Re: [boost] [boost/spirit] numeric policy

Subject: Re: [boost] [boost/spirit] numeric policy
From: Brian Schrom (brian.schrom_at_[hidden])
Date: 2013-08-01 17:27:35

On 07/31/2013 10:03 PM, Agustín K-ballo Bergé wrote:

> ...
> For  Spirit  V2,  policies  are  not required  to  inherit  from  the
> default  ones,  so any  change  to  them  is potentially  a  breaking
> change. Although I have yet to see a case where this would happen. If
> you think this approach would work for  your use case and you want to
> try and make  the necessary source and documentation  changes, I will
> be happy to help you through the process.

Thank you.  The patch (against svn trunk as of 2013-08-01) I have so far
is quite  small, so I  in-lined it below.   Without knowing a  whole lot
more about Spirit, this  is my best guess for the  files that would need
changing.

I built  the resultant quickbook and  ran the spirit unit  tests, though
some are failing trunk  on my gcc version 4.7.3 setup,  but appear to be
unrelated to this change.  I'm still not 100% sure that this is a proper
approach  to the  problem.  For  instance,  maybe the  policy should  be
initializing the value, and that  is incorporated into the begin/init or
separate.

If  you feel  that this  approach is  appropriate, I  can do  some small
additional changes to  conform to the code base properly  if I know what
they are.  If there  is substantial effort to get where  it needs to be,
it  is probably  not worth  your time  right now  and the  patch can  be
archived in the mailing list if it's useful for a later time.

Regards,

Brian

Index: libs/spirit/doc/qi/numeric.qbk
===================================================================
--- libs/spirit/doc/qi/numeric.qbk      (revision 85183)
+++ libs/spirit/doc/qi/numeric.qbk      (working copy)
@@ -765,6 +765,7 @@
 
 [table
     [[Expression]                           [Semantics]]
+    [[`RP::begin(n)`]                       [Allows the policy to
initialize]]
     [[`RP::allow_leading_dot`]              [Allow leading dot.]]
     [[`RP::allow_trailing_dot`]             [Allow trailing dot.]]
     [[`RP::expect_dot`]                     [Require a dot.]]
@@ -789,6 +790,8 @@
     [[`RP::parse_inf(f, l, n)`]             [Parse an Inf.
                                             Return `true` if
successful, otherwise `false`.
                                             If successful, place the
result into `n`.]]
+    [[`RP::end(n,b)`]                       [Allows the policy to
finalize, b is result of parsing.
+                                            Return `true` if
successful, otherwise `false`.]]
 ]
 
 The `parse_nan` and `parse_inf` functions get called whenever:
Index: boost/spirit/home/qi/numeric/detail/real_impl.hpp
===================================================================
--- boost/spirit/home/qi/numeric/detail/real_impl.hpp   (revision 85183)
+++ boost/spirit/home/qi/numeric/detail/real_impl.hpp   (working copy)
@@ -133,6 +133,7 @@
         parse(Iterator& first, Iterator const& last, Attribute& attr,
             RealPolicies const& p)
         {
+            p.begin(attr);
             if (first == last)
                 return false;
             Iterator save = first;
@@ -258,7 +259,7 @@
             traits::assign_to(traits::negate(neg, n), attr);
 
             // Success!!!
-            return true;
+            return p.end(attr, true);
         }
     };
 
Index: boost/spirit/home/qi/numeric/real_policies.hpp
===================================================================
--- boost/spirit/home/qi/numeric/real_policies.hpp      (revision 85183)
+++ boost/spirit/home/qi/numeric/real_policies.hpp      (working copy)
@@ -28,6 +28,13 @@
         static bool const allow_trailing_dot = true;
         static bool const expect_dot = false;
 
+        //  Do any initialization of attr that is needed
+        template <typename Attribute>
+        static void
+        begin(Attribute& attr_)
+        {
+        }
+
         template <typename Iterator>
         static bool
         parse_sign(Iterator& /*first*/, Iterator const& /*last*/)
@@ -154,6 +161,14 @@
             }
             return false;
         }
+
+        //  Do any finalization of attr that is needed
+        template <typename Attribute>
+        static bool
+        end(Attribute& attr_,bool result)
+        {
+            return result;
+        }
     };
 
    
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