Sum of specific tuple elements

Question

I am trying to compute the sum of specific elements of a tuple, but cannot get it to compile.

Here is what the function I want to write would look like:

template <int ...ids>
float sum(std::tuple<int, int, std::string, float> things)
{
    return "<sum of elements of ids>";
}

And I would like to call it, like this:

std::tuple<int, int, std::string, float> my_things= { 1, 2, "3", 4.0f };
float sum_numbers = sum<0, 1, 3>(my_things);

I couldn't get it to work using folding. :/ Is this even possible? If yes, how so?

Thanks in advance!

Answer 1

I couldn't get it to work using folding. :/ Is this even possible? If yes, how so?

What about as follows ?

template <int ...ids>
float sum (std::tuple<int, int, std::string, float> things)
{
    return ( 0.0f + ... + std::get<ids>(things) );
}

But I suggest (1) to be more generic and (2) to use std::size_t for indexes

template <std::size_t ... Ids, template T>
auto sum (T const & things)
 { return ( 0 + ... + std::get<Ids>(things) ); }