How can you pretty-print a `std::tuple` using variadic templates in C 11?

Pretty-Printing std::tuple

In a previous question on embellishing STL containers, we successfully developed a comprehensive and graceful solution. This next step delves into the realm of pretty-printing std::tuple using variadic templates (exclusively C 11).

Background

For std::pair, the syntax is straightforward:

std::ostream & operator<<(std::ostream &, const std::pair<S, T> &);

Analogous Construction for Tuple

The puzzle lies in defining an analogous construction for printing a tuple. We aim for this behavior:

auto a = std::make_tuple(5, "Hello", -0.1);
std::cout << a << std::endl; // prints: (5, "Hello", -0.1)

Solution

The answer lies in exploiting indices:

namespace aux{
template<std::size_t... Is> struct seq{};

template<std::size_t N, std::size_t... Is>
struct gen_seq : gen_seq<N-1, N-1, Is...>{};

template<std::size_t... Is>
struct gen_seq<0, Is...>: seq<Is...>{};

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: Delimiters

For added flexibility, we provide partial specializations:

// 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")" };

Conclusion

With the above modifications, pretty-printing std::tuple can be customized to accommodate different character sets and delimiter preferences, offering an elegant and versatile solution.

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