[GitHub] [arrow] westonpace commented on a diff in pull request #14666: ARROW-18350: [C++] Use std::to_chars instead of std::to_string

[GitBox <gi...@apache.org>]

westonpace commented on code in PR #14666:
URL: https://github.com/apache/arrow/pull/14666#discussion_r1025684123


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cpp/src/arrow/util/string.h:
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@@ -97,5 +100,31 @@ std::optional<std::string> Replace(std::string_view s, std::string_view token,
 ARROW_EXPORT
 arrow::Result<bool> ParseBoolean(std::string_view value);
 
+/// \brief An ergonomic wrapper around std::to_chars, returning a std::string
+///
+/// For most inputs, the std::string result will not incur any heap allocation
+/// thanks to small string optimization.
+///
+/// Compared to std::to_string, this function gives locale-agnostic results
+/// and might also be faster.
+template <typename T, typename... Args>
+std::string ToChars(T value, Args&&... args) {
+  // According to various sources, the GNU libstdc++ and Microsoft's C++ STL
+  // allow up to 15 bytes of small string optimization, while clang's libc++
+  // goes up to 22 bytes. Choose the pessimistic value.
+  std::string out(15, 0);
+  auto res = std::to_chars(&out.front(), &out.back(), value, args...);
+  while (res.ec != std::errc{}) {
+    assert(res.ec == std::errc::value_too_large);
+    out.resize(out.capacity() * 2);
+    res = std::to_chars(&out.front(), &out.back(), value, args...);
+  }
+  const auto length = res.ptr - out.data();
+  assert(length <= static_cast<int64_t>(out.length()));
+  out[length] = 0;

Review Comment:
   I believe this is redundant: https://godbolt.org/z/6vcW1jEfh
   
   The call to resize *should* set `out[length]` to 0 so that `std::string` can fulfill its requirements for `c_str`.
   
   That being said, it should be harmless and I imagine would optimize out.



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