In Python, we can do nested lists right out of the box. E.g. [1, [2, 3], 4, [5, [6]], 7]. In C++, we must jump through some hoops, but it can be done.

nested_list.cc

#include <iostream>
#include <initializer_list>
#include <vector>

using namespace ::std;

template <typename E>
class NestedList {
public:
  class Element {
  public:
    Element(const E& e)
      : holding_primitive_type_(true), element_(e) {}
    Element(initializer_list<Element>&& children)
      : holding_primitive_type_(false), children_(children) {}

    friend ostream& operator<< (ostream& out, const Element& e) {
      if (e.holding_primitive_type_) {
        out << e.element_;
        return out;
      }

      NestedList::Print(&out, e.children_);
      return out;
    }
    
  private:

    // Determines which of the next two variables holds our contents
    bool holding_primitive_type_;

    E element_;
    vector<Element> children_;
  };
  friend class Element;

  NestedList() = default;
  NestedList(initializer_list<Element>&& initial_contents)
    : contents_(initial_contents) {}

  void push_back(const E& e);
  vector<Element>& push_back_empty();

  friend ostream& operator<< (ostream& out, const NestedList& lst) {
    Print(&out, lst.contents_);
    return out;
  }

  static void Print(ostream* os, const vector<Element>& lst) {
    auto& out = *os;

    out << "{";
    bool first = true;
    for (const auto& child : lst) {
      // Separating comma.
      if (first) {
        first = false;
      } else {
        out << ", ";
      }

      out << child;
    }
    out << "}";
  }

private:
  vector<Element> contents_;
};

int main(int argc, char* argv[]) {
  NestedList<int> lst = {1, {2, 3}, 4, {5, {6}}, 7};
  cout << "NestedList: " << lst << endl;

  return 0;
}

I think variant could help us clean this up...