Weird behaviour of Koenig Lookup

Question

Hi, consider the following program:

    namespace NS2 {
      class base { };

      template<typename T>
      int size(T& t) {
        std::cout << "size NS2 called!" << std::endl;
        return sizeof(t);
      } 
    };

    namespace NS1 {
      class X : NS2::base { };
    }

    namespace NS3 {
      template<typename T>
      int size(T& t) {
        std::cout << "size NS3 called!" << std::endl;
        return sizeof(t) + 1;
      }

      template<typename T>
      class tmpl 
      {
      public:
        void operator()() { size(*this); }
      };
    };

int main() +{
  NS3::tmpl<NS1::X> t;
  t();
  return 0;
}

My compiler (gcc 4.3.3) does not compile the program because the call to size is ambigous. The namespace NS2 seems to be added to the set of associate namespaces for the size call in the class tmpl. Even after reading the section about Koenig Lookup in the ISI Standard I am not sure if this behaviour is standard conform. Is it? Does any one know a way to work around this behaviour without qualifying the size call with the NS3 prefix?

Thanks in advance!

Answer 1

Template arguments and base classes both affect ADL, so I think GCC is correct, here: NS3 comes from the current scope, NS1 from the X template argument, and NS2 from the base class of the template argument.

You have to disambiguate somehow; I'd suggest renaming one or more of the functions, if feasible, or perhaps use SFINAE to disambiguate the functions.

(Similar Situation: Note that boost::noncopyable is actually "typedef noncopyable_::noncopyable noncopyable;" so that the boost namespace doesn't get added to the ADL set of types that derive from it.)