X-Git-Url: http://plrg.eecs.uci.edu/git/?p=folly.git;a=blobdiff_plain;f=folly%2FConv.h;h=61e50e3733b316a4254c6bac094962efe2e80e93;hp=d6553373fa0203dfb120466ce42a0b1a50a454c8;hb=3d9eb7ffc32c2f5f878bb4aee61d30cb8c62a146;hpb=ad2f872bf354b62dbde66937da096565ef9ed663

diff --git a/folly/Conv.h b/folly/Conv.h
index d6553373..61e50e37 100644
--- a/folly/Conv.h
+++ b/folly/Conv.h
@@ -1,5 +1,5 @@
 /*
- * Copyright 2016 Facebook, Inc.
+ * Copyright 2011-present Facebook, Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -35,79 +35,92 @@
 #include <typeinfo>
 #include <utility>
 
-#include <boost/implicit_cast.hpp>
 #include <double-conversion/double-conversion.h> // V8 JavaScript implementation
 
 #include <folly/Demangle.h>
+#include <folly/Expected.h>
 #include <folly/FBString.h>
 #include <folly/Likely.h>
 #include <folly/Range.h>
+#include <folly/Traits.h>
+#include <folly/Unit.h>
 #include <folly/portability/Math.h>
 
 namespace folly {
 
-class ConversionError : public std::range_error {
- public:
-  // Keep this in sync with kErrorStrings in Conv.cpp
-  enum Code {
-    SUCCESS,
-    EMPTY_INPUT_STRING,
-    NO_DIGITS,
-    BOOL_OVERFLOW,
-    BOOL_INVALID_VALUE,
-    NON_DIGIT_CHAR,
-    INVALID_LEADING_CHAR,
-    POSITIVE_OVERFLOW,
-    NEGATIVE_OVERFLOW,
-    STRING_TO_FLOAT_ERROR,
-    NON_WHITESPACE_AFTER_END,
-    ARITH_POSITIVE_OVERFLOW,
-    ARITH_NEGATIVE_OVERFLOW,
-    ARITH_LOSS_OF_PRECISION,
-    NUM_ERROR_CODES, // has to be the last entry
-  };
+// Keep this in sync with kErrorStrings in Conv.cpp
+enum class ConversionCode : unsigned char {
+  SUCCESS,
+  EMPTY_INPUT_STRING,
+  NO_DIGITS,
+  BOOL_OVERFLOW,
+  BOOL_INVALID_VALUE,
+  NON_DIGIT_CHAR,
+  INVALID_LEADING_CHAR,
+  POSITIVE_OVERFLOW,
+  NEGATIVE_OVERFLOW,
+  STRING_TO_FLOAT_ERROR,
+  NON_WHITESPACE_AFTER_END,
+  ARITH_POSITIVE_OVERFLOW,
+  ARITH_NEGATIVE_OVERFLOW,
+  ARITH_LOSS_OF_PRECISION,
+  NUM_ERROR_CODES, // has to be the last entry
+};
 
-  ConversionError(const std::string& str, Code code)
-      : std::range_error(str), code_(code) {}
+struct ConversionErrorBase : std::range_error {
+  using std::range_error::range_error;
+};
 
-  ConversionError(const char* str, Code code)
-      : std::range_error(str), code_(code) {}
+class ConversionError : public ConversionErrorBase {
+ public:
+  ConversionError(const std::string& str, ConversionCode code)
+      : ConversionErrorBase(str), code_(code) {}
+
+  ConversionError(const char* str, ConversionCode code)
+      : ConversionErrorBase(str), code_(code) {}
 
-  Code errorCode() const { return code_; }
+  ConversionCode errorCode() const {
+    return code_;
+  }
 
  private:
-  Code code_;
+  ConversionCode code_;
 };
 
-namespace detail {
-
-ConversionError makeConversionError(
-    ConversionError::Code code,
-    const char* input,
-    size_t inputLen);
-
-inline ConversionError makeConversionError(
-    ConversionError::Code code,
-    const std::string& str) {
-  return makeConversionError(code, str.data(), str.size());
-}
-
-inline ConversionError makeConversionError(
-    ConversionError::Code code,
-    StringPiece sp) {
-  return makeConversionError(code, sp.data(), sp.size());
-}
+/*******************************************************************************
+ * Custom Error Translation
+ *
+ * Your overloaded parseTo() function can return a custom error code on failure.
+ * ::folly::to() will call makeConversionError to translate that error code into
+ * an object to throw. makeConversionError is found by argument-dependent
+ * lookup. It should have this signature:
+ *
+ * namespace other_namespace {
+ * enum YourErrorCode { BAD_ERROR, WORSE_ERROR };
+ *
+ * struct YourConversionError : ConversionErrorBase {
+ *   YourConversionError(const char* what) : ConversionErrorBase(what) {}
+ * };
+ *
+ * YourConversionError
+ * makeConversionError(YourErrorCode code, ::folly::StringPiece sp) {
+ *   ...
+ *   return YourConversionError(messageString);
+ * }
+ ******************************************************************************/
+ConversionError makeConversionError(ConversionCode code, StringPiece sp);
 
+namespace detail {
 /**
  * Enforce that the suffix following a number is made up only of whitespace.
  */
-inline ConversionError::Code enforceWhitespaceErr(StringPiece sp) {
+inline ConversionCode enforceWhitespaceErr(StringPiece sp) {
   for (auto c : sp) {
-    if (!std::isspace(c)) {
-      return ConversionError::NON_WHITESPACE_AFTER_END;
+    if (UNLIKELY(!std::isspace(c))) {
+      return ConversionCode::NON_WHITESPACE_AFTER_END;
     }
   }
-  return ConversionError::SUCCESS;
+  return ConversionCode::SUCCESS;
 }
 
 /**
@@ -115,42 +128,57 @@ inline ConversionError::Code enforceWhitespaceErr(StringPiece sp) {
  */
 inline void enforceWhitespace(StringPiece sp) {
   auto err = enforceWhitespaceErr(sp);
-  if (err != ConversionError::SUCCESS) {
-    throw detail::makeConversionError(err, sp);
+  if (err != ConversionCode::SUCCESS) {
+    throw makeConversionError(err, sp);
   }
 }
+} // namespace detail
 
 /**
- * A simple std::pair-like wrapper to wrap both a value and an error
+ * The identity conversion function.
+ * tryTo<T>(T) returns itself for all types T.
  */
-template <typename T>
-struct ConversionResult {
-  explicit ConversionResult(T v) : value(v) {}
-  explicit ConversionResult(ConversionError::Code e) : error(e) {}
-
-  bool success() const {
-    return error == ConversionError::SUCCESS;
-  }
+template <class Tgt, class Src>
+typename std::enable_if<
+    std::is_same<Tgt, typename std::decay<Src>::type>::value,
+    Expected<Tgt, ConversionCode>>::type
+tryTo(Src&& value) {
+  return std::forward<Src>(value);
+}
 
