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

From: Martin Apel (martin.apel_at_[hidden])
Date: 2007-08-14 02:38:03

Steven Watanabe wrote:
> AMDG
>
> Martin Apel <martin.apel <at> simpack.de> writes:
>
>   
>> Hi all,
>>
>> 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.
>>
>>     
>
> joint_veiw is not intended to work that way for
> maps.  It just takes the two sequences and sticks
> them together.  The default implementation of at
> which is used for joint_view returns the Nth element
> of the sequence.  Since you had the keys in order
> starting at 1 it appeared that it was adding one.
> The value of the key is irrelevant in determining
> the result.
>
>   
Thanks for the information, Steve. I didn't understand this from the
available docs. So is there another way of
joining two maps, so they create a single map, that can be used with
mpl::at?

Regards,

Martin

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