How can you pretty-print `std::tuple` using variadic templates?

Pretty-Printing of std::tuple Using Variadic Templates

In a follow-up to a previous discussion on pretty-printing STL containers, we aim to extend this capability to include std::tuple using variadic templates.

The Problem

For std::pair, we previously implemented a custom operator overload for pretty-printing:

std::ostream & operator &p) {
  return o <p>The goal is to create a similar construction for std::tuple.</p><p><strong>The Solution</strong></p><pre class="brush:php;toolbar:false">namespace aux {
// Index sequence utility
template<:size_t...> struct seq {};
template<:size_t n std::size_t... is>
struct gen_seq : gen_seq<n-1 n-1 is...> {};
template struct gen_seq : seq {};

// Recursive printing function using indices
template<class ch class tr tuple std::size_t... is>
void print_tuple(std::basic_ostream<ch>& os, Tuple const& t, seq<is...>) {
  using swallow = int[];
  (void)swallow{0, (void(os (t)), 0)...};
}
} // aux::

template<class ch class tr class... args>
auto operator& os, std::tuple<args...> const& t)
    -> std::basic_ostream<ch tr>&
{
  os ());
  return os <p><strong>Bonus Generality</strong></p>
<p>To enhance generality, add partial specializations for delimiter handling:</p>
<pre class="brush:php;toolbar:false">// Delimiters for tuple
template<class... args>
struct delimiters<:tuple>, char> {
  static const delimiters_values<char> values;
};
template<class... args>
const delimiters_values<char> delimiters<:tuple>, char>::values = { "(", ", ", ")" };

template<class... args>
struct delimiters<:tuple>, wchar_t> {
  static const delimiters_values<wchar_t> values;
};
template<class... args>
const delimiters_values<wchar_t> delimiters<:tuple>, wchar_t>::values = { L"(", L", ", L")" };</:tuple></wchar_t></class...></wchar_t></:tuple></class...></:tuple></char></class...></char></:tuple></class...>

Integrate these changes into operator

template<class ch class tr class... args>
auto operator& os, std::tuple<args...> const& t)
    -> std::basic_ostream<ch tr>&
{
  typedef std::tuple<args...> tuple_t;
  if(delimiters<tuple_t ch>::values.prefix != 0)
    os ::values.prefix;

  aux::print_tuple(os, t, aux::gen_seq<sizeof...>());

  if(delimiters<tuple_t ch>::values.postfix != 0)
    os ::values.postfix;

  return os;
}

template<class ch class tr tuple std::size_t... is>
void print_tuple(std::basic_ostream<ch tr>& os, Tuple const& t, seq<is...>) {
  using swallow = int[];
  char const* delim = delimiters<tuple ch>::values.delimiter;
  if(!delim) delim = "";
  (void)swallow{0, (void(os (t)), 0)...};
}</tuple></is...></ch></class></tuple_t></sizeof...></tuple_t></args...></ch></args...></class>

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