-  T value;
-  ConversionError::Code error{ConversionError::SUCCESS};
-};
+template <class Tgt, class Src>
+typename std::enable_if<
+    std::is_same<Tgt, typename std::decay<Src>::type>::value,
+    Tgt>::type
+to(Src&& value) {
+  return std::forward<Src>(value);
 }
 
+/*******************************************************************************
+ * Arithmetic to boolean
+ ******************************************************************************/
+
 /**
- * The identity conversion function.
- * to<T>(T) returns itself for all types T.
+ * Unchecked conversion from arithmetic to boolean. This is different from the
+ * other arithmetic conversions because we use the C convention of treating any
+ * non-zero value as true, instead of range checking.
  */
 template <class Tgt, class Src>
-typename std::enable_if<std::is_same<Tgt, Src>::value, Tgt>::type
-to(const Src & value) {
-  return value;
+typename std::enable_if<
+    std::is_arithmetic<Src>::value && !std::is_same<Tgt, Src>::value &&
+        std::is_same<Tgt, bool>::value,
+    Expected<Tgt, ConversionCode>>::type
+tryTo(const Src& value) {
+  return value != Src();
 }
 
 template <class Tgt, class Src>
-typename std::enable_if<std::is_same<Tgt, Src>::value, Tgt>::type
-to(Src && value) {
-  return std::forward<Src>(value);
+typename std::enable_if<
+    std::is_arithmetic<Src>::value && !std::is_same<Tgt, Src>::value &&
+        std::is_same<Tgt, bool>::value,
+    Tgt>::type
+to(const Src& value) {
+  return value != Src();
 }
 
 /*******************************************************************************
@@ -159,35 +187,57 @@ to(Src && value) {
 
 namespace detail {
 
-template <class T>
-const T& getLastElement(const T & v) {
-  return v;
-}
+#ifdef _MSC_VER
+// MSVC can't quite figure out the LastElementImpl::call() stuff
+// in the base implementation, so we have to use tuples instead,
+// which result in significantly more templates being compiled,
+// though the runtime performance is the same.
 
-template <class T, class... Ts>
-typename std::tuple_element<
-  sizeof...(Ts),
-  std::tuple<T, Ts...> >::type const&
-  getLastElement(const T&, const Ts&... vs) {
-  return getLastElement(vs...);
+template <typename... Ts>
+auto getLastElement(Ts&&... ts) -> decltype(
+    std::get<sizeof...(Ts)-1>(std::forward_as_tuple(std::forward<Ts>(ts)...))) {
+  return std::get<sizeof...(Ts)-1>(
+      std::forward_as_tuple(std::forward<Ts>(ts)...));
 }
 
-// This class exists to specialize away std::tuple_element in the case where we
-// have 0 template arguments. Without this, Clang/libc++ will blow a
-// static_assert even if tuple_element is protected by an enable_if.
+inline void getLastElement() {}
+
+template <size_t size, typename... Ts>
+struct LastElementType : std::tuple_element<size - 1, std::tuple<Ts...>> {};
+
+template <>
+struct LastElementType<0> {
+  using type = void;
+};
+
 template <class... Ts>
-struct last_element {
-  typedef typename std::enable_if<
-    sizeof...(Ts) >= 1,
-    typename std::tuple_element<
-      sizeof...(Ts) - 1, std::tuple<Ts...>
-    >::type>::type type;
+struct LastElement
+    : std::decay<typename LastElementType<sizeof...(Ts), Ts...>::type> {};
+#else
+template <typename... Ts>
+struct LastElementImpl {
+  static void call(Ignored<Ts>...) {}
 };
 
-template <>
-struct last_element<> {
-  typedef void type;
+template <typename Head, typename... Ts>
+struct LastElementImpl<Head, Ts...> {
+  template <typename Last>
+  static Last call(Ignored<Ts>..., Last&& last) {
+    return std::forward<Last>(last);
+  }
+};
+
+template <typename... Ts>
+auto getLastElement(const Ts&... ts)
+    -> decltype(LastElementImpl<Ts...>::call(ts...)) {
+  return LastElementImpl<Ts...>::call(ts...);
+}
+
+template <class... Ts>
+struct LastElement : std::decay<decltype(
+                         LastElementImpl<Ts...>::call(std::declval<Ts>()...))> {
 };
+#endif
 
 } // namespace detail
 
@@ -232,7 +282,7 @@ unsafeTelescope128(char * buffer, size_t room, unsigned __int128 x) {
   return p;
 }
 
-}
+} // namespace detail
 #endif
 
 /**
@@ -252,7 +302,7 @@ inline uint32_t digits10(uint64_t v) {
   // 10^i, defined for i 0 through 19.
   // This is 20 * 8 == 160 bytes, which fits neatly into 5 cache lines
   // (assuming a cache line size of 64).
-  static const uint64_t powersOf10[20] FOLLY_ALIGNED(64) = {
+  alignas(64) static const uint64_t powersOf10[20] = {
       1,
       10,
       100,
@@ -276,7 +326,7 @@ inline uint32_t digits10(uint64_t v) {
   };
 
   // "count leading zeroes" operation not valid; for 0; special case this.
-  if UNLIKELY (! v) {
+  if (UNLIKELY(!v)) {
     return 1;
   }
 
@@ -291,16 +341,24 @@ inline uint32_t digits10(uint64_t v) {
 
   // return that log_10 lower bound, plus adjust if input >= 10^(that bound)
   // in case there's a small error and we misjudged length.
-  return minLength + (uint32_t) (UNLIKELY (v >= powersOf10[minLength]));
+  return minLength + uint32_t(v >= powersOf10[minLength]);
 
 #else
 
   uint32_t result = 1;
-  for (;;) {
-    if (LIKELY(v < 10)) return result;
-    if (LIKELY(v < 100)) return result + 1;
-    if (LIKELY(v < 1000)) return result + 2;
-    if (LIKELY(v < 10000)) return result + 3;
+  while (true) {
+    if (LIKELY(v < 10)) {
+      return result;
+    }
+    if (LIKELY(v < 100)) {
+      return result + 1;
+    }
+    if (LIKELY(v < 1000)) {
+      return result + 2;
+    }
+    if (LIKELY(v < 10000)) {
+      return result + 3;
+    }
     // Skip ahead by 4 orders of magnitude
     v /= 10000U;
     result += 4;
@@ -332,12 +390,12 @@ inline uint32_t uint64ToBufferUnsafe(uint64_t v, char *const buffer) {
     // Keep these together so a peephole optimization "sees" them and
     // computes them in one shot.
     auto const q = v / 10;
-    auto const r = static_cast<uint32_t>(v % 10);
+    auto const r = static_cast<char>(v % 10);
     buffer[pos--] = '0' + r;
     v = q;
   }
   // Last digit is trivial to handle
-  buffer[pos] = static_cast<uint32_t>(v) + '0';
+  buffer[pos] = static_cast<char>(v) + '0';
   return result;
 }
 
