Re: [Boost-users] [Fusion] fusion::traits::is_sequence <-> mpl::is_sequence?

From: Tobias Schwinger (tschwinger_at_[hidden])
Date: 2007-01-19 13:24:27

Hi,

Sohail Somani wrote:
> If I am trying to make a fusion sequence mpl compatible, do I need to 
> specialize mpl::is_sequence as well?  

No. mpl::is_sequence just checks whether mpl::begin returns mpl::void_ 
(in this case it's not an MPL Sequence).

> I had expected that the following should be true:
> 
> BOOST_MPL_ASSERT(( fusion::traits::is_sequence<myseq> ));
> BOOST_MPL_ASSERT(( mpl::is_sequence<myseq> ));

It is - once you bring the part of MPL support in that enables you to 
use MPL Tag Dispatched Metafunctions on Fusion sequences:

    #include <boost/fusion/sequence/intrinsic/mpl.hpp>

> 
> But the compilation fails on the second line. 

That breakage should be intentional to make sure that mpl::is_sequence 
won't start lying, otherwise a Fusion Sequence can be both an MPL 
Sequence and not an MPL Sequence in the same program (see attached code).

Note that I fixed that problem very recently, so maybe your version of 
mpl::is_sequence is still just lying.

For a recent CVS snapshot, Fusion sequences are always MPL Sequences. 
They can be incompletely defined, however (and in this case your 
compiler hopefully tells you that begin_impl<fusion_sequence_tag> is an 
incomplete type).

Regards,
Tobias

P.S.: The attached code will intentionally fail to compile if your 
snapshot of Fusion is up to date.


#include <boost/mpl/is_sequence.hpp>
#include <boost/fusion/sequence/container/vector.hpp>
#include <iostream>

bool is_seq_tu1()
{
    return boost::mpl::is_sequence< boost::fusion::vector<int> >::value;
}

bool is_seq_tu2();

int main()
{
    if (is_seq_tu1() != is_seq_tu2())
    {
        std::cout << "ODR violation missed by the linker" << std::endl;
        return 1;
    }
    else
    {
        std::cout << "Fine." << std::endl;
        return 0;
    }
}


#include <boost/mpl/is_sequence.hpp>
#include <boost/fusion/sequence/container/vector.hpp>
#include <boost/fusion/sequence/intrinsic/mpl.hpp>

bool is_seq_tu2()
{
    return boost::mpl::is_sequence< boost::fusion::vector<int> >::value;
}