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<<(std::ostream &o, const std::pair<S,T> &p) {
  return o << "(" << p.first << ", " << p.second << ")";
}

The goal is to create a similar construction for std::tuple.

The Solution

namespace aux {
// Index sequence utility
template<std::size_t...> struct seq {};
template<std::size_t N, std::size_t... Is>
struct gen_seq : gen_seq<N-1, N-1, Is...> {};
template<> struct gen_seq<0> : seq<> {};

// Recursive printing function using indices
template<class Ch, class Tr, class Tuple, std::size_t... Is>
void print_tuple(std::basic_ostream<Ch,Tr>& os, Tuple const& t, seq<Is...>) {
  using swallow = int[];
  (void)swallow{0, (void(os << (Is == 0? "" : ", ") << std::get<Is>(t)), 0)...};
}
} // aux::

template<class Ch, class Tr, class... Args>
auto operator<<(std::basic_ostream<Ch, Tr>& os, std::tuple<Args...> const& t)
    -> std::basic_ostream<Ch, Tr>&
{
  os << "(";
  aux::print_tuple(os, t, aux::gen_seq<sizeof...(Args)>());
  return os << ")";
}

Bonus Generality

To enhance generality, add partial specializations for delimiter handling:

// Delimiters for tuple
template<class... Args>
struct delimiters<std::tuple<Args...>, char> {
  static const delimiters_values<char> values;
};
template<class... Args>
const delimiters_values<char> delimiters<std::tuple<Args...>, char>::values = { "(", ", ", ")" };

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

Integrate these changes into operator<< and print_tuple:

template<class Ch, class Tr, class... Args>
auto operator<<(std::basic_ostream<Ch, Tr>& 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 << delimiters<tuple_t,char>::values.prefix;

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

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

  return os;
}

template<class Ch, class Tr, class 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 << (Is == 0? "" : delim) << std::get<Is>(t)), 0)...};
}

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