Template tuple - calling a function on each element

My question is in the code:

template<typename... Ts>
struct TupleOfVectors {
  std::tuple<std::vector<Ts>...> tuple;

  void do_something_to_each_vec() {
    //Question: I want to do this:
    //  "for each (N)": do_something_to_vec<N>()
    //How?
  }

  template<size_t N>
  void do_something_to_vec() {
    auto &vec = std::get<N>(tuple);
    //do something to vec
  }
};

Solution 1:

You can quite easily do that with some indices machinery. Given a meta-function gen_seq for generating compile-time integer sequences (encapsulated by the seq class template):

namespace detail
{
    template<int... Is>
    struct seq { };

    template<int N, int... Is>
    struct gen_seq : gen_seq<N - 1, N - 1, Is...> { };

    template<int... Is>
    struct gen_seq<0, Is...> : seq<Is...> { };
}

And the following function templates:

#include <tuple>

namespace detail
{
    template<typename T, typename F, int... Is>
    void for_each(T&& t, F f, seq<Is...>)
    {
        auto l = { (f(std::get<Is>(t)), 0)... };
    }
}

template<typename... Ts, typename F>
void for_each_in_tuple(std::tuple<Ts...> const& t, F f)
{
    detail::for_each(t, f, detail::gen_seq<sizeof...(Ts)>());
}

You can use the for_each_in_tuple function above this way:

#include <string>
#include <iostream>

struct my_functor
{
    template<typename T>
    void operator () (T&& t)
    {
        std::cout << t << std::endl;
    }
};

int main()
{
    std::tuple<int, double, std::string> t(42, 3.14, "Hello World!");
    for_each_in_tuple(t, my_functor());
}

Here is a live example.

In your concrete situation, this is how you could use it:

template<typename... Ts>
struct TupleOfVectors
{
    std::tuple<std::vector<Ts>...> t;

    void do_something_to_each_vec()
    {
        for_each_in_tuple(t, tuple_vector_functor());
    }

    struct tuple_vector_functor
    {
        template<typename T>
        void operator () (T const &v)
        {
            // Do something on the argument vector...
        }
    };
};

And once again, here is a live example.

Update

If you're using C++14 or later, you can replace the seq and gen_seq classes above with std::integer_sequence like so:

namespace detail
{
    template<typename T, typename F, int... Is>
    void
    for_each(T&& t, F f, std::integer_sequence<int, Is...>)
    {
        auto l = { (f(std::get<Is>(t)), 0)... };
    }
} // namespace detail

template<typename... Ts, typename F>
void
for_each_in_tuple(std::tuple<Ts...> const& t, F f)
{
    detail::for_each(t, f, std::make_integer_sequence<int, sizeof...(Ts)>());
}

If you're using C++17 or later you can do this (from this comment below):

std::apply([](auto ...x){std::make_tuple(some_function(x)...);} , the_tuple);

Solution 2:

In C++17 you can do this:

std::apply([](auto ...x){std::make_tuple(some_function(x)...);} , the_tuple);

given that some_function has suitable overloads for all the types in the tuple.

This already works in Clang++ 3.9, using std::experimental::apply.

Solution 3:

In addition to the answer of @M. Alaggan, if you need to call a function on tuple elements in order of their appearance^† in the tuple, in C++17 you can also use a fold expression like this:

std::apply([](auto& ...x){(..., some_function(x));}, the_tuple);

(live example).

^{†_{Because otherwise order of evaluation of function arguments is unspecified.}}

Solution 4:

Here's one approach which may work well in your case:

template<typename... Ts>
struct TupleOfVectors {
    std::tuple<std::vector<Ts>...> tuple;

    void do_something_to_each_vec()
    {
        // First template parameter is just a dummy.
        do_something_to_each_vec_helper<0,Ts...>();
    }

    template<size_t N>
    void do_something_to_vec()
    {
        auto &vec = std::get<N>(tuple);
        //do something to vec
    }

private:
    // Anchor for the recursion
    template <int>
    void do_something_to_each_vec_helper() { }

    // Execute the function for each template argument.
    template <int,typename Arg,typename...Args>
    void do_something_to_each_vec_helper()
    {
        do_something_to_each_vec_helper<0,Args...>();
        do_something_to_vec<sizeof...(Args)>();
    }
};

The only thing that is a bit messy here is the extra dummy int template parameter to do_something_to_each_vec_helper. It is necessary to make the do_something_to_each_vec_helper still be a template when no arguments remain. If you had another template parameter you wanted to use, you could use it there instead.