Re: template template parameter compilation errors!!!

aaragon <>
Sun, 23 Mar 2008 11:16:05 -0700 (PDT)
On Mar 22, 1:51 am, Andrey Tarasevich <>

aaragon wrote:

What about default parameters? I could use for that class template
either the std::map, or a hash_map. They both have different number of
parameters, so why should I provide the total number of parameters?

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
     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;;

     TriangleGaussMap<hash_map_adaptor> map2;;

Best regards,
Andrey Tarasevich

Yes, I see the trick, that'll do it. However, it's kind of annoying
having to write code to support something that I've been using for a
while. With an earlier version of GCC, I was able to chose either a
std::map or a gnu::hash_map very easily. Thanks for the info.


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