Re: Function Template Specialization

Hi,

I recently came across some not so much but a bit surprising behaviour
when using a function template and some specializations. The code snippet
below is a bit verbose, maybe I'm missing something obvious.

I do have a template function get_value(), which returns a selected value
 from an aggregate.
The template parameter is used to generalize of different types of such an
aggregate.

Then I do have a template function is_valid(), which returns a boolean
flag, depending on the aggregate content, and the set of the parameter.
So it has two template parameters (ValueSet, AggregateType).
//---------------------------------------------------------------------
//---------------------------------------------------------------------
#include <iostream>

typedef enum { id_1, id_2, id_3 } index_t;
struct FooSet {}; struct BarSet {};
typedef std::pair<int, bool> baz_t; // (value, currently-valid-flag)
//---------------------------------------------------------------------
template<class ParameterSet, class Aggregate>
bool is_valid(const Aggregate& dat) { return false; }

template<>
bool is_valid<FooSet, class Aggregate> (const Aggregate& dat) {
   return true;
}

template<>
bool is_valid<BarSet, class Aggregate> (const Aggregate& dat) {
   return true;
}

//---------------------------------------------------------------------
template<class Aggregate>
baz_t get_value(const Aggregate& dat, index_t id) {
   switch(id) {
   case id_1: return baz_t(dat.member_1, is_valid<FooSet>(dat));
   case id_2: return baz_t(dat.member_2, is_valid<BarSet>(dat));
   case id_3: return baz_t(dat.member_3, is_valid<BarSet>(dat));
   default: throw std::exception();
   }
}
//---------------------------------------------------------------------
struct some_aggregate {
   int member_1; // element of set foo
   int member_2; // element of set bar
   int member_3; // element of set bar
};
//---------------------------------------------------------------------
int main() {
   using namespace std;
   some_aggregate blub = { 1, 2 };
   baz_t one(get_value(blub, id_1));
   baz_t two(get_value(blub, id_2));
   cout << "id_1: " << one.first << " | " << one.second << endl;
   cout << "id_2: " << two.first << " | " << two.second << endl;
   return 0;
}
//---------------------------------------------------------------------
//---------------------------------------------------------------------
The program prints:
   id_1: 1 | 0
   id_2: 2 | 0
But I would like it to print
   id_1: 1 | 1
   id_2: 2 | 1

I have solved my specific task just using class and member templates quite
easily, but I would like to know if there would have been just some
syntactic sugar (I tried different syntaxes, but without understanding
this leads to nothing). I think I've read about the this issue, but the
C++ FAQ Template section is silent about it, and I can't remember any
keywords. Any reading suggestions would be very nice.