X-Git-Url: http://plrg.eecs.uci.edu/git/?p=folly.git;a=blobdiff_plain;f=folly%2FConv.h;h=72518d808f5ad895ad04c33eadb5a87a711f5cc9;hp=de369c0a29c3cb0d10b5d980a0650ee6729095ec;hb=7b116ffe3b116ffc3c2089ca0b864ca7ebb1d28c;hpb=4f0bc5272f90bfcb7a18e6a1a4359a202a1b8cf9

diff --git a/folly/Conv.h b/folly/Conv.h
index de369c0a..72518d80 100644
--- a/folly/Conv.h
+++ b/folly/Conv.h
@@ -1,5 +1,5 @@
 /*
- * Copyright 2012 Facebook, Inc.
+ * Copyright 2017 Facebook, Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -21,75 +21,165 @@
  * @author Andrei Alexandrescu (andrei.alexandrescu@fb.com)
  */
 
-#ifndef FOLLY_BASE_CONV_H_
-#define FOLLY_BASE_CONV_H_
+#pragma once
 
-#include "folly/FBString.h"
-#include "folly/Likely.h"
-#include "folly/Preprocessor.h"
-#include "folly/Range.h"
-
-#include <boost/implicit_cast.hpp>
-#include <type_traits>
+#include <algorithm>
+#include <cctype>
+#include <climits>
+#include <cstddef>
 #include <limits>
+#include <stdexcept>
 #include <string>
 #include <tuple>
-#include <stdexcept>
+#include <type_traits>
 #include <typeinfo>
+#include <utility>
 
-#include "double-conversion.h"   // V8 JavaScript implementation
+#include <boost/implicit_cast.hpp>
+#include <double-conversion/double-conversion.h> // V8 JavaScript implementation
 
-#define FOLLY_RANGE_CHECK(condition, message)                           \
-  ((condition) ? (void)0 : throw std::range_error(                      \
-    (__FILE__ "(" + std::to_string((long long int) __LINE__) + "): "    \
-     + (message)).c_str()))
+#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 {
 
+// 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
+};
+
+struct ConversionErrorBase : std::range_error {
+  using std::range_error::range_error;
+};
+
+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) {}
+
+  ConversionCode errorCode() const {
+    return code_;
+  }
+
+ private:
+  ConversionCode code_;
+};
+
 /*******************************************************************************
- * Integral to integral
+ * 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 {
 /**
- * Checked conversion from integral to integral. The checks are only
- * performed when meaningful, e.g. conversion from int to long goes
- * unchecked.
+ * Enforce that the suffix following a number is made up only of whitespace.
  */
-template <class Tgt, class Src>
-typename std::enable_if<
-  std::is_integral<Src>::value && std::is_integral<Tgt>::value,
-  Tgt>::type
-to(const Src & value) {
-  /* static */ if (std::numeric_limits<Tgt>::max()
-                   < std::numeric_limits<Src>::max()) {
-    FOLLY_RANGE_CHECK(value <= std::numeric_limits<Tgt>::max(),
-                      "Overflow");
+inline ConversionCode enforceWhitespaceErr(StringPiece sp) {
+  for (auto c : sp) {
+    if (UNLIKELY(!std::isspace(c))) {
+      return ConversionCode::NON_WHITESPACE_AFTER_END;
+    }
   }
-  /* static */ if (std::is_signed<Src>::value &&
-                   (!std::is_signed<Tgt>::value || sizeof(Src) > sizeof(Tgt))) {
-    FOLLY_RANGE_CHECK(value >= std::numeric_limits<Tgt>::min(),
-                      "Negative overflow");
+  return ConversionCode::SUCCESS;
+}
+
+/**
+ * Keep this implementation around for prettyToDouble().
+ */
+inline void enforceWhitespace(StringPiece sp) {
+  auto err = enforceWhitespaceErr(sp);
+  if (err != ConversionCode::SUCCESS) {
+    throw makeConversionError(err, sp);
   }
-  return static_cast<Tgt>(value);
+}
+}
+
+/**
+ * The identity conversion function.
+ * tryTo<T>(T) returns itself for all types T.
+ */
+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);
+}
+
+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);
 }
 
 /*******************************************************************************
- * Floating point to floating point
+ * Arithmetic to boolean
  ******************************************************************************/
 