@@ -349,7 +407,7 @@ void toAppend(char value, Tgt * result) {
   *result += value;
 }
 
-template<class T>
+template <class T>
 constexpr typename std::enable_if<
   std::is_same<T, char>::value,
   size_t>::type
@@ -357,13 +415,10 @@ estimateSpaceNeeded(T) {
   return 1;
 }
 
-/**
- * Ubiquitous helper template for writing string appenders
- */
-template <class T> struct IsSomeString {
-  enum { value = std::is_same<T, std::string>::value
-         || std::is_same<T, fbstring>::value };
-};
+template <size_t N>
+constexpr size_t estimateSpaceNeeded(const char (&)[N]) {
+  return N;
+}
 
 /**
  * Everything implicitly convertible to const char* gets appended.
@@ -381,11 +436,10 @@ toAppend(Src value, Tgt * result) {
   }
 }
 
-template<class Src>
-typename std::enable_if<
-  std::is_convertible<Src, const char*>::value,
-  size_t>::type
-estimateSpaceNeeded(Src value) {
+template <class Src>
+typename std::enable_if<std::is_convertible<Src, const char*>::value, size_t>::
+    type
+    estimateSpaceNeeded(Src value) {
   const char *c = value;
   if (c) {
     return folly::StringPiece(value).size();
@@ -393,12 +447,18 @@ estimateSpaceNeeded(Src value) {
   return 0;
 }
 
-template<class Src>
+template <class Src>
+typename std::enable_if<IsSomeString<Src>::value, size_t>::type
+estimateSpaceNeeded(Src const& value) {
+  return value.size();
+}
+
+template <class Src>
 typename std::enable_if<
-  (std::is_convertible<Src, folly::StringPiece>::value ||
-  IsSomeString<Src>::value) &&
-  !std::is_convertible<Src, const char*>::value,
-  size_t>::type
+    std::is_convertible<Src, folly::StringPiece>::value &&
+        !IsSomeString<Src>::value &&
+        !std::is_convertible<Src, const char*>::value,
+    size_t>::type
 estimateSpaceNeeded(Src value) {
   return folly::StringPiece(value).size();
 }
@@ -408,7 +468,7 @@ inline size_t estimateSpaceNeeded(std::nullptr_t /* value */) {
   return 0;
 }
 
-template<class Src>
+template <class Src>
 typename std::enable_if<
   std::is_pointer<Src>::value &&
   IsSomeString<std::remove_pointer<Src>>::value,
@@ -481,7 +541,7 @@ toAppend(unsigned __int128 value, Tgt * result) {
   result->append(buffer + p, buffer + sizeof(buffer));
 }
 
-template<class T>
+template <class T>
 constexpr typename std::enable_if<
   std::is_same<T, __int128>::value,
   size_t>::type
@@ -489,7 +549,7 @@ estimateSpaceNeeded(T) {
   return detail::digitsEnough<__int128>();
 }
 
-template<class T>
+template <class T>
 constexpr typename std::enable_if<
   std::is_same<T, unsigned __int128>::value,
   size_t>::type
@@ -515,9 +575,11 @@ toAppend(Src value, Tgt * result) {
   char buffer[20];
   if (value < 0) {
     result->push_back('-');
-    result->append(buffer, uint64ToBufferUnsafe(-uint64_t(value), buffer));
+    result->append(
+        buffer,
+        uint64ToBufferUnsafe(~static_cast<uint64_t>(value) + 1, buffer));
   } else {
-    result->append(buffer, uint64ToBufferUnsafe(value, buffer));
+    result->append(buffer, uint64ToBufferUnsafe(uint64_t(value), buffer));
   }
 }
 
@@ -612,7 +674,7 @@ estimateSpaceNeeded(Src value) {
 namespace detail {
 constexpr int kConvMaxDecimalInShortestLow = -6;
 constexpr int kConvMaxDecimalInShortestHigh = 21;
-} // folly::detail
+} // namespace detail
 
 /** Wrapper around DoubleToStringConverter **/
 template <class Tgt, class Src>
@@ -639,14 +701,14 @@ toAppend(
       conv.ToShortest(value, &builder);
       break;
     case DoubleToStringConverter::FIXED:
-      conv.ToFixed(value, numDigits, &builder);
+      conv.ToFixed(value, int(numDigits), &builder);
       break;
     default:
       CHECK(mode == DoubleToStringConverter::PRECISION);
-      conv.ToPrecision(value, numDigits, &builder);
+      conv.ToPrecision(value, int(numDigits), &builder);
       break;
   }
-  const size_t length = builder.position();
+  const size_t length = size_t(builder.position());
   builder.Finalize();
   result->append(buffer, length);
 }
@@ -688,7 +750,9 @@ estimateSpaceNeeded(Src value) {
       // so 21 is the longest non-exponential number > 1.
       detail::kConvMaxDecimalInShortestHigh
     });
-  return kMaxPositiveSpace + (value < 0);  // +1 for minus sign, if negative
+  return size_t(
+      kMaxPositiveSpace +
+      (value < 0 ? 1 : 0)); // +1 for minus sign, if negative
 }
 
 /**
@@ -696,13 +760,13 @@ estimateSpaceNeeded(Src value) {
  * for estimateSpaceNeed for your type, so that we allocate
  * as much as you need instead of the default
  */
-template<class Src>
+template <class Src>
 struct HasLengthEstimator : std::false_type {};
 
 template <class Src>
 constexpr typename std::enable_if<
   !std::is_fundamental<Src>::value
-#ifdef FOLLY_HAVE_INT128_T
+#if FOLLY_HAVE_INT128_T
   // On OSX 10.10, is_fundamental<__int128> is false :-O
   && !std::is_same<__int128, Src>::value
   && !std::is_same<unsigned __int128, Src>::value
@@ -730,12 +794,12 @@ size_t estimateSpaceToReserve(size_t sofar, const T& v, const Ts&... vs) {
   return estimateSpaceToReserve(sofar + estimateSpaceNeeded(v), vs...);
 }
 
-template<class...Ts>
+template <class... Ts>
 void reserveInTarget(const Ts&...vs) {
   getLastElement(vs...)->reserve(estimateSpaceToReserve(0, vs...));
 }
 
