Re: [boost] Possible bug with mpl::joint_view working on two mpl::maps

From: Martin Apel (martin.apel_at_[hidden])
Date: 2007-08-14 08:04:31

Aleksey Gurtovoy wrote:
> Hi Martin,
>
>   
>> I have two mpl maps like this:
>>
>> typedef
>> map<pair<int_<1>, int_<42> >,
>>     pair<int_<2>, int_<43> >,
>>     pair<int_<3>, int_<44> >
>>     
>>> Map1;
>>>       
>> typedef
>> map<pair<int_<4>, int_<44> >,
>>     pair<int_<5>, int_<45> >,
>>     pair<int_<6>, int_<46> >
>>     
>>> Map2;
>>>       
>> I try to join them using "joint_view" as follows:
>>
>> typedef joint_view<Map1, Map2>::type JointMap;
>>
>> which works fine. Then I do
>>
>> std::cout << at<JointMap, int_<2> >::type::value << "\n";
>>
>> which gives the following compiler error:
>>
>> error: 'value' is not a member of 'boost::mpl::pair<mpl_::int_<3>,
>> mpl_::int_<44> >'
>>
>> I tried this with gcc 3.4.6 and 4.2 under Linux, Boost version is
>> 1.34.0.  It seems "at" returns the entry after the searched entry,
>> and not only the right half, but the complete pair.
>>
>> Has anybody got any hints on this? To me it looks like a bug in MPL,
>> but I might be wrong.
>>     
>
> Both sides of this behavior steam from nongenericity of the 'at'
> metafunction, which treats Associated Sequences differently from
> "plain" Forward Sequences [1]. 
>
> 'joint_view' of two maps is simply a Forward Sequence [2], which
> changes the 'at' semantics from a value lookup in the associative
> sequence to an equivalent of 'deref< advance< begin<s>::type,n >::type
>   
>> ::type'. Thus the result you observed: 'at<JointMap, int_<2> >::type'
>>     
> simply returns JointMap's third element.
>
> This can be definitely argued to be a design bug. 
>
> However, suppose that requesting the map's value by key was spelled
> differently, say, 'get'. Well, even in this case, unless the
> 'joint_view' of two Associated Sequences becomes in an Associated
> Sequence as well, applying 'get' to JointMap still won't work --
> although you'd most likely get an error instead of a puzzling result.
>
> All in all, to get the behavior you want with the current library you
> need to actually merge two maps into a third one.
>
> [1] http://boost.org/libs/mpl/doc/refmanual/at.html
> [2] http://boost.org/libs/mpl/doc/refmanual/joint-view.html
>
> HTH,
>   
I'm not sure, if I understand this correctly. What is the recommended
way of merging two maps into
a third one? Is it possible with the current MPL implementation? I only
found an map::insert metafunction
to insert a single element, but not a whole range of pairs.

Martin
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