+/**
+ * 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_floating_point<Tgt>::value && std::is_floating_point<Src>::value,
-  Tgt>::type
-to(const Src & value) {
-  /* static */ if (std::numeric_limits<Tgt>::max() <
-                   std::numeric_limits<Src>::max()) {
-    FOLLY_RANGE_CHECK(value <= std::numeric_limits<Tgt>::max(),
-                      "Overflow");
-    FOLLY_RANGE_CHECK(value >= -std::numeric_limits<Tgt>::max(),
-                      "Negative overflow");
-  }
-  return boost::implicit_cast<Tgt>(value);
+    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_arithmetic<Src>::value && !std::is_same<Tgt, Src>::value &&
+        std::is_same<Tgt, bool>::value,
+    Tgt>::type
+to(const Src& value) {
+  return value != Src();
 }
 
 /*******************************************************************************
@@ -98,30 +188,104 @@ to(const Src & value) {
 
 namespace detail {
 
-template <class T> struct IsSomeString {
-  enum { value = std::is_same<T, std::string>::value
-         || std::is_same<T, fbstring>::value };
+#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 <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)...));
+}
+
+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 T>
-const T& getLastElement(const T & v) {
-  return v;
-}
+template <class... Ts>
+struct LastElement
+    : std::decay<typename LastElementType<sizeof...(Ts), Ts...>::type> {};
+#else
+template <typename... Ts>
+struct LastElementImpl {
+  static void call(Ignored<Ts>...) {}
+};
 
-template <class T, class... Ts>
-typename std::tuple_element<
-  sizeof...(Ts),
-  std::tuple<T, Ts...> >::type const&
-  getLastElement(const T& v, const Ts&... vs) {
-  return getLastElement(vs...);
+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
 
 /*******************************************************************************
  * Conversions from integral types to string types.
  ******************************************************************************/
 
+#if FOLLY_HAVE_INT128_T
+namespace detail {
+
+template <typename IntegerType>
+constexpr unsigned int
+digitsEnough() {
+  return (unsigned int)(ceil(sizeof(IntegerType) * CHAR_BIT * M_LN2 / M_LN10));
+}
+
+inline size_t
+unsafeTelescope128(char * buffer, size_t room, unsigned __int128 x) {
+  typedef unsigned __int128 Usrc;
+  size_t p = room - 1;
+
+  while (x >= (Usrc(1) << 64)) { // Using 128-bit division while needed
+    const auto y = x / 10;
+    const auto digit = x % 10;
+
+    buffer[p--] = '0' + digit;
+    x = y;
+  }
+
+  uint64_t xx = x; // Moving to faster 64-bit division thereafter
+
+  while (xx >= 10) {
+    const auto y = xx / 10ULL;
+    const auto digit = xx % 10ULL;
+
+    buffer[p--] = '0' + digit;
+    xx = y;
+  }
+
+  buffer[p] = '0' + xx;
+
+  return p;
+}
+
+}
+#endif
+
 /**
  * Returns the number of digits in the base 10 representation of an
  * uint64_t. Useful for preallocating buffers and such. It's also used
@@ -130,6 +294,58 @@ typename std::tuple_element<
  */
 
 inline uint32_t digits10(uint64_t v) {
+#ifdef __x86_64__
+
+  // For this arch we can get a little help from specialized CPU instructions
+  // which can count leading zeroes; 64 minus that is appx. log (base 2).
+  // Use that to approximate base-10 digits (log_10) and then adjust if needed.
+
+  // 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) = {
+      1,
+      10,
+      100,
+      1000,
+      10000,
+      100000,
+      1000000,
+      10000000,
+      100000000,
+      1000000000,
+      10000000000,
+      100000000000,
+      1000000000000,
+      10000000000000,
+      100000000000000,
+      1000000000000000,
+      10000000000000000,
+      100000000000000000,
+      1000000000000000000,
+      10000000000000000000UL,
+  };
+
+  // "count leading zeroes" operation not valid; for 0; special case this.
+  if UNLIKELY (! v) {
+    return 1;
+  }
+
+  // bits is in the ballpark of log_2(v).
+  const uint8_t leadingZeroes = __builtin_clzll(v);
+  const auto bits = 63 - leadingZeroes;
+
+  // approximate log_10(v) == log_10(2) * bits.
+  // Integer magic below: 77/256 is appx. 0.3010 (log_10(2)).
+  // The +1 is to make this the ceiling of the log_10 estimate.
+  const uint32_t minLength = 1 + ((bits * 77) >> 8);
+
+  // 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]));
+
+#else
+
   uint32_t result = 1;
   for (;;) {
     if (LIKELY(v < 10)) return result;
@@ -140,6 +356,8 @@ inline uint32_t digits10(uint64_t v) {
     v /= 10000U;
     result += 4;
   }
+
+#endif
 }
 