-template<class Delimiter, class...Ts>
+template <class Delimiter, class... Ts>
 void reserveInTargetDelim(const Delimiter& d, const Ts&...vs) {
   static_assert(sizeof...(vs) >= 2, "Needs at least 2 args");
   size_t fordelim = (sizeof...(vs) - 2) *
@@ -755,11 +819,10 @@ toAppendStrImpl(const T& v, Tgt result) {
 }
 
 template <class T, class... Ts>
-typename std::enable_if<sizeof...(Ts) >= 2
-  && IsSomeString<
-  typename std::remove_pointer<
-    typename detail::last_element<Ts...>::type
-  >::type>::value>::type
+typename std::enable_if<
+    sizeof...(Ts) >= 2 &&
+    IsSomeString<typename std::remove_pointer<
+        typename detail::LastElement<const Ts&...>::type>::type>::value>::type
 toAppendStrImpl(const T& v, const Ts&... vs) {
   toAppend(v, getLastElement(vs...));
   toAppendStrImpl(vs...);
@@ -773,11 +836,10 @@ toAppendDelimStrImpl(const Delimiter& /* delim */, const T& v, Tgt result) {
 }
 
 template <class Delimiter, class T, class... Ts>
-typename std::enable_if<sizeof...(Ts) >= 2
-  && IsSomeString<
-  typename std::remove_pointer<
-    typename detail::last_element<Ts...>::type
-  >::type>::value>::type
+typename std::enable_if<
+    sizeof...(Ts) >= 2 &&
+    IsSomeString<typename std::remove_pointer<
+        typename detail::LastElement<const Ts&...>::type>::type>::value>::type
 toAppendDelimStrImpl(const Delimiter& delim, const T& v, const Ts&... vs) {
   // we are really careful here, calling toAppend with just one element does
   // not try to estimate space needed (as we already did that). If we call
@@ -786,8 +848,7 @@ toAppendDelimStrImpl(const Delimiter& delim, const T& v, const Ts&... vs) {
   toAppend(delim, detail::getLastElement(vs...));
   toAppendDelimStrImpl(delim, vs...);
 }
-} // folly::detail
-
+} // namespace detail
 
 /**
  * Variadic conversion to string. Appends each element in turn.
@@ -811,11 +872,10 @@ toAppendDelimStrImpl(const Delimiter& delim, const T& v, const Ts&... vs) {
  * }
  */
 template <class... Ts>
-typename std::enable_if<sizeof...(Ts) >= 3
-  && IsSomeString<
-  typename std::remove_pointer<
-    typename detail::last_element<Ts...>::type
-  >::type>::value>::type
+typename std::enable_if<
+    sizeof...(Ts) >= 3 &&
+    IsSomeString<typename std::remove_pointer<
+        typename detail::LastElement<const Ts&...>::type>::type>::value>::type
 toAppend(const Ts&... vs) {
   ::folly::detail::toAppendStrImpl(vs...);
 }
@@ -841,11 +901,8 @@ void toAppend(const pid_t a, Tgt* res) {
  * will probably save a few calls to malloc.
  */
 template <class... Ts>
-typename std::enable_if<
-  IsSomeString<
-  typename std::remove_pointer<
-    typename detail::last_element<Ts...>::type
-  >::type>::value>::type
+typename std::enable_if<IsSomeString<typename std::remove_pointer<
+    typename detail::LastElement<const Ts&...>::type>::type>::value>::type
 toAppendFit(const Ts&... vs) {
   ::folly::detail::reserveInTarget(vs...);
   toAppend(vs...);
@@ -882,11 +939,10 @@ typename std::enable_if<IsSomeString<Tgt>::value>::type toAppendDelim(
  * comments for toAppend for details about memory allocation.
  */
 template <class Delimiter, class... Ts>
-typename std::enable_if<sizeof...(Ts) >= 3
-  && IsSomeString<
-  typename std::remove_pointer<
-    typename detail::last_element<Ts...>::type
-  >::type>::value>::type
+typename std::enable_if<
+    sizeof...(Ts) >= 3 &&
+    IsSomeString<typename std::remove_pointer<
+        typename detail::LastElement<const Ts&...>::type>::type>::value>::type
 toAppendDelim(const Delimiter& delim, const Ts&... vs) {
   detail::toAppendDelimStrImpl(delim, vs...);
 }
@@ -895,11 +951,8 @@ toAppendDelim(const Delimiter& delim, const Ts&... vs) {
  * Detail in comment for toAppendFit
  */
 template <class Delimiter, class... Ts>
-typename std::enable_if<
-  IsSomeString<
-  typename std::remove_pointer<
-    typename detail::last_element<Ts...>::type
-  >::type>::value>::type
+typename std::enable_if<IsSomeString<typename std::remove_pointer<
+    typename detail::LastElement<const Ts&...>::type>::type>::value>::type
 toAppendDelimFit(const Delimiter& delim, const Ts&... vs) {
   detail::reserveInTargetDelim(delim, vs...);
   toAppendDelim(delim, vs...);
@@ -914,25 +967,48 @@ void toAppendDelimFit(const De&, const Ts&) {}
  */
 template <class Tgt, class... Ts>
 typename std::enable_if<
-  IsSomeString<Tgt>::value && (
-    sizeof...(Ts) != 1 ||
-    !std::is_same<Tgt, typename detail::last_element<Ts...>::type>::value),
-  Tgt>::type
+    IsSomeString<Tgt>::value &&
+        (sizeof...(Ts) != 1 ||
+         !std::is_same<Tgt, typename detail::LastElement<const Ts&...>::type>::
+             value),
+    Tgt>::type
 to(const Ts&... vs) {
   Tgt result;
   toAppendFit(vs..., &result);
   return result;
 }
 
+/**
+ * Special version of to<SomeString> for floating point. When calling
+ * folly::to<SomeString>(double), generic implementation above will
+ * firstly reserve 24 (or 25 when negative value) bytes. This will
+ * introduce a malloc call for most mainstream string implementations.
+ *
+ * But for most cases, a floating point doesn't need 24 (or 25) bytes to
+ * be converted as a string.
+ *
+ * This special version will not do string reserve.
+ */
+template <class Tgt, class Src>
+typename std::enable_if<
+    IsSomeString<Tgt>::value && std::is_floating_point<Src>::value,
+    Tgt>::type
+to(Src value) {
+  Tgt result;
+  toAppend(value, &result);
+  return result;
+}
+
 /**
  * toDelim<SomeString>(SomeString str) returns itself.
  */
 template <class Tgt, class Delim, class Src>
-typename std::enable_if<IsSomeString<Tgt>::value &&
-                            std::is_same<Tgt, Src>::value,
-                        Tgt>::type
-toDelim(const Delim& /* delim */, const Src& value) {
-  return value;
+typename std::enable_if<
+    IsSomeString<Tgt>::value &&
+        std::is_same<Tgt, typename std::decay<Src>::type>::value,
+    Tgt>::type
+toDelim(const Delim& /* delim */, Src&& value) {
+  return std::forward<Src>(value);
 }
 
