[Boost-bugs] [Boost C++ Libraries] #4967: apply_visitor_delayed_t 's operator() is not const correct

Subject: [Boost-bugs] [Boost C++ Libraries] #4967: apply_visitor_delayed_t 's operator() is not const correct
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2010-12-08 16:23:51

#4967: apply_visitor_delayed_t 's operator() is not const correct
----------------------------------+-----------------------------------------
 Reporter:  zhuo.qiang@…          |       Owner:  ebf    
     Type:  Patches               |      Status:  new    
Milestone:  To Be Determined      |   Component:  variant
  Version:  Boost 1.45.0          |    Severity:  Problem
 Keywords:                        |  
----------------------------------+-----------------------------------------
 The operator() overload in Boost.Variant's boost::apply_visitor_delayed_t
 is not const qualified which I think could be safely add.

 The benefit of adding const is as following:

 There are some generic function need to store and call the
 apply_visitor_delayed_t instance, transform_iterator & transform_range,
 for example. The following code will not compile without the const
 quality:

 ----------------------------------------------------
 using namespace boost;
 using namespace std;

 typedef variant<int, string> V;

 struct  times_2_t : public static_visitor<int>
 {
     template<class T>
     int operator()(T const& t) const
     {
         return 2 * t;
     };

     int operator()(string const& s) const
     {
         return 2 * s.size();
     };
 };

 BOOST_AUTO_TEST_CASE(Test)
 {
     vector<V> all;
     all.push_back(1);
     all.push_back(2);

     times_2_t times_2;
     int const result = boost::accumulate(
         all | boost::adaptors::transformed(apply_visitor(times_2)),
         0);

     BOOST_CHECK_EQUAL(6, result);
 }

 ---------------------------------------

 One way to fix this is using a boost::funciton to hold the
 apply_visitor_delayed_t instance and pass the wrapper to transformed:
 ---------------------------------------------------------
     times_2_t times_2;
     function<int(V const&)> f = apply_visitor(times_2);
     int const result = boost::accumulate(
         all | boost::adaptors::transformed(apply_visitor(f)),
         0);
 -------------------------------------------------------
 However, this will make simple things complex and have runtime cost.

 So a more direct fix would be add the harmless const qualify to the
 operator() for apply_visitor_delayed_t

 The attached is a simply patch:

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/4967>
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