Template tuple - calling a function on each element

Question

My question is in the code:

template
struct TupleOfVectors {
  std::tuple...> tuple;

  void do_something_t         


        

Answer 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
    struct seq { };

    template
    struct gen_seq : gen_seq { };

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

And the following function templates:

#include 

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

template
void for_each_in_tuple(std::tuple const& t, F f)
{
    detail::for_each(t, f, detail::gen_seq());
}

You can use the for_each_in_tuple function above this way:

#include 
#include 

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

int main()
{
    std::tuple 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
struct TupleOfVectors
{
    std::tuple...> t;

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

    struct tuple_vector_functor
    {
        template
        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
    void
    for_each(T&& t, F f, std::integer_sequence)
    {
        auto l = { (f(std::get(t)), 0)... };
    }
} // namespace detail

template
void
for_each_in_tuple(std::tuple const& t, F f)
{
    detail::for_each(t, f, std::make_integer_sequence());
}

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);