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

From: Steven Watanabe (steven_at_[hidden])
Date: 2007-08-13 15:59:15

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.

In Christ,
Steven Watanabe