[boost] type traits / linkage and generic programming

From: Herve Bronnimann (hervebronnimann_at_[hidden])
Date: 2006-10-16 14:59:02

[Apologies for duplicates, I don't know if an earlier posting went through because I wasn't subscribed under this email address]
 
Dear boosters: According to the C++ standard, language linkage is part of the specification of a type and even though on many platforms two function types with different linkage specs (eg. C and C++ language linkage) are the same, the standard states rather unambiguously ([7.5 decl.link]) "Two function types with different language linkages are distinct types even if they are otherwise identical." 

My question: does anyone know of a technique to determine the linkage of a function type at compile time? I'm not sure that that would matter for my purposes, but the question arose and I couldn't find an answer.  Thanks for any feedback, 
--Herve Bronnimann 

PS: here's a concrete example of what I'm asking, a metafunction LinkageTraits that would make the following example compile (code obviously untested - all errors mine): 

   #include <boost/static_assert.h> 
   #include <boost/type_traits.h> 

   struct CPPLinkageTag {}; 
   struct CLinkageTag {}; 

   extern "C" { 
     typedef void functionWithCLinkageType(); 
   } 
   typedef void functionWithCPPLinkageType(); 

   main() 
   { 
     BOOST_STATIC_ASSERT( 
       boost::is_same<LinkageTraits<functionWithCLinkageType>::type, 
                      CLinkageTag >::value 
     ); 
     BOOST_STATIC_ASSERT( 
       boost::is_same<LinkageTraits<functionWithCPPLinkageType>::type, 
                      CPPLinkageTag >::value 
     ); 
   }