 /**
@@ -165,12 +383,12 @@ 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;
 }
 
@@ -182,13 +400,21 @@ void toAppend(char value, Tgt * result) {
   *result += value;
 }
 
+template<class T>
+constexpr typename std::enable_if<
+  std::is_same<T, char>::value,
+  size_t>::type
+estimateSpaceNeeded(T) {
+  return 1;
+}
+
 /**
  * Everything implicitly convertible to const char* gets appended.
  */
 template <class Tgt, class Src>
 typename std::enable_if<
   std::is_convertible<Src, const char*>::value
-  && detail::IsSomeString<Tgt>::value>::type
+  && IsSomeString<Tgt>::value>::type
 toAppend(Src value, Tgt * result) {
   // Treat null pointers like an empty string, as in:
   // operator<<(std::ostream&, const char*).
@@ -198,12 +424,47 @@ 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) {
+  const char *c = value;
+  if (c) {
+    return folly::StringPiece(value).size();
+  };
+  return 0;
+}
+
+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
+estimateSpaceNeeded(Src value) {
+  return folly::StringPiece(value).size();
+}
+
+template <>
+inline size_t estimateSpaceNeeded(std::nullptr_t /* value */) {
+  return 0;
+}
+
+template<class Src>
+typename std::enable_if<
+  std::is_pointer<Src>::value &&
+  IsSomeString<std::remove_pointer<Src>>::value,
+  size_t>::type
+estimateSpaceNeeded(Src value) {
+  return value->size();
+}
+
 /**
  * Strings get appended, too.
  */
 template <class Tgt, class Src>
 typename std::enable_if<
-  detail::IsSomeString<Src>::value && detail::IsSomeString<Tgt>::value>::type
+  IsSomeString<Src>::value && IsSomeString<Tgt>::value>::type
 toAppend(const Src& value, Tgt * result) {
   result->append(value);
 }
@@ -213,7 +474,7 @@ toAppend(const Src& value, Tgt * result) {
  */
 template <class Tgt>
 typename std::enable_if<
-   detail::IsSomeString<Tgt>::value>::type
+   IsSomeString<Tgt>::value>::type
 toAppend(StringPiece value, Tgt * result) {
   result->append(value.data(), value.size());
 }
@@ -224,33 +485,100 @@ toAppend(StringPiece value, Tgt * result) {
  */
 template <class Tgt>
 typename std::enable_if<
-   detail::IsSomeString<Tgt>::value>::type
+   IsSomeString<Tgt>::value>::type
 toAppend(const fbstring& value, Tgt * result) {
   result->append(value.data(), value.size());
 }
 
