Convert tuple to pair

Question

Have educational task: write template function, which takes arbitrary std::tuple and 2 indexes inside and returns std::pair, containing elements of given std::tuple with correspondent indexes.

Example:

auto t = std::make_tuple(0, 3.5, "Hello");
std::pair p = to_pair<1,2>(t);
// p contains 3.5 and "Hello"

Written something like this:

template
auto to_pair(std::tuple t) -> decltype(std::make_pair(std::get(t), std::get(t))) {
    return std::make_pair(std::get(t), std::get(t));
}

However got an error:

r: missing template arguments before ‘t’ auto to_pair(std::tuple t) -> decltype(std::make_pair(gett, gett)) ^

What I'm doing wrong and what is correct syntax here?

user_255803 · Answer

std::tuple is a template class, so there's no std::tuple, only std::tuple. In your case, the type of t is std::tuple.

Also, you're calling std::get without an argument (there's missing braces).

You're almost there, the function should be something along the lines of:

template
auto to_pair(std::tuple t)
    -> decltype(std::make_pair(std::get(t), std::get(t))) {
  return std::make_pair(std::get(t), std::get(t));
}

Convert tuple to pair

1 Answers