如何使用可變參數模板和自訂分隔符號漂亮地列印“std::tuple”？

漂亮打印 std::tuple

在上一個問題中，我們為漂亮打印 STL 容器創建了一個優雅的解決方案。現在，讓我們解決 std::tuple 的漂亮列印問題使用可變參數模板。

對於std::pair，我們可以使用自訂運算子進行列印：

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

要將其推廣到元組，我們可以利用模板參數解套件和索引產生：

namespace aux{
template<std::size_t... Is> struct seq{}; // dummy helper type

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...>){ // overload to print tuples
  using swallow = int[]; // silence unused warning
  (void)swallow{0, (void(os << (Is == 0? "" : ", ") << std::get<Is>(t)), 0)...}; 
}
}

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)>()); // call template overload with index helper to print
  return os << ")";
}

新增自訂分隔符號（例如，對於char 和wchar_t)，我們可以使用部分特化：

// 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")" };

並調整運算子

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;

  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...>){ 
  char const* delim = delimiters<Tuple, Ch>::values.delimiter;
  if(!delim) delim = "";
  (void)swallow{0, (void(os << (Is == 0? "" : delim) << std::get<Is>(t)), 0)...};
}

現在，使用自訂分隔符號列印std::tuple 非常簡單：

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

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