+#if FOLLY_HAVE_INT128_T
+/**
+ * Special handling for 128 bit integers.
+ */
+
+template <class Tgt>
+void
+toAppend(__int128 value, Tgt * result) {
+  typedef unsigned __int128 Usrc;
+  char buffer[detail::digitsEnough<unsigned __int128>() + 1];
+  size_t p;
+
+  if (value < 0) {
+    p = detail::unsafeTelescope128(buffer, sizeof(buffer), -Usrc(value));
+    buffer[--p] = '-';
+  } else {
+    p = detail::unsafeTelescope128(buffer, sizeof(buffer), value);
+  }
+
+  result->append(buffer + p, buffer + sizeof(buffer));
+}
+
+template <class Tgt>
+void
+toAppend(unsigned __int128 value, Tgt * result) {
+  char buffer[detail::digitsEnough<unsigned __int128>()];
+  size_t p;
+
+  p = detail::unsafeTelescope128(buffer, sizeof(buffer), value);
+
+  result->append(buffer + p, buffer + sizeof(buffer));
+}
+
+template<class T>
+constexpr typename std::enable_if<
+  std::is_same<T, __int128>::value,
+  size_t>::type
+estimateSpaceNeeded(T) {
+  return detail::digitsEnough<__int128>();
+}
+
+template<class T>
+constexpr typename std::enable_if<
+  std::is_same<T, unsigned __int128>::value,
+  size_t>::type
+estimateSpaceNeeded(T) {
+  return detail::digitsEnough<unsigned __int128>();
+}
+
+#endif
+
 /**
  * int32_t and int64_t to string (by appending) go through here. The
  * result is APPENDED to a preexisting string passed as the second
- * parameter. For convenience, the function also returns a reference
- * to *result. This should be efficient with fbstring because fbstring
+ * parameter. This should be efficient with fbstring because fbstring
  * incurs no dynamic allocation below 23 bytes and no number has more
  * than 22 bytes in its textual representation (20 for digits, one for
  * sign, one for the terminating 0).
  */
 template <class Tgt, class Src>
 typename std::enable_if<
-  std::is_integral<Src>::value && std::is_signed<Src>::value
-  && detail::IsSomeString<Tgt>::value && sizeof(Src) >= 4>::type
+  std::is_integral<Src>::value && std::is_signed<Src>::value &&
+  IsSomeString<Tgt>::value && sizeof(Src) >= 4>::type
 toAppend(Src value, Tgt * result) {
-  typedef typename std::make_unsigned<Src>::type Usrc;
   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));
+  }
+}
+
+template <class Src>
+typename std::enable_if<
+  std::is_integral<Src>::value && std::is_signed<Src>::value
+  && sizeof(Src) >= 4 && sizeof(Src) < 16,
+  size_t>::type
+estimateSpaceNeeded(Src value) {
+  if (value < 0) {
+    // When "value" is the smallest negative, negating it would evoke
+    // undefined behavior, so, instead of writing "-value" below, we write
+    // "~static_cast<uint64_t>(value) + 1"
+    return 1 + digits10(~static_cast<uint64_t>(value) + 1);
   }
+
+  return digits10(static_cast<uint64_t>(value));
 }
 
 /**
@@ -259,10 +587,19 @@ toAppend(Src value, Tgt * result) {
 template <class Tgt, class Src>
 typename std::enable_if<
   std::is_integral<Src>::value && !std::is_signed<Src>::value
-  && detail::IsSomeString<Tgt>::value && sizeof(Src) >= 4>::type
+  && IsSomeString<Tgt>::value && sizeof(Src) >= 4>::type
 toAppend(Src value, Tgt * result) {
   char buffer[20];
-  result->append(buffer, buffer + uint64ToBufferUnsafe(value, buffer));
+  result->append(buffer, uint64ToBufferUnsafe(value, buffer));
+}
+
+template <class Src>
+typename std::enable_if<
+  std::is_integral<Src>::value && !std::is_signed<Src>::value
+  && sizeof(Src) >= 4 && sizeof(Src) < 16,
+  size_t>::type
+estimateSpaceNeeded(Src value) {
+  return digits10(value);
 }
 
 /**
@@ -272,7 +609,7 @@ toAppend(Src value, Tgt * result) {
 template <class Tgt, class Src>
 typename std::enable_if<
   std::is_integral<Src>::value
-  && detail::IsSomeString<Tgt>::value && sizeof(Src) < 4>::type
+  && IsSomeString<Tgt>::value && sizeof(Src) < 4>::type
 toAppend(Src value, Tgt * result) {
   typedef typename
     std::conditional<std::is_signed<Src>::value, int64_t, uint64_t>::type
@@ -280,55 +617,52 @@ toAppend(Src value, Tgt * result) {
   toAppend<Tgt>(static_cast<Intermediate>(value), result);
 }
 
-#if defined(__GNUC__) && __GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 7)
-// std::underlying_type became available by gcc 4.7.0
+template <class Src>
+typename std::enable_if<
+  std::is_integral<Src>::value
+  && sizeof(Src) < 4
+  && !std::is_same<Src, char>::value,
+  size_t>::type
+estimateSpaceNeeded(Src value) {
+  typedef typename
+    std::conditional<std::is_signed<Src>::value, int64_t, uint64_t>::type
+    Intermediate;
+  return estimateSpaceNeeded(static_cast<Intermediate>(value));
+}
 
