[Boost-users] How to fwd decl boost::variant (with type sequences)?

Subject: [Boost-users] How to fwd decl boost::variant (with type sequences)?
From: Paul (peebor_at_[hidden])
Date: 2010-07-17 16:56:57

In a large project (winxp, boost 1.38.0, 10K sources) we are using the 
boost::variant at some places. The variant types are visible to a large 
amount of cpp’s. Many of these source-file are not ‘using’ the variant 
at all, a small amount only ‘passes’ a variant type, and an even smaller 
amount actually uses it (copy, static-visitor, etc). We have noticed 
however that the compile-time increases for all sources-files, even when 
the variant is not used. On a single source-file its maybe split of a 
second extra but with 10K sources it quickly becomes problem.

First attempt to solve this is to fwd declare the variant using provided 
header variant_fwd.hpp.

Example:

[v.h]

#include <boost/variant/variant_fwd.hpp>

class I001{ public: };
class I002{ public: };
class I003{ public: };

typedef boost::variant<
   boost::shared_ptr<C001>,
   boost::shared_ptr<C002>,
   boost::shared_ptr<C003> > v;

[a.h]

#include “v.h”

class CA
{
public:
   void foo(const v& ref);
};

Source-files now compile faster while only the sources that actually use 
the variant need to add #include <boost/variant/variant.hpp> to get the 
definition of the variant class.

However it doesn’t really fit our needs because we need the ‘counted’ 
variant types (using typelists). Reasons for this is simply that some 
variants are only 3, 5 or 10 elements while another holds 39 element 
types. We don’t want to extend the length of each type to say 50. 
Therefore we first compose an mpl::vector (and even compose subsets 
using joint_view) before defining the variant types.

Example of use:

#include <boost/mpl/vector.hpp>
#include <boost/variant.hpp>

   typedef boost::mpl::vector<
     boost::shared_ptr<I001>,
     boost::shared_ptr<I002>,
     boost::shared_ptr<I003>
   > MplV;
   typedef boost::make_variant_over<MplV>::type v;

Now each cpp file must not only include the variant definitions again 
but also (part of) mpl.

How could we solve this issue?

One idea we had is to wrap the variant in a class and fwd decl that. 
Although it works I’m not sure if that is the way to go?

Thanks for any advise,
Paul