How to write a variadic method which replaces chained method calls?

Question

I am working on a recursive map class called group_by which models the SQL namesake.

For example, gb is a group_by object

Answer 1

Chained method calls are essentially recursive, so you need to implement at recursively:

child_type& at( I const& key ) {
    return m_map.at( key );
}

template<typename J, typename... Ks>
auto at(const I &i, const J &j, const Ks &...ks)
-> decltype(m_map.at(i).at(j, ks...)) {
    return m_map.at(i).at(j, ks...);
}

Note that since at requires at least 1 argument, the variadic form takes at least 2 parameters. This is significantly easier to implement than dispatching on sizeof..., and should be easier to read.