 /**
  * Enumerated values get appended as integers.
  */
 template <class Tgt, class Src>
 typename std::enable_if<
-  std::is_enum<Src>::value && detail::IsSomeString<Tgt>::value>::type
+  std::is_enum<Src>::value && IsSomeString<Tgt>::value>::type
 toAppend(Src value, Tgt * result) {
   toAppend(
       static_cast<typename std::underlying_type<Src>::type>(value), result);
 }
 
-#else
-
-/**
- * Enumerated values get appended as integers.
- */
-template <class Tgt, class Src>
+template <class Src>
 typename std::enable_if<
-  std::is_enum<Src>::value && detail::IsSomeString<Tgt>::value>::type
-toAppend(Src value, Tgt * result) {
-  /* static */ if (Src(-1) < 0) {
-    /* static */ if (sizeof(Src) <= sizeof(int)) {
-      toAppend(static_cast<int>(value), result);
-    } else {
-      toAppend(static_cast<long>(value), result);
-    }
-  } else {
-    /* static */ if (sizeof(Src) <= sizeof(int)) {
-      toAppend(static_cast<unsigned int>(value), result);
-    } else {
-      toAppend(static_cast<unsigned long>(value), result);
-    }
-  }
+  std::is_enum<Src>::value, size_t>::type
+estimateSpaceNeeded(Src value) {
+  return estimateSpaceNeeded(
+      static_cast<typename std::underlying_type<Src>::type>(value));
 }
 
-#endif // gcc 4.7 onwards
-
 /*******************************************************************************
  * Conversions from floating-point types to string types.
  ******************************************************************************/
 
+namespace detail {
+constexpr int kConvMaxDecimalInShortestLow = -6;
+constexpr int kConvMaxDecimalInShortestHigh = 21;
+} // folly::detail
+
 /** Wrapper around DoubleToStringConverter **/
 template <class Tgt, class Src>
 typename std::enable_if<
   std::is_floating_point<Src>::value
-  && detail::IsSomeString<Tgt>::value>::type
+  && IsSomeString<Tgt>::value>::type
 toAppend(
   Src value,
   Tgt * result,
@@ -337,9 +671,9 @@ toAppend(
   using namespace double_conversion;
   DoubleToStringConverter
     conv(DoubleToStringConverter::NO_FLAGS,
-         "infinity", "NaN", 'E',
-         -6,  // decimal in shortest low
-         21,  // decimal in shortest high
+         "Infinity", "NaN", 'E',
+         detail::kConvMaxDecimalInShortestLow,
+         detail::kConvMaxDecimalInShortestHigh,
          6,   // max leading padding zeros
          1);  // max trailing padding zeros
   char buffer[256];
@@ -349,14 +683,14 @@ 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);
 }
@@ -367,631 +701,861 @@ toAppend(
 template <class Tgt, class Src>
 typename std::enable_if<
   std::is_floating_point<Src>::value
-  && detail::IsSomeString<Tgt>::value>::type
+  && IsSomeString<Tgt>::value>::type
 toAppend(Src value, Tgt * result) {
   toAppend(
     value, result, double_conversion::DoubleToStringConverter::SHORTEST, 0);
 }
 