 /**
@@ -941,10 +1017,11 @@ toDelim(const Delim& /* delim */, const Src& value) {
  */
 template <class Tgt, class Delim, class... Ts>
 typename std::enable_if<
-  IsSomeString<Tgt>::value && (
-    sizeof...(Ts) != 1 ||
-    !std::is_same<Tgt, typename detail::last_element<Ts...>::type>::value),
-  Tgt>::type
+    IsSomeString<Tgt>::value &&
+        (sizeof...(Ts) != 1 ||
+         !std::is_same<Tgt, typename detail::LastElement<const Ts&...>::type>::
+             value),
+    Tgt>::type
 toDelim(const Delim& delim, const Ts&... vs) {
   Tgt result;
   toAppendDelimFit(delim, vs..., &result);
@@ -957,130 +1034,121 @@ toDelim(const Delim& delim, const Ts&... vs) {
 
 namespace detail {
 
-ConversionResult<bool> str_to_bool(StringPiece* src);
+Expected<bool, ConversionCode> str_to_bool(StringPiece* src) noexcept;
 
 template <typename T>
-ConversionResult<T> str_to_floating(StringPiece* src);
+Expected<T, ConversionCode> str_to_floating(StringPiece* src) noexcept;
 
-extern template ConversionResult<float> str_to_floating<float>(
-    StringPiece* src);
-extern template ConversionResult<double> str_to_floating<double>(
-    StringPiece* src);
+extern template Expected<float, ConversionCode> str_to_floating<float>(
+    StringPiece* src) noexcept;
+extern template Expected<double, ConversionCode> str_to_floating<double>(
+    StringPiece* src) noexcept;
 
 template <class Tgt>
-ConversionResult<Tgt> digits_to(const char* b, const char* e);
+Expected<Tgt, ConversionCode> digits_to(const char* b, const char* e) noexcept;
 
-extern template ConversionResult<char> digits_to<char>(
-    const char*,
-    const char*);
-extern template ConversionResult<signed char> digits_to<signed char>(
+extern template Expected<char, ConversionCode> digits_to<char>(
     const char*,
-    const char*);
-extern template ConversionResult<unsigned char> digits_to<unsigned char>(
+    const char*) noexcept;
+extern template Expected<signed char, ConversionCode> digits_to<signed char>(
     const char*,
-    const char*);
+    const char*) noexcept;
+extern template Expected<unsigned char, ConversionCode>
+digits_to<unsigned char>(const char*, const char*) noexcept;
 
-extern template ConversionResult<short> digits_to<short>(
-    const char*,
-    const char*);
-extern template ConversionResult<unsigned short> digits_to<unsigned short>(
+extern template Expected<short, ConversionCode> digits_to<short>(
     const char*,
-    const char*);
+    const char*) noexcept;
+extern template Expected<unsigned short, ConversionCode>
+digits_to<unsigned short>(const char*, const char*) noexcept;
 
-extern template ConversionResult<int> digits_to<int>(const char*, const char*);
-extern template ConversionResult<unsigned int> digits_to<unsigned int>(
+extern template Expected<int, ConversionCode> digits_to<int>(
     const char*,
-    const char*);
-
-extern template ConversionResult<long> digits_to<long>(
+    const char*) noexcept;
+extern template Expected<unsigned int, ConversionCode> digits_to<unsigned int>(
     const char*,
-    const char*);
-extern template ConversionResult<unsigned long> digits_to<unsigned long>(
+    const char*) noexcept;
+
+extern template Expected<long, ConversionCode> digits_to<long>(
     const char*,
-    const char*);
+    const char*) noexcept;
+extern template Expected<unsigned long, ConversionCode>
+digits_to<unsigned long>(const char*, const char*) noexcept;
 
-extern template ConversionResult<long long> digits_to<long long>(
+extern template Expected<long long, ConversionCode> digits_to<long long>(
     const char*,
-    const char*);
-extern template ConversionResult<unsigned long long>
-digits_to<unsigned long long>(const char*, const char*);
+    const char*) noexcept;
+extern template Expected<unsigned long long, ConversionCode>
+digits_to<unsigned long long>(const char*, const char*) noexcept;
 
 #if FOLLY_HAVE_INT128_T
-extern template ConversionResult<__int128> digits_to<__int128>(
+extern template Expected<__int128, ConversionCode> digits_to<__int128>(
     const char*,
-    const char*);
-extern template ConversionResult<unsigned __int128>
-digits_to<unsigned __int128>(const char*, const char*);
+    const char*) noexcept;
+extern template Expected<unsigned __int128, ConversionCode>
+digits_to<unsigned __int128>(const char*, const char*) noexcept;
 #endif
 
 template <class T>
-ConversionResult<T> str_to_integral(StringPiece* src);
-
-extern template ConversionResult<char> str_to_integral<char>(StringPiece* src);
-extern template ConversionResult<signed char> str_to_integral<signed char>(
-    StringPiece* src);
-extern template ConversionResult<unsigned char> str_to_integral<unsigned char>(
-    StringPiece* src);
-
-extern template ConversionResult<short> str_to_integral<short>(
-    StringPiece* src);
-extern template ConversionResult<unsigned short>
-str_to_integral<unsigned short>(StringPiece* src);
-
-extern template ConversionResult<int> str_to_integral<int>(StringPiece* src);
-extern template ConversionResult<unsigned int> str_to_integral<unsigned int>(
-    StringPiece* src);
-
-extern template ConversionResult<long> str_to_integral<long>(StringPiece* src);
-extern template ConversionResult<unsigned long> str_to_integral<unsigned long>(
-    StringPiece* src);
-
-extern template ConversionResult<long long> str_to_integral<long long>(
-    StringPiece* src);
-extern template ConversionResult<unsigned long long>
-str_to_integral<unsigned long long>(StringPiece* src);
+Expected<T, ConversionCode> str_to_integral(StringPiece* src) noexcept;
+
+extern template Expected<char, ConversionCode> str_to_integral<char>(
+    StringPiece* src) noexcept;
+extern template Expected<signed char, ConversionCode>
+str_to_integral<signed char>(StringPiece* src) noexcept;
+extern template Expected<unsigned char, ConversionCode>
+str_to_integral<unsigned char>(StringPiece* src) noexcept;
+
+extern template Expected<short, ConversionCode> str_to_integral<short>(
+    StringPiece* src) noexcept;
+extern template Expected<unsigned short, ConversionCode>
+str_to_integral<unsigned short>(StringPiece* src) noexcept;
+
+extern template Expected<int, ConversionCode> str_to_integral<int>(
+    StringPiece* src) noexcept;
+extern template Expected<unsigned int, ConversionCode>
+str_to_integral<unsigned int>(StringPiece* src) noexcept;
+
+extern template Expected<long, ConversionCode> str_to_integral<long>(
+    StringPiece* src) noexcept;
+extern template Expected<unsigned long, ConversionCode>
+str_to_integral<unsigned long>(StringPiece* src) noexcept;
+
+extern template Expected<long long, ConversionCode> str_to_integral<long long>(
+    StringPiece* src) noexcept;
+extern template Expected<unsigned long long, ConversionCode>
+str_to_integral<unsigned long long>(StringPiece* src) noexcept;
 
