C++ convert variables to template arguments

I decided to have some more fun with the code, here's an improved version over my first attempt which has the following benefits:

• Supports enum types
• Explicitly specify how many parameters should be converted
• Generic implementation for the complicated part, one small helper for each function that uses it.

The code:

#include <iostream>
#include <utility>
#include <type_traits>

// an enum we would like to support
enum class tribool { FALSE, TRUE, FILE_NOT_FOUND };

// declare basic generic template
// (independent of a specific function you'd like to call)
template< template< class > class CB, std::size_t N, typename = std::tuple<> >
struct var_to_template;

// register types that should be supported
template< template< class > class CB, std::size_t N, typename... Cs >
struct var_to_template< CB, N, std::tuple< Cs... > >
{
    // bool is pretty simple, there are only two values
    template< typename R, typename... Args >
    static R impl( bool b, Args&&... args )
    {
        return b
          ? var_to_template< CB, N-1, std::tuple< Cs..., std::true_type > >::template impl< R >( std::forward< Args >( args )... )
          : var_to_template< CB, N-1, std::tuple< Cs..., std::false_type > >::template impl< R >( std::forward< Args >( args )... );
    }

    // for each enum, you need to register all its values
    template< typename R, typename... Args >
    static R impl( tribool tb, Args&&... args )
    {
        switch( tb ) {
        case tribool::FALSE:
          return var_to_template< CB, N-1, std::tuple< Cs..., std::integral_constant< tribool, tribool::FALSE > > >::template impl< R >( std::forward< Args >( args )... );
        case tribool::TRUE:
          return var_to_template< CB, N-1, std::tuple< Cs..., std::integral_constant< tribool, tribool::TRUE > > >::template impl< R >( std::forward< Args >( args )... );
        case tribool::FILE_NOT_FOUND:
          return var_to_template< CB, N-1, std::tuple< Cs..., std::integral_constant< tribool, tribool::FILE_NOT_FOUND > > >::template impl< R >( std::forward< Args >( args )... );
        }
        throw "unreachable";
    }

    // in theory you could also add int, long, ... but
    // you'd have to switch on every possible value that you want to support!
};

// terminate the recursion
template< template< class > class CB, typename... Cs >
struct var_to_template< CB, 0, std::tuple< Cs... > >
{
    template< typename R, typename... Args >
    static R impl( Args&&... args )
    {
        return CB< std::tuple< Cs... > >::template impl< R >( std::forward< Args >( args )... );
    }
};

// here's your function with the template parameters
template< bool B, tribool TB >
int HeavyLoop_impl( int arg )
{
    for( int i = 0; i < 10000000; i++ ) {
        arg += B ? 1 : 2;
        arg += ( TB == tribool::TRUE ) ? 10 : ( TB == tribool::FALSE ) ? 20 : 30;
    }
    return arg;
}

// a helper class, required once per function that you'd like to forward
template< typename > struct HeavyLoop_callback;
template< typename... Cs >
struct HeavyLoop_callback< std::tuple< Cs... > >
{
    template< typename R, typename... Args >
    static R impl( Args&&... args )
    {
        return HeavyLoop_impl< Cs::value... >( std::forward< Args >( args )... );
    }
};

// and here, everything comes together:
int HeavyLoop( bool b, tribool tb, int arg )
{
    // you provide the helper and the number of arguments
    // that should be converted to var_to_template<>
    // and you provide the return type to impl<>
    return var_to_template< HeavyLoop_callback, 2 >::impl< int >( b, tb, arg );
}

int main()
{
    bool b = true;
    tribool tb = tribool::FALSE;
    int arg = 0;
    int res = HeavyLoop( b, tb, arg );
    std::cout << "res: " << res << std::endl;
    return 0;
}

And here's a live example in case you want to play with it.

Here's how you can do it:

#include <iostream>
using namespace std;

template <bool b1, bool b2>
struct HeavyLoopImpl
{
    static int func(int arg)
    {
        for (int i = 0; i < 10000000; i++) {
            arg += b1 ? 1 : 2;
            arg += b2 ? 10 : 20;
        }
        return arg;
    }
};

template <template<bool...> class Impl,bool...Bs>
struct GenericJump
{
    template<typename... Args>
    static int impl(Args&&... args)
    {
        return Impl<Bs...>::func(std::forward<Args>(args)...);
    }

    template<typename... Args>
    static int impl(bool b, Args&&... args)
    {
        return b
            ? GenericJump<Impl,Bs...,true >::impl(std::forward<Args>(args)...)
            : GenericJump<Impl,Bs...,false>::impl(std::forward<Args>(args)...);
    }
};

int HeavyLoop(bool b1, bool b2, int arg)
{
    return GenericJump<HeavyLoopImpl>::impl(b1,b2,arg);
}

int main()
{
    bool b1 = true;
    bool b2 = false;
    int arg = 0;
    int res = HeavyLoop(b1, b2, arg);
    cout << "res: "<<res<<endl;
    return 0;
}

This is basically Daniels solution, but it allows you to use functions other than HeavyLoop_impl() as implementation. Only being able to call a single template function kind of defeats the purpose of being a generic solution. The GenericJump template class can call other functions also. You only have to change the HeavyLoop_impl() template function into a template class with a static function func(). It works marvellously. It compiles with gcc 4.7.3 and gives the correct output.