 /**
- * Variadic conversion to string. Appends each element in turn.
+ * Upper bound of the length of the output from
+ * DoubleToStringConverter::ToShortest(double, StringBuilder*),
+ * as used in toAppend(double, string*).
+ */
+template <class Src>
+typename std::enable_if<
+  std::is_floating_point<Src>::value, size_t>::type
+estimateSpaceNeeded(Src value) {
+  // kBase10MaximalLength is 17. We add 1 for decimal point,
+  // e.g. 10.0/9 is 17 digits and 18 characters, including the decimal point.
+  constexpr int kMaxMantissaSpace =
+    double_conversion::DoubleToStringConverter::kBase10MaximalLength + 1;
+  // strlen("E-") + digits10(numeric_limits<double>::max_exponent10)
+  constexpr int kMaxExponentSpace = 2 + 3;
+  static const int kMaxPositiveSpace = std::max({
+      // E.g. 1.1111111111111111E-100.
+      kMaxMantissaSpace + kMaxExponentSpace,
+      // E.g. 0.000001.1111111111111111, if kConvMaxDecimalInShortestLow is -6.
+      kMaxMantissaSpace - detail::kConvMaxDecimalInShortestLow,
+      // If kConvMaxDecimalInShortestHigh is 21, then 1e21 is the smallest
+      // number > 1 which ToShortest outputs in exponential notation,
+      // so 21 is the longest non-exponential number > 1.
+      detail::kConvMaxDecimalInShortestHigh
+    });
+  return size_t(
+      kMaxPositiveSpace +
+      (value < 0 ? 1 : 0)); // +1 for minus sign, if negative
+}
+
+/**
+ * This can be specialized, together with adding specialization
+ * for estimateSpaceNeed for your type, so that we allocate
+ * as much as you need instead of the default
+ */
+template<class Src>
+struct HasLengthEstimator : std::false_type {};
+
+template <class Src>
+constexpr typename std::enable_if<
+  !std::is_fundamental<Src>::value
+#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
+#endif
+  && !IsSomeString<Src>::value
+  && !std::is_convertible<Src, const char*>::value
+  && !std::is_convertible<Src, StringPiece>::value
+  && !std::is_enum<Src>::value
+  && !HasLengthEstimator<Src>::value,
+  size_t>::type
+estimateSpaceNeeded(const Src&) {
+  return sizeof(Src) + 1; // dumbest best effort ever?
+}
+
+namespace detail {
+
+template <class Tgt>
+typename std::enable_if<IsSomeString<Tgt>::value, size_t>::type
+estimateSpaceToReserve(size_t sofar, Tgt*) {
+  return sofar;
+}
+
+template <class T, class... Ts>
+size_t estimateSpaceToReserve(size_t sofar, const T& v, const Ts&... vs) {
+  return estimateSpaceToReserve(sofar + estimateSpaceNeeded(v), vs...);
+}
+
+template<class...Ts>
+void reserveInTarget(const Ts&...vs) {
+  getLastElement(vs...)->reserve(estimateSpaceToReserve(0, vs...));
+}
+
+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) *
+      estimateSpaceToReserve(0, d, static_cast<std::string*>(nullptr));
+  getLastElement(vs...)->reserve(estimateSpaceToReserve(fordelim, vs...));
+}
+
+/**
+ * Variadic base case: append one element
  */
+template <class T, class Tgt>
+typename std::enable_if<
+  IsSomeString<typename std::remove_pointer<Tgt>::type>
+  ::value>::type
+toAppendStrImpl(const T& v, Tgt result) {
+  toAppend(v, result);
+}
+
 template <class T, class... Ts>
-typename std::enable_if<sizeof...(Ts) >= 2
-  && detail::IsSomeString<
-  typename std::remove_pointer<
-    typename std::tuple_element<
-      sizeof...(Ts) - 1, std::tuple<Ts...>
-      >::type>::type>::value>::type
-toAppend(const T& v, const Ts&... vs) {
+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...);
+}
+
+template <class Delimiter, class T, class Tgt>
+typename std::enable_if<
+    IsSomeString<typename std::remove_pointer<Tgt>::type>::value>::type
+toAppendDelimStrImpl(const Delimiter& /* delim */, const T& v, Tgt result) {
+  toAppend(v, result);
+}
+
+template <class Delimiter, class T, class... Ts>
+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
+  // toAppend(v, delim, ....) we would do unnecesary size calculation
   toAppend(v, detail::getLastElement(vs...));
+  toAppend(delim, detail::getLastElement(vs...));
+  toAppendDelimStrImpl(delim, vs...);
+}
+} // folly::detail
+
+
+/**
+ * Variadic conversion to string. Appends each element in turn.
+ * If we have two or more things to append, we it will not reserve
+ * the space for them and will depend on strings exponential growth.
+ * If you just append once consider using toAppendFit which reserves
+ * the space needed (but does not have exponential as a result).
+ *
+ * Custom implementations of toAppend() can be provided in the same namespace as
+ * the type to customize printing. estimateSpaceNeed() may also be provided to
+ * avoid reallocations in toAppendFit():
+ *
+ * namespace other_namespace {
+ *
+ * template <class String>
+ * void toAppend(const OtherType&, String* out);
+ *
+ * // optional
+ * size_t estimateSpaceNeeded(const OtherType&);
+ *
+ * }
+ */
+template <class... Ts>
+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...);
+}
+
+#ifdef _MSC_VER
+// Special case pid_t on MSVC, because it's a void* rather than an
+// integral type. We can't do a global special case because this is already
+// dangerous enough (as most pointers will implicitly convert to a void*)
+// just doing it for MSVC.
+template <class Tgt>
+void toAppend(const pid_t a, Tgt* res) {
+  toAppend(uint64_t(a), res);
+}
+#endif
+
+/**
+ * Special version of the call that preallocates exaclty as much memory
+ * as need for arguments to be stored in target. This means we are
+ * not doing exponential growth when we append. If you are using it
+ * in a loop you are aiming at your foot with a big perf-destroying
+ * bazooka.
+ * On the other hand if you are appending to a string once, this
+ * will probably save a few calls to malloc.
+ */
+template <class... Ts>
+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...);
 }
 