 #if FOLLY_HAVE_INT128_T
-extern template ConversionResult<__int128> str_to_integral<__int128>(
-    StringPiece* src);
-extern template ConversionResult<unsigned __int128>
-str_to_integral<unsigned __int128>(StringPiece* src);
+extern template Expected<__int128, ConversionCode> str_to_integral<__int128>(
+    StringPiece* src) noexcept;
+extern template Expected<unsigned __int128, ConversionCode>
+str_to_integral<unsigned __int128>(StringPiece* src) noexcept;
 #endif
 
 template <typename T>
-typename std::enable_if<std::is_same<T, bool>::value, ConversionResult<T>>::type
-convertTo(StringPiece* src) {
+typename std::
+    enable_if<std::is_same<T, bool>::value, Expected<T, ConversionCode>>::type
+    convertTo(StringPiece* src) noexcept {
   return str_to_bool(src);
 }
 
 template <typename T>
 typename std::enable_if<
-    std::is_floating_point<T>::value, ConversionResult<T>>::type
-convertTo(StringPiece* src) {
+    std::is_floating_point<T>::value,
+    Expected<T, ConversionCode>>::type
+convertTo(StringPiece* src) noexcept {
   return str_to_floating<T>(src);
 }
 
 template <typename T>
 typename std::enable_if<
     std::is_integral<T>::value && !std::is_same<T, bool>::value,
-    ConversionResult<T>>::type
-convertTo(StringPiece* src) {
+    Expected<T, ConversionCode>>::type
+convertTo(StringPiece* src) noexcept {
   return str_to_integral<T>(src);
 }
 
-template <typename T>
-struct WrapperInfo { using type = T; };
-
-template <typename T, typename Gen>
-typename std::enable_if<
-    std::is_same<typename WrapperInfo<T>::type, T>::value, T>::type
-inline wrap(ConversionResult<typename WrapperInfo<T>::type> res, Gen&& gen) {
-  if (LIKELY(res.success())) {
-    return res.value;
-  }
-  throw detail::makeConversionError(res.error, gen());
-}
-
 } // namespace detail
 
 /**
@@ -1089,12 +1157,22 @@ inline wrap(ConversionResult<typename WrapperInfo<T>::type> res, Gen&& gen) {
  */
 template <typename Tgt>
 typename std::enable_if<
-    std::is_integral<typename detail::WrapperInfo<Tgt>::type>::value &&
-    !std::is_same<typename detail::WrapperInfo<Tgt>::type, bool>::value,
+    std::is_integral<Tgt>::value && !std::is_same<Tgt, bool>::value,
+    Expected<Tgt, ConversionCode>>::type
+tryTo(const char* b, const char* e) {
+  return detail::digits_to<Tgt>(b, e);
+}
+
+template <typename Tgt>
+typename std::enable_if<
+    std::is_integral<Tgt>::value && !std::is_same<Tgt, bool>::value,
     Tgt>::type
 to(const char* b, const char* e) {
-  auto res = detail::digits_to<typename detail::WrapperInfo<Tgt>::type>(b, e);
-  return detail::wrap<Tgt>(res, [&] { return StringPiece(b, e); });
+  return tryTo<Tgt>(b, e).thenOrThrow(
+      [](Tgt res) { return res; },
+      [=](ConversionCode code) {
+        return makeConversionError(code, StringPiece(b, e));
+      });
 }
 
 /*******************************************************************************
@@ -1102,49 +1180,37 @@ to(const char* b, const char* e) {
  ******************************************************************************/
 
 /**
- * Parsing strings to numeric types. These routines differ from
- * parseTo(str, numeric) routines in that they take a POINTER TO a StringPiece
- * and alter that StringPiece to reflect progress information.
+ * Parsing strings to numeric types.
  */
 template <typename Tgt>
-typename std::enable_if<
-    std::is_arithmetic<typename detail::WrapperInfo<Tgt>::type>::value>::type
-parseTo(StringPiece* src, Tgt& out) {
-  auto res = detail::convertTo<typename detail::WrapperInfo<Tgt>::type>(src);
-  out = detail::wrap<Tgt>(res, [&] { return *src; });
-}
-
-template <typename Tgt>
-typename std::enable_if<
-    std::is_arithmetic<typename detail::WrapperInfo<Tgt>::type>::value>::type
+FOLLY_NODISCARD inline typename std::enable_if<
+    std::is_arithmetic<Tgt>::value,
+    Expected<StringPiece, ConversionCode>>::type
 parseTo(StringPiece src, Tgt& out) {
-  auto res = detail::convertTo<typename detail::WrapperInfo<Tgt>::type>(&src);
-  if (LIKELY(res.success())) {
-    res.error = detail::enforceWhitespaceErr(src);
-  }
-  out = detail::wrap<Tgt>(res, [&] { return src; });
+  return detail::convertTo<Tgt>(&src).then(
+      [&](Tgt res) { return void(out = res), src; });
 }
 
 /*******************************************************************************
  * Integral / Floating Point to integral / Floating Point
  ******************************************************************************/
 
+namespace detail {
+
 /**
- * Unchecked conversion from arithmetic to boolean. This is different from the
- * other arithmetic conversions because we use the C convention of treating any
- * non-zero value as true, instead of range checking.
+ * Bool to integral/float doesn't need any special checks, and this
+ * overload means we aren't trying to see if a bool is less than
+ * an integer.
  */
-template <class Tgt, class Src>
+template <class Tgt>
 typename std::enable_if<
-    std::is_arithmetic<Src>::value && !std::is_same<Tgt, Src>::value &&
-        std::is_same<Tgt, bool>::value,
-    Tgt>::type
-to(const Src& value) {
-  return value != Src();
+    !std::is_same<Tgt, bool>::value &&
+        (std::is_integral<Tgt>::value || std::is_floating_point<Tgt>::value),
+    Expected<Tgt, ConversionCode>>::type
+convertTo(const bool& value) noexcept {
+  return static_cast<Tgt>(value ? 1 : 0);
 }
 
