Re: [boost] Cycle between Graph and GraphParallel

Subject: Re: [boost] Cycle between Graph and GraphParallel
From: Marc Glisse (marc.glisse_at_[hidden])
Date: 2018-10-06 07:01:36

On Sat, 6 Oct 2018, Peter Dimov via Boost wrote:

> Currently we have a dependency cycle between `graph` and `graph_parallel`. 
> It's not the only one, but it's notable with the fact that both libraries are 
> buildable and we therefore can't ignore it as we can ignore cycles between 
> header-only libraries that at release time become a single directory.
>
> In the particular, this impedes efforts to generate CMake config files. Under 
> one scheme I'm currently working on, for instance, Boost.Python can't be 
> installed because it tries to install Graph, which tries to install 
> GraphParallel, which tries to install Graph.
>
> The cycle between these two is an interesting one. 
> boost/graph/breadth_first_search.hpp does:
>
> #ifdef BOOST_GRAPH_USE_MPI
> #  include <boost/graph/distributed/breadth_first_search.hpp>
> #endif
>
> at its end, and boost/graph/distributed/breadth_first_search.hpp does:
>
> #ifndef BOOST_GRAPH_USE_MPI
> #error "Parallel BGL files should not be included unless 
> <boost/graph/use_mpi.hpp> has been included"
> #endif
>
> #include <boost/graph/breadth_first_search.hpp>
>
> at its very start. So these headers include each other.
>
> One would naively think that only the parallel header should include the 
> normal one, but one is not well versed in graph library architectures, so one 
> might quite possibly be missing something, or a lot.
>
> Comments?

I don't know why you are calling this a dependency. The file declares the 
sequential stuff, and if the parallel version is available/requested 
(BOOST_GRAPH_USE_MPI) makes it available as well. So you don't need the 
parallel version to use the sequential one.

-- 
Marc Glisse