C++17 structured binding that also includes an existing variable

Question

This SO answer lists some shortcomings of C++17 decomposition declarations (the feature formerly known as \"structured binding\"). For example, you can\'t give explicit type

Answer 1

This is a really dumb idea that I wouldn't seriously suggest unless there ends up being no sane workaround... but consider the following code.

template
auto plus(std::index_sequence)
{
    return std::index_sequence{};
}

template
auto tuple_select(RHS&& rhs, std::index_sequence)
{
    return std::forward_as_tuple(std::get(std::forward(rhs))...);
}

template
struct AndTie {
    std::tuple v;
    AndTie(Ts&... vs) : v(vs...) {}

    template
    auto operator=(RHS&& rhs) && {
        constexpr int N = std::tuple_size_v;
        constexpr int K = sizeof...(Ts);
        v = tuple_select(std::forward(rhs), plus(std::make_index_sequence{}));
        return tuple_select(std::forward(rhs), std::make_index_sequence{});
    }
};

This gives us

auto [key] =AndTie(p)= load_string(p, end);
auto [value] =AndTie(p)= load_value(p, end);

It still has the limitation that the "tied" lvalues are constrained to appear last and the "declared" variables are constrained to appear first, but I don't think there's much way to get around that. And something like tuple_shuffle could handle that if you needed it.