+template <class Ts>
+void toAppendFit(const Ts&) {}
+
 /**
  * Variadic base case: do nothing.
  */
 template <class Tgt>
-typename std::enable_if<detail::IsSomeString<Tgt>::value>::type
-toAppend(Tgt* result) {
+typename std::enable_if<IsSomeString<Tgt>::value>::type toAppend(
+    Tgt* /* result */) {}
+
+/**
+ * Variadic base case: do nothing.
+ */
+template <class Delimiter, class Tgt>
+typename std::enable_if<IsSomeString<Tgt>::value>::type toAppendDelim(
+    const Delimiter& /* delim */, Tgt* /* result */) {}
+
+/**
+ * 1 element: same as toAppend.
+ */
+template <class Delimiter, class T, class Tgt>
+typename std::enable_if<IsSomeString<Tgt>::value>::type toAppendDelim(
+    const Delimiter& /* delim */, const T& v, Tgt* tgt) {
+  toAppend(v, tgt);
+}
+
+/**
+ * Append to string with a delimiter in between elements. Check out
+ * 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::LastElement<const Ts&...>::type>::type>::value>::type
+toAppendDelim(const Delimiter& delim, const Ts&... vs) {
+  detail::toAppendDelimStrImpl(delim, vs...);
 }
 
+/**
+ * Detail in comment for toAppendFit
+ */
+template <class Delimiter, class... Ts>
+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...);
+}
+
+template <class De, class Ts>
+void toAppendDelimFit(const De&, const Ts&) {}
+
 /**
  * to<SomeString>(v1, v2, ...) uses toAppend() (see below) as back-end
  * for all types.
  */
 template <class Tgt, class... Ts>
-typename std::enable_if<detail::IsSomeString<Tgt>::value, Tgt>::type
+typename std::enable_if<
+    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;
-  toAppend(vs..., &result);
+  toAppendFit(vs..., &result);
   return result;
 }
 
-/*******************************************************************************
- * Conversions from string types to integral types.
- ******************************************************************************/
-
-namespace detail {
-
 /**
- * Finds the first non-digit in a string. The number of digits
- * searched depends on the precision of the Tgt integral. Assumes the
- * string starts with NO whitespace and NO sign.
+ * 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.
  *
- * The semantics of the routine is:
- *   for (;; ++b) {
- *     if (b >= e || !isdigit(*b)) return b;
- *   }
+ * But for most cases, a floating point doesn't need 24 (or 25) bytes to
+ * be converted as a string.
  *
- *  Complete unrolling marks bottom-line (i.e. entire conversion)
- *  improvements of 20%.
- */
-  template <class Tgt>
-  const char* findFirstNonDigit(const char* b, const char* e) {
-    for (; b < e; ++b) {
-      auto const c = static_cast<unsigned>(*b) - '0';
-      if (c >= 10) break;
-    }
-    return b;
-  }
-
-  // Maximum value of number when represented as a string
-  template <class T> struct MaxString {
-    static const char*const value;
-  };
-
+ * 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;
+}
 
