>>>>
>>>
>>> Well, I'd consider it fair game to edit the zip_iterator.hpp header file
>>> directly :)
>>>
>>> - Jeff
>>>
>> Looks like I found a working solution. I transformed the tuple_xxx
>> template functions into template
>> structs, for which overloading seemingly works fine. Since I am not
>> familiar with boost::mpl I used
>> my own versions of the meta transformations based on C++11. For tuple this
>> may not be an issue, since
>> its a C++11 feature, but for std::pair it could be.
>>
>> Furthermore I would suggest to implement these functions exclusively for
>> boost::tuple and leave the
>> general template unimplemented. This may ease specialisations of other
>> classes. Using C++11 for
>> example the specialisations for std::pair and std::tuple are almost
>> identical, but have to be
>> specialised individually. Just my 0.02$ .
>>
>> Shall I post the modified code directly to the mailing list or somewhere
>> else?
>>
> 
> Create a trac ticket, attach any patches, and I'll take a look at it when I
> get a chance.
> 
> - Jeff
> 
I just created ticket #7526 with a proposed patch. I am not sure, whether I got the different
compiler macros right, though, since I do not have access to the compilers they are intented for.
Furthermore you may want to enclose the std::tuple stuff (including namespace helper) into a macro,
which enables it if C++11 is used (I presume such a macro does already exist, but I did not know its
exact syntax).

Hope that helps.

Regards
Claas