Re: template template parameter compilation errors!!!

aaragon wrote:


Well, the thing is that in tis case you _can't_ use "either the std::map, or a
hash_map" if these templates accept different number of template arguments.

Just like default arguments in a function declaration don't change the fact that
a two-parameter function is still a two-parameter function (according to its type)

   void foo(int, int = 2);

   void (*p1)(int) = &foo; // ERROR
   void (*p2)(int, int) = &foo; // OK

default template arguments in template declaration don't change the fact that a
4-parameter template is a 4-parameter template and it will not pass as an
argument for a 2-parameter template template parameter.

If you want to stick with 2-parameter template template parameter and still be
able to use 'std::map' or 'hash_map' as arguments, you'll have to use an
appropriate template trick, of which there are many.

For example, consider how I do it in the following code sample

   template <typename S, typename D> struct std_map_adaptor {
     typedef std::map<S, D> type;
   };

   template <typename S, typename D> struct hash_map_adaptor {
     typedef hash_map<S, D> type;
   };

   template < template <class,class> class MapPolicy = std_map_adaptor>
   class TriangleGaussMap
   {
   public:
     void foo()
     {
       typename MapPolicy<int, double>::type map;
       // Note the use of nested typename 'type'

       map[0] = 5;
       map[1] = 3.5;
       map[2] = 8.1;
     }
   };

   int main()
   {
     TriangleGaussMap<> map1;
     map1.foo();

     TriangleGaussMap<hash_map_adaptor> map2;
     map2.foo();
   }

--
Best regards,
Andrey Tarasevich