-namespace detail {
-
 /**
  * Checked conversion from integral to integral. The checks are only
  * performed when meaningful, e.g. conversion from int to long goes
@@ -1155,22 +1221,23 @@ typename std::enable_if<
     std::is_integral<Src>::value && !std::is_same<Tgt, Src>::value &&
         !std::is_same<Tgt, bool>::value &&
         std::is_integral<Tgt>::value,
-    ConversionResult<Tgt>>::type
-convertTo(const Src& value) {
+    Expected<Tgt, ConversionCode>>::type
+convertTo(const Src& value) noexcept {
   /* static */ if (
-      std::numeric_limits<Tgt>::max() < std::numeric_limits<Src>::max()) {
+      folly::_t<std::make_unsigned<Tgt>>(std::numeric_limits<Tgt>::max()) <
+      folly::_t<std::make_unsigned<Src>>(std::numeric_limits<Src>::max())) {
     if (greater_than<Tgt, std::numeric_limits<Tgt>::max()>(value)) {
-      return ConversionResult<Tgt>(ConversionError::ARITH_POSITIVE_OVERFLOW);
+      return makeUnexpected(ConversionCode::ARITH_POSITIVE_OVERFLOW);
     }
   }
   /* static */ if (
       std::is_signed<Src>::value &&
       (!std::is_signed<Tgt>::value || sizeof(Src) > sizeof(Tgt))) {
     if (less_than<Tgt, std::numeric_limits<Tgt>::min()>(value)) {
-      return ConversionResult<Tgt>(ConversionError::ARITH_NEGATIVE_OVERFLOW);
+      return makeUnexpected(ConversionCode::ARITH_NEGATIVE_OVERFLOW);
     }
   }
-  return ConversionResult<Tgt>(static_cast<Tgt>(value));
+  return static_cast<Tgt>(value);
 }
 
 /**
@@ -1182,18 +1249,18 @@ template <class Tgt, class Src>
 typename std::enable_if<
     std::is_floating_point<Tgt>::value && std::is_floating_point<Src>::value &&
         !std::is_same<Tgt, Src>::value,
-    ConversionResult<Tgt>>::type
-convertTo(const Src& value) {
+    Expected<Tgt, ConversionCode>>::type
+convertTo(const Src& value) noexcept {
   /* static */ if (
       std::numeric_limits<Tgt>::max() < std::numeric_limits<Src>::max()) {
     if (value > std::numeric_limits<Tgt>::max()) {
-      return ConversionResult<Tgt>(ConversionError::ARITH_POSITIVE_OVERFLOW);
+      return makeUnexpected(ConversionCode::ARITH_POSITIVE_OVERFLOW);
     }
     if (value < std::numeric_limits<Tgt>::lowest()) {
-      return ConversionResult<Tgt>(ConversionError::ARITH_NEGATIVE_OVERFLOW);
+      return makeUnexpected(ConversionCode::ARITH_NEGATIVE_OVERFLOW);
     }
   }
-  return ConversionResult<Tgt>(boost::implicit_cast<Tgt>(value));
+  return static_cast<Tgt>(value);
 }
 
 /**
@@ -1260,18 +1327,18 @@ template <typename Tgt, typename Src>
 typename std::enable_if<
     (std::is_integral<Src>::value && std::is_floating_point<Tgt>::value) ||
         (std::is_floating_point<Src>::value && std::is_integral<Tgt>::value),
-    ConversionResult<Tgt>>::type
-convertTo(const Src& value) {
+    Expected<Tgt, ConversionCode>>::type
+convertTo(const Src& value) noexcept {
   if (LIKELY(checkConversion<Tgt>(value))) {
     Tgt result = static_cast<Tgt>(value);
     if (LIKELY(checkConversion<Src>(result))) {
       Src witness = static_cast<Src>(result);
       if (LIKELY(value == witness)) {
-        return ConversionResult<Tgt>(result);
+        return result;
       }
     }
   }
-  return ConversionResult<Tgt>(ConversionError::ARITH_LOSS_OF_PRECISION);
+  return makeUnexpected(ConversionCode::ARITH_LOSS_OF_PRECISION);
 }
 
 template <typename Tgt, typename Src>
@@ -1294,13 +1361,20 @@ using IsArithToArith = std::integral_constant<
 
 template <typename Tgt, typename Src>
 typename std::enable_if<
-    detail::IsArithToArith<
-        typename detail::WrapperInfo<Tgt>::type, Src>::value, Tgt>::type
-to(const Src& value) {
-  auto res = detail::convertTo<typename detail::WrapperInfo<Tgt>::type>(value);
-  return detail::wrap<Tgt>(res, [&] {
-    return detail::errorValue<typename detail::WrapperInfo<Tgt>::type>(value);
-  });
+    detail::IsArithToArith<Tgt, Src>::value,
+    Expected<Tgt, ConversionCode>>::type
+tryTo(const Src& value) noexcept {
+  return detail::convertTo<Tgt>(value);
+}
+
+template <typename Tgt, typename Src>
+typename std::enable_if<detail::IsArithToArith<Tgt, Src>::value, Tgt>::type to(
+    const Src& value) {
+  return tryTo<Tgt>(value).thenOrThrow(
+      [](Tgt res) { return res; },
+      [&](ConversionCode e) {
+        return makeConversionError(e, detail::errorValue<Tgt>(value));
+      });
 }
 
 /*******************************************************************************
@@ -1311,49 +1385,166 @@ to(const Src& value) {
  * argument-dependent lookup:
  *
  * namespace other_namespace {
- * void parseTo(::folly::StringPiece, OtherType&);
+ * ::folly::Expected<::folly::StringPiece, SomeErrorCode>
+ *   parseTo(::folly::StringPiece, OtherType&) noexcept;
  * }
  ******************************************************************************/
 template <class T>
-typename std::enable_if<std::is_enum<T>::value>::type
-parseTo(StringPiece in, T& out) {
-  typename std::underlying_type<T>::type tmp;
-  parseTo(in, tmp);
-  out = static_cast<T>(tmp);
-}
-
-inline void parseTo(StringPiece in, StringPiece& out) {
+FOLLY_NODISCARD typename std::enable_if<
+    std::is_enum<T>::value,
+    Expected<StringPiece, ConversionCode>>::type
+parseTo(StringPiece in, T& out) noexcept {
+  typename std::underlying_type<T>::type tmp{};
+  auto restOrError = parseTo(in, tmp);
+  out = static_cast<T>(tmp); // Harmless if parseTo fails
+  return restOrError;
+}
+
+FOLLY_NODISCARD
+inline Expected<StringPiece, ConversionCode> parseTo(
+    StringPiece in,
+    StringPiece& out) noexcept {
   out = in;
+  return StringPiece{in.end(), in.end()};
 }
 
-inline void parseTo(StringPiece in, std::string& out) {
+FOLLY_NODISCARD
+inline Expected<StringPiece, ConversionCode> parseTo(
+    StringPiece in,
+    std::string& out) {
   out.clear();
-  out.append(in.data(), in.size());
+  out.append(in.data(), in.size()); // TODO try/catch?
+  return StringPiece{in.end(), in.end()};
 }
 
