Elegant template specialization

Question

Is there an elegant way to specialize a template based on one of its template parameters?

Ie.

template<int N> struct Junk {
    static int foo() {
        // stuff
        return Junk<N - 1>::foo();
    }
};

// compile error: template argument '(size * 5)' involves template parameter(s)
template<int N> struct Junk<N*5> {
    static int foo() {
        // stuff
        return N;
    }
};

template<> struct Junk<0> {
    static int foo() {
        // stuff
        return 0;
    }
};

Ie. I am trying to specialize a template based on the parameter being divisible by 5. The only way I can seem to do it is like below:

template<int N> struct JunkDivisibleBy5 {
    static int foo() {
        // stuff
        return N;
    }
};

template<int N> struct Junk {
    static int foo() {
        // stuff
        if ((N - 1) % 5 == 0 && N != 1)
            return JunkDivisibleBy5<N - 1>::foo();
        else
            return Junk<N - 1>::foo();
    }
};


template<> struct Junk<0> {
    static int foo() {
        // stuff
        return 0;
    }
};

But this is significantly less elegant, and also necessitates instantiation of all templates even if the template argument shouldn't require it.

Answer 1

How's this:

#include <iostream>
using namespace std;

template < typename T, T N, T D >
struct fraction {
    typedef T value_type;
    static const value_type num = N;
    static const value_type denom = D;
    static const bool is_div = (num % denom == 0);
};

template< typename T, T N, T D, bool P >
struct do_if {
    static void op() { cout << N << " NOT divisible by " << D << endl; }
};

template< typename T, T N, T D >
struct do_if< T, N, D, true > {
    static void op() { cout << N << " divisible by " << D << endl; }
};

template < int N >
void foo() {
    typedef fraction< int, N, 5 > f;
    do_if< typename f::value_type, f::num, f::denom, f::is_div >::op();
}

int main() {
    foo< -5 >();
    foo< -1 >();
    foo< 0 >();
    foo< 1 >();
    foo< 5 >();
    foo< 10000005 >();
    return 0;
}

Answer 2

Using D programming language templates, one could write it as:

struct Junk(int N)
{
    static int foo()
    {
        static if (N == 0)
            return 0;
        else static if ((N % 5) == 0)
            return N;
        else
            return Junk!(N - 1).foo();
    }
}

static if's are executed at compile time.