如何使用可變參數模板漂亮地列印“std::tuple”？

使用可變參數模板漂亮地打印std::tuple

之前關於漂亮打印STL 容器的討論的後續內容，我們的目標是使用可變參數擴展此功能以包括std::tuple

問題

對於std::pair，我們之前實現了一個用於漂亮打印的自訂運算子重載：

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

目標是為std::tuple 建立類似的構造。

解決方案

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 << ")";
}

額外的通用性

要增強通用性，請為分隔符號處理添加部分專業化：

// 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")" };

整合這些變更進入運算符

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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