Re: [boost] [variant] make_variant_over requirement

From: Tobias Schwinger (tschwinger_at_[hidden])
Date: 2007-02-17 14:01:32

Joel de Guzman wrote:
> Hi,
> 
> In the variant docs, in the make_variant_over section
> (Using a type sequence to specify bounded types), it is
> not clear what MPL sequence concept it is referring to.
> 
> I was hoping it is a Forward Sequence, but I realized that
> when I try to use make_variant_over with a Forward Sequence,
> it fails trying to use mpl::clear, which hints that
> make_variant_over requires an MPL Extensible Sequence.
> This is rather undesirable. Is there a reason why this is so?
> Can't we lessen the requirement? The situation makes it not
> possible to use mpl views or fusion views for example.

Well, one could slowly but surely create an extensible sequence with 
mpl::copy.

I had a quick take at this one to make things work without wasting 
compile time, see attached patch (tested with GCC4).

Regards,
Tobias

Index: variant.hpp
===================================================================
RCS file: /cvsroot/boost/boost/boost/variant/variant.hpp,v
retrieving revision 1.97
diff -u -r1.97 variant.hpp
--- variant.hpp	5 Jun 2006 02:00:58 -0000	1.97
+++ variant.hpp	17 Feb 2007 18:38:01 -0000
@@ -68,11 +68,12 @@
 #include "boost/mpl/deref.hpp"
 #include "boost/mpl/pair.hpp"
 #include "boost/mpl/protect.hpp"
-#include "boost/mpl/push_front.hpp"
 #include "boost/mpl/same_as.hpp"
 #include "boost/mpl/size_t.hpp"
 #include "boost/mpl/sizeof.hpp"
-#include "boost/mpl/transform.hpp"
+#include "boost/mpl/transform_view.hpp"
+#include "boost/mpl/joint_view.hpp"
+#include "boost/mpl/single_view.hpp"
 #include "boost/mpl/assert.hpp"
 
 ///////////////////////////////////////////////////////////////////////////////
@@ -114,10 +115,8 @@
 {
 private: // helpers, for metafunction result (below)
 
-    typedef typename mpl::transform1<Sequence, F>::type transformed_;
-    typedef typename mpl::max_element<transformed_
-          
-        >::type max_it;
+    typedef mpl::transform_view<Sequence, F> transformed_;
+    typedef typename mpl::max_element<transformed_>::type max_it;
 
 public: // metafunction result
 
@@ -216,12 +215,10 @@
 {
 private: // helpers, for metafunction result (below)
 
-    typedef typename mpl::eval_if<
+    typedef typename mpl::if_<
           NeverUsesBackupFlag
-        , mpl::identity< Types >
-        , mpl::push_front<
-              Types, backup_holder<void*>
-            >
+        , Types
+        , mpl::joint_view< mpl::single_view< backup_holder<void*> >, Types> 
         >::type types;
 
     typedef typename max_value<
@@ -943,30 +940,30 @@
           ::boost::mpl::not_< mpl::empty<specified_types> >::value
         ));
 
-    typedef typename mpl::eval_if<
+    typedef typename mpl::if_<
           is_recursive_
-        , mpl::transform<
+        , mpl::transform_view<
               specified_types
             , mpl::protect<
                   detail::variant::quoted_enable_recursive<wknd_self_t>
                 >
             >
-        , mpl::identity< specified_types >
+        , specified_types 
         >::type recursive_enabled_types;
 
 public: // public typedefs
 
-    typedef typename mpl::transform<
+    typedef mpl::transform_view<
           recursive_enabled_types
         , unwrap_recursive<mpl::_1>
-        >::type types;
+        > types;
 
 private: // internal typedefs
 
-    typedef typename mpl::transform<
+    typedef mpl::transform_view<
           recursive_enabled_types
         , mpl::protect< detail::make_reference_content<> >
-        >::type internal_types;
+        > internal_types;
 
     typedef typename mpl::front<
           internal_types