How can you use C template metaprogramming to apply a specific function to each vector within a tuple of vectors?

Using a Function Template to Manipulate Tuple Elements

Imagine a scenario where you have a tuple containing different vector types and desire to perform specific operations on each vector. For instance, you want to call a function on each vector within the tuple, denoted as "do_something_to_vec()" for each N. Here's how you can achieve this using C template trickery:

1. Generating a Compile-Time Sequence of Indices:

First, we need a way to generate a sequence of indices that will correspond to the positions of the vectors within the tuple. This can be done using the seq and gen_seq meta-functions:

<code class="cpp">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...> { };
}</code>

2. Defining a Custom Functor to Apply the Function:

Next, we create a functor that can apply the desired function to each vector. The functor could look something like this:

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

3. Utilizing the for_each_in_tuple Function:

Now, we can define a generic for_each_in_tuple function that uses the seq and gen_seq meta-functions to iterate over each element of the tuple and apply our custom functor:

<code class="cpp">#include <tuple>

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

4. Applying the Function to the Tuple of Vectors:

Within your TupleOfVectors struct, you can utilize the for_each_in_tuple function to call your specific function on each vector within the tuple:

<code class="cpp">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());
    }
};</code>

5. Live Example:

Here's a live example demonstrating how to use the solution: https://godbolt.org/z/q4wXTqPa6

Note:

If you're using C 14 or later, you can replace the seq and gen_seq meta-functions with std::integer_sequence. For C 17 and beyond, an even simpler solution using std::apply is possible:

<code class="cpp">std::apply([](auto ...x){std::make_tuple(some_function(x)...);} , the_tuple);</code>

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