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<int, double, bool> InstrumentTuple;

Cache cache;
InstrumentTuple tuple = cache.get<InstrumentTuple>();

I know I could "simply" pass the types of the tuple. This is what I do know but it is quite cumbersome since I make a lot of calls to this function and since the tuples are quite long:

InstrumentTuple tuple = c.get<int, double, bool>(); // syntax I'd like to avoid

So I tried multiple implementations of the get method, but with no success :

Enabling via a template parameter

#include <tuple>

class Cache
{
private:
    template<int I, typename T, typename = typename std::enable_if<I == std::tuple_size<T>::value>::type>
    std::tuple<> get() // line 6
    {
        return std::tuple<>();
    }

    template<int I, typename T, typename = typename std::enable_if<I != std::tuple_size<T>::value>::type>
    std::tuple<typename std::tuple_element<I,T>::type, decltype(get<I+1, T>())> get() // line 12
    {
        std::tuple<typename std::tuple_element<I,T>::type> value;
        return std::tuple_cat(value, get<I+1, T>());
    }

public:
    template<typename T>
    T get()
    {
        return get<0, T>(); // line 22
    }
};

int main(int argc, char** argv)
{
    Cache cache;
    typedef std::tuple<int, double, bool> InstrumentTuple;
    InstrumentTuple tuple = cache.get<InstrumentTuple>(); // line 30
}

Which gives me this error :

main.cpp: In instantiation of 'T Cache::get() [with T = std::tuple<int, double, bool>]':
main.cpp:30:56:   required from here
main.cpp:22:26: error: no matching function for call to 'Cache::get()'
main.cpp:22:26: note: candidates are:
main.cpp:6:18: note: template<int I, class T, class> std::tuple<> Cache::get()
main.cpp:6:18: note:   template argument deduction/substitution failed:
main.cpp:5:33: error: no type named 'type' in 'struct std::enable_if<false, void>'
main.cpp:12:81: note: template<int I, class T, class> std::tuple<typename std::tuple_element<I, T>::type, decltype (get<(I + 1), T>())> Cache::get()
// ----- Important part
main.cpp:12:81: note:   template argument deduction/substitution failed:
main.cpp: In substitution of 'template<int I, class T, class> std::tuple<typename std::tuple_element<I, T>::type, decltype (get<(I + 1), T>())> Cache::get() [with int I = 0; T = std::tuple<int, double, bool>; <template-parameter-1-3> = <missing>]':
// -----
main.cpp:22:26:   required from 'T Cache::get() [with T = std::tuple<int, double, bool>]'
main.cpp:30:56:   required from here
main.cpp:12:81: error: no matching function for call to 'Cache::get()'
main.cpp:12:81: note: candidate is:
main.cpp:6:18: note: template<int I, class T, class> std::tuple<> Cache::get()
main.cpp:6:18: note:   template argument deduction/substitution failed:
main.cpp:5:33: error: no type named 'type' in 'struct std::enable_if<false, void>'
main.cpp: In instantiation of 'T Cache::get() [with T = std::tuple<int, double, bool>]':
main.cpp:30:56:   required from here
main.cpp:20:7: note: template<class T> T Cache::get()
main.cpp:20:7: note:   template argument deduction/substitution failed:
main.cpp:22:26: error: wrong number of template arguments (2, should be 1)

I don't get why is there a missing template parameter.

So I tried another implementation :

Template-template named parameter

#include <tuple>

class Cache
{
private:
    template<int>
    std::tuple<> get() // line 7
    {
        return std::tuple<>();
    }

    template<int index, typename type, typename... rest>
    std::tuple<type, rest...> get() // line 13
    {
        return std::tuple_cat(std::tuple<type>(), get<index+1, rest...>());
    }

public:
    template<template<typename... types> class tuple>
    typename std::tuple<(tuple::types)...> get()
    {
        return get<0, (tuple::types)...>();
    }
};  // line 24

int main(int argc, char** argv)
{
    Cache cache;
    typedef std::tuple<int, double, bool> InstrumentTuple;
    InstrumentTuple tuple = cache.get<InstrumentTuple>(); // line 30
}

