Why take arguments by reference?

Relevant GitHub issue: https://github.com/eliaskosunen/scnlib/issues/2

Another frequent complaint is how the library requires default-constructing your arguments, and then passing them by reference. A proposed alternative is returning the arguments as a tuple, and then unpacking them at call site.

This is covered pretty well by the above GitHub issue, but to summarize:

• std::tuple has measurable overhead (~5% slowdown)

• it still would require your arguments to be default-constructible

To elaborate on the second bullet point, consider this example:

auto [result, i, str] =
    scn::scan_tuple<int, non_default_constructible_string>(
        range, scn::default_tag);

Now, consider what would happen if an error occurs during scanning the integer. The function would need to return, but what to do with the string? It must be default-constructed (std::tuple doesn’t allow unconstructed members).

Would it be more convenient, especially with C++17 structured bindings? One could argue that, and that’s why an alternative API, returning a tuple, is available, in the header <scn/tuple_return.h>. The rationale of putting it in a separate header is to avoid pulling in the entirety of very heavy standard headers <tuple> and <functional>.

TODO: flesh this section out