Re: Incorrect stream operator called for a template class

On Apr 18, 11:10 am, Ryan wrote:


Let's me cut down your example ...

   #include <iostream>

   template <typename T>
   class B
   {
   public:
      typedef int footype;
      footype foovalue;
   };

   template<class T>
   std::ostream& operator<<(std::ostream& stream,
      typename B<T>::footype value)
   {
      return stream << "foo";
   }

   int main()
   {
      B<int> b;
      std::cout << b.foovalue << std::endl;
      return 0;
   }

The compiler will see your operator<< but the problem is that it won't
try to figure out what T is because none of your function's parameter
types are "deducible contexts" with respect to T. At the op<< call
site you have a stream on the left hand side and an int on the right
hand side. Should the compiler now try every possible T to see whether
B<T>::foo is an int? What if there are many such Ts like in your case?
B<int>::foo is an int as well as B<double>::foo is an int.

HTH,
SG

--
      [ See http://www.gotw.ca/resources/clcm.htm for info about ]
      [ comp.lang.c++.moderated. First time posters: Do this! ]