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

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

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