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How can you pretty-print a `std::tuple` using variadic templates in C 11?
How can you pretty-print a `std::tuple` using variadic templates in C 11?
Nov 10, 2024 am 12:21 AM
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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