-inline void parseTo(StringPiece in, fbstring& out) {
+FOLLY_NODISCARD
+inline Expected<StringPiece, ConversionCode> parseTo(
+    StringPiece in,
+    fbstring& out) {
   out.clear();
-  out.append(in.data(), in.size());
+  out.append(in.data(), in.size()); // TODO try/catch?
+  return StringPiece{in.end(), in.end()};
+}
+
+namespace detail {
+template <typename Tgt>
+using ParseToResult = decltype(parseTo(StringPiece{}, std::declval<Tgt&>()));
+
+struct CheckTrailingSpace {
+  Expected<Unit, ConversionCode> operator()(StringPiece sp) const {
+    auto e = enforceWhitespaceErr(sp);
+    if (UNLIKELY(e != ConversionCode::SUCCESS)) {
+      return makeUnexpected(e);
+    }
+    return unit;
+  }
+};
+
+template <class Error>
+struct ReturnUnit {
+  template <class T>
+  constexpr Expected<Unit, Error> operator()(T&&) const {
+    return unit;
+  }
+};
+
+// Older versions of the parseTo customization point threw on error and
+// returned void. Handle that.
+template <class Tgt>
+inline typename std::enable_if<
+    std::is_void<ParseToResult<Tgt>>::value,
+    Expected<StringPiece, ConversionCode>>::type
+parseToWrap(StringPiece sp, Tgt& out) {
+  parseTo(sp, out);
+  return StringPiece(sp.end(), sp.end());
+}
+
+template <class Tgt>
+inline typename std::enable_if<
+    !std::is_void<ParseToResult<Tgt>>::value,
+    ParseToResult<Tgt>>::type
+parseToWrap(StringPiece sp, Tgt& out) {
+  return parseTo(sp, out);
 }
 
+template <typename Tgt>
+using ParseToError = ExpectedErrorType<decltype(
+    detail::parseToWrap(StringPiece{}, std::declval<Tgt&>()))>;
+
+} // namespace detail
+
 /**
  * String or StringPiece to target conversion. Accepts leading and trailing
  * whitespace, but no non-space trailing characters.
  */
 
 template <class Tgt>
-typename std::enable_if<!std::is_same<StringPiece, Tgt>::value, Tgt>::type
-to(StringPiece src) {
+inline typename std::enable_if<
+    !std::is_same<StringPiece, Tgt>::value,
+    Expected<Tgt, detail::ParseToError<Tgt>>>::type
+tryTo(StringPiece src) {
+  Tgt result{};
+  using Error = detail::ParseToError<Tgt>;
+  using Check = typename std::conditional<
+      std::is_arithmetic<Tgt>::value,
+      detail::CheckTrailingSpace,
+      detail::ReturnUnit<Error>>::type;
+  return parseTo(src, result).then(Check(), [&](Unit) {
+    return std::move(result);
+  });
+}
+
+template <class Tgt, class Src>
+inline typename std::enable_if<
+    IsSomeString<Src>::value && !std::is_same<StringPiece, Tgt>::value,
+    Tgt>::type
+to(Src const& src) {
+  return to<Tgt>(StringPiece(src.data(), src.size()));
+}
+
+template <class Tgt>
+inline
+    typename std::enable_if<!std::is_same<StringPiece, Tgt>::value, Tgt>::type
+    to(StringPiece src) {
+  Tgt result{};
+  using Error = detail::ParseToError<Tgt>;
+  using Check = typename std::conditional<
+      std::is_arithmetic<Tgt>::value,
+      detail::CheckTrailingSpace,
+      detail::ReturnUnit<Error>>::type;
+  auto tmp = detail::parseToWrap(src, result);
+  return tmp
+      .thenOrThrow(Check(), [&](Error e) { throw makeConversionError(e, src); })
+      .thenOrThrow(
+          [&](Unit) { return std::move(result); },
+          [&](Error e) { throw makeConversionError(e, tmp.value()); });
+}
+
+/**
+ * tryTo/to that take the strings by pointer so the caller gets information
+ * about how much of the string was consumed by the conversion. These do not
+ * check for trailing whitepsace.
+ */
+template <class Tgt>
+Expected<Tgt, detail::ParseToError<Tgt>> tryTo(StringPiece* src) {
   Tgt result;
-  parseTo(src, result);
-  return result;
+  return parseTo(*src, result).then([&, src](StringPiece sp) -> Tgt {
+    *src = sp;
+    return std::move(result);
+  });
 }
 
 template <class Tgt>
 Tgt to(StringPiece* src) {
-  Tgt result;
-  parseTo(src, result);
-  return result;
+  Tgt result{};
+  using Error = detail::ParseToError<Tgt>;
+  return parseTo(*src, result)
+      .thenOrThrow(
+          [&, src](StringPiece sp) -> Tgt {
+            *src = sp;
+            return std::move(result);
+          },
+          [=](Error e) { return makeConversionError(e, *src); });
 }
 
 /*******************************************************************************
@@ -1362,15 +1553,39 @@ Tgt to(StringPiece* src) {
 
 template <class Tgt, class Src>
 typename std::enable_if<
-  std::is_enum<Src>::value && !std::is_same<Src, Tgt>::value, Tgt>::type
-to(const Src & value) {
+    std::is_enum<Src>::value && !std::is_same<Src, Tgt>::value &&
+        !std::is_convertible<Tgt, StringPiece>::value,
+    Expected<Tgt, ConversionCode>>::type
+tryTo(const Src& value) {
+  using I = typename std::underlying_type<Src>::type;
+  return tryTo<Tgt>(static_cast<I>(value));
+}
+
+template <class Tgt, class Src>
+typename std::enable_if<
+    !std::is_convertible<Src, StringPiece>::valuea &&
+        std::is_enum<Tgt>::value && !std::is_same<Src, Tgt>::value,
+    Expected<Tgt, ConversionCode>>::type
+tryTo(const Src& value) {
+  using I = typename std::underlying_type<Tgt>::type;
+  return tryTo<I>(value).then([](I i) { return static_cast<Tgt>(i); });
+}
+
+template <class Tgt, class Src>
+typename std::enable_if<
+    std::is_enum<Src>::value && !std::is_same<Src, Tgt>::value &&
+        !std::is_convertible<Tgt, StringPiece>::value,
+    Tgt>::type
+to(const Src& value) {
   return to<Tgt>(static_cast<typename std::underlying_type<Src>::type>(value));
 }
 
 template <class Tgt, class Src>
 typename std::enable_if<
-  std::is_enum<Tgt>::value && !std::is_same<Src, Tgt>::value, Tgt>::type
-to(const Src & value) {
+    !std::is_convertible<Src, StringPiece>::value && std::is_enum<Tgt>::value &&
+        !std::is_same<Src, Tgt>::value,
+    Tgt>::type
+to(const Src& value) {
   return static_cast<Tgt>(to<typename std::underlying_type<Tgt>::type>(value));
 }