But then I get this error :

// ----- Important part
main.cpp:24:1: error: expected identifier before '}' token
main.cpp:24:1: error: expected unqualified-id before '}' token
// -----
main.cpp: In function 'int main(int, char**)':
main.cpp:30:56: error: no matching function for call to 'Cache::get()'
main.cpp:30:56: note: candidates are:
main.cpp:7:18: note: template<int <anonymous> > std::tuple<> Cache::get()
main.cpp:7:18: note:   template argument deduction/substitution failed:
main.cpp:13:31: note: template<int index, class type, class ... rest> std::tuple<_Head, _Tail ...> Cache::get()
main.cpp:13:31: note:   template argument deduction/substitution failed:

Again, I don't understand the errors due to a missing identifier.

I'm wondering now if what I want to achieve is even possible. Is it possible to use a std::tuple like I want to ? Or is there a better way ?

Answer 1

Your first solution is failing because the second overload to get is not visible at the point of its own return type declaration; to get around this you would need to separate out the return type computation into its own subprogram.

The second solution is closer; the problem is that you're only inferring the template std::tuple, not its arguments. An easy way to infer variadic arguments (e.g. type arguments to tuple) is through an empty tag structure, requiring one extra level of indirection:

template<typename T> struct type_tag {};

class Cache {
    // ... (as above)

    template<typename... Ts> std::tuple<Ts...> get(type_tag<std::tuple<Ts...>>) {
        return get<0, Ts...>();
    }

public:
    template<typename T> T get() {
        return get(type_tag<T>{});
    }
};

You should check to see whether you can write the solution using pack expansion instead of recursion, for example:

template<typename T> struct type_tag {};

class Cache {
    template<typename... Ts> std::tuple<Ts...> get(type_tag<std::tuple<Ts...>>) {
        return std::tuple<Ts...>{Ts{}...};
    }

public:
    template<typename T> T get() {
        return get(type_tag<T>{});
    }
};

Answer 2

Here is what I came up with:

#include <tuple>

struct Cache;

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

template<int I, typename... Ts> struct cache_getter<I, std::tuple<Ts...> > {
    static std::tuple<Ts...> get(Cache & c);
};

struct Cache {
protected:
    template<int, typename...> friend struct tuple_walker;
private:
    /* here T is a type from within a std::tuple<...> */
    template<int I, typename T> std::tuple<T> get_ex() {
        return std::tuple<T>();
    }
public:
    /* here T is actually a std::tuple<...> */
    template<typename T> 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<typename...> struct my_tuple_cat;
template<typename H, typename... T> struct my_tuple_cat<H, T...> {
    static auto cat(H h, T... t) -> decltype(std::tuple_cat(h, my_tuple_cat<T...>::cat(t...)))
    { return std::tuple_cat(h, my_tuple_cat<T...>::cat(t...)); }
};
template<typename T> struct my_tuple_cat<T> {
    static T cat(T t) { return t; }
};

/* this one is used to call Cache.get_ex<int I, typename T>() with incrementing values for I */
template<int I, typename H, typename... T> struct tuple_walker<I, H, T...> {
    static std::tuple<H, T...> get(Cache & c) {
        return my_tuple_cat<std::tuple<H>, std::tuple<T...>>::cat(c.get_ex<I, H>(), tuple_walker<I + 1, T...>::get(c));
    }
};
template<int I, typename H> struct tuple_walker<I, H> {
    static std::tuple<H> get(Cache & c) {
        return c.get_ex<I, H>();
    }
};
/* this one will forward the types in std::tuple<...> to tuple_walker to get each tuple separately */
template<int I, typename... Ts> std::tuple<Ts...> cache_getter<I, std::tuple<Ts...> >::get(Cache & c) {
    return tuple_walker<I, Ts...>::get(c);
}

int main(int argc, char ** argv) {
    Cache cache;
    typedef std::tuple<int, double, bool> InstrumentTuple;
    InstrumentTuple tuple = cache.get<InstrumentTuple>();
    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