Use std::tuple for template parameter list instead of list of types

Question

I\'m trying to make a call to a templated function like this :

typedef std::tuple Instrument         


        

Answer 1

Here is what I came up with:

#include 

struct Cache;

/* typename = std::tuple<...> */
template struct cache_getter;
/* typename = parameters from std::tuple<...> */
template struct tuple_walker;

template struct cache_getter > {
    static std::tuple get(Cache & c);
};

struct Cache {
protected:
    template friend struct tuple_walker;
private:
    /* here T is a type from within a std::tuple<...> */
    template std::tuple get_ex() {
        return std::tuple();
    }
public:
    /* here T is actually a std::tuple<...> */
    template T get() {
        return cache_getter<0, T>::get(*this);
    }
};

/* since std::tuple_cat only accepts 2 std::tuples per call but we don't have control over the number of types in the passed in std::tuple, we'll need to chain our calls */
template struct my_tuple_cat;
template struct my_tuple_cat {
    static auto cat(H h, T... t) -> decltype(std::tuple_cat(h, my_tuple_cat::cat(t...)))
    { return std::tuple_cat(h, my_tuple_cat::cat(t...)); }
};
template struct my_tuple_cat {
    static T cat(T t) { return t; }
};

/* this one is used to call Cache.get_ex() with incrementing values for I */
template struct tuple_walker {
    static std::tuple get(Cache & c) {
        return my_tuple_cat, std::tuple>::cat(c.get_ex(), tuple_walker::get(c));
    }
};
template struct tuple_walker {
    static std::tuple get(Cache & c) {
        return c.get_ex();
    }
};
/* this one will forward the types in std::tuple<...> to tuple_walker to get each tuple separately */
template std::tuple cache_getter >::get(Cache & c) {
    return tuple_walker::get(c);
}

int main(int argc, char ** argv) {
    Cache cache;
    typedef std::tuple InstrumentTuple;
    InstrumentTuple tuple = cache.get();
    return 0;
}

I hope this is worth something. I haven't done much in C++11 yet, so maybe this isn't an optimal solution.

Proof that it compiles can be found here