Re: [Boost-bugs] [Boost C++ Libraries] #7378: lookup_one_property fails if property tag not found

Subject: Re: [Boost-bugs] [Boost C++ Libraries] #7378: lookup_one_property fails if property tag not found
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2012-09-14 17:55:51

#7378: lookup_one_property fails if property tag not found
-----------------------------------------------+----------------------------
  Reporter:  Andreas Hehn <hehn@…>             |       Owner:  dgregor                           
      Type:  Bugs                              |      Status:  closed                            
 Milestone:  To Be Determined                  |   Component:  graph                             
   Version:  Boost 1.52.0                      |    Severity:  Problem                           
Resolution:  wontfix                           |    Keywords:  lookup_one_property property_value
-----------------------------------------------+----------------------------

Comment (by Andreas Hehn <hehn@…>):

 The problem is that in the enable_if of the lookup(...) member function

 {{{
 template <typename Tag, typename T, typename Base, typename PropName>
   struct lookup_one_property_internal<boost::property<Tag, T, Base>,
 PropName>: lookup_one_property_internal<Base, PropName> {
     private:
     typedef lookup_one_property_internal<Base, PropName> base_type;
     public:
     template <typename PL>
     static typename enable_if<is_same<PL, boost::property<Tag, T, Base> >,
 typename base_type::type&>::type
     lookup(PL& prop, const PropName& tag)
 }}}

 typename base_type::type& does not depend on the template parameter of the
 template member function but only on the template parameters of the class.
 Therefore base_type::type is required to exist, even if lookup(...) is not
 used (or the enable_if<...> disables the function).

 A solution could be to add a indirection which depeneds on {{{PL}}}, like
 {{{
  template <class First, class Second>
  struct second_type {
      typedef Second type;
  };

 ...

     static typename enable_if<is_same<PL, boost::property<Tag, T, Base> >,
 typename second_type<PL,base_type>::type::type&>::type
     lookup(PL& prop, const PropName& tag)
 }}}

 which will prevent the compiler from resolving base_type::type unless
 lookup(...) is explicitly instantiated.

 This way the general template {{{lookup_one_property_internal}}} does not
 need to define a {{{type}}} member to use
 {{{lookup_one_property<...>::found}}}.

 But I have to admit that this solution looks a little ugly.

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/7378#comment:7>
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