-/*
- * Lookup tables that converts from a decimal character value to an integral
- * binary value, shifted by a decimal "shift" multiplier.
- * For all character values in the range '0'..'9', the table at those
- * index locations returns the actual decimal value shifted by the multiplier.
- * For all other values, the lookup table returns an invalid OOR value.
- */
-// Out-of-range flag value, larger than the largest value that can fit in
-// four decimal bytes (9999), but four of these added up together should
-// still not overflow uint16_t.
-constexpr int32_t OOR = 10000;
-
-__attribute__((aligned(16))) constexpr uint16_t shift1[] = {
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 0-9
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  //  10
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  //  20
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  //  30
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, 0,         //  40
-  1, 2, 3, 4, 5, 6, 7, 8, 9, OOR, OOR,
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  //  60
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  //  70
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  //  80
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  //  90
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 100
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 110
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 120
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 130
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 140
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 150
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 160
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 170
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 180
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 190
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 200
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 210
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 220
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 230
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 240
-  OOR, OOR, OOR, OOR, OOR, OOR                       // 250
-};
+/**
+ * toDelim<SomeString>(SomeString str) returns itself.
+ */
+template <class Tgt, class Delim, class Src>
+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);
+}
 
-__attribute__((aligned(16))) constexpr uint16_t shift10[] = {
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 0-9
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  //  10
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  //  20
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  //  30
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, 0,         //  40
-  10, 20, 30, 40, 50, 60, 70, 80, 90, OOR, OOR,
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  //  60
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  //  70
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  //  80
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  //  90
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 100
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 110
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 120
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 130
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 140
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 150
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 160
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 170
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 180
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 190
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 200
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 210
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 220
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 230
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 240
-  OOR, OOR, OOR, OOR, OOR, OOR                       // 250
-};
+/**
+ * toDelim<SomeString>(delim, v1, v2, ...) uses toAppendDelim() as
+ * back-end for all types.
+ */
+template <class Tgt, class Delim, class... Ts>
+typename std::enable_if<
+    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);
+  return result;
+}
 
-__attribute__((aligned(16))) constexpr uint16_t shift100[] = {
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 0-9
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  //  10
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  //  20
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  //  30
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, 0,         //  40
-  100, 200, 300, 400, 500, 600, 700, 800, 900, OOR, OOR,
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  //  60
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  //  70
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  //  80
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  //  90
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 100
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 110
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 120
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 130
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 140
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 150
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 160
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 170
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 180
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 190
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 200
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 210
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 220
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 230
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 240
-  OOR, OOR, OOR, OOR, OOR, OOR                       // 250
-};
+/*******************************************************************************
+ * Conversions from string types to integral types.
+ ******************************************************************************/
 
-__attribute__((aligned(16))) constexpr uint16_t shift1000[] = {
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 0-9
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  //  10
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  //  20
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  //  30
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, 0,         //  40
-  1000, 2000, 3000, 4000, 5000, 6000, 7000, 8000, 9000, OOR, OOR,
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  //  60
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  //  70
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  //  80
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  //  90
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 100
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 110
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 120
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 130
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 140
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 150
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 160
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 170
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 180
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 190
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 200
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 210
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 220
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 230
-  OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR, OOR,  // 240
-  OOR, OOR, OOR, OOR, OOR, OOR                       // 250
-};
+namespace detail {
 
-/**
- * String represented as a pair of pointers to char to unsigned
- * integrals. Assumes NO whitespace before or after, and also that the
- * string is composed entirely of digits. Tgt must be unsigned, and no
- * sign is allowed in the string (even it's '+'). String may be empty,
- * in which case digits_to throws.
- */
-  template <class Tgt>
-  Tgt digits_to(const char * b, const char * e) {
-
-    static_assert(!std::is_signed<Tgt>::value, "Unsigned type expected");
-    assert(b <= e);
-
-    const size_t size = e - b;
-
-    /* Although the string is entirely made of digits, we still need to
-     * check for overflow.
-     */
-    if (size >= std::numeric_limits<Tgt>::digits10 + 1) {
-      // Leading zeros? If so, recurse to keep things simple
-      if (b < e && *b == '0') {
-        for (++b;; ++b) {
-          if (b == e) return 0; // just zeros, e.g. "0000"
-          if (*b != '0') return digits_to<Tgt>(b, e);
-        }
-      }
-      FOLLY_RANGE_CHECK(size == std::numeric_limits<Tgt>::digits10 + 1 &&
-                        strncmp(b, detail::MaxString<Tgt>::value, size) <= 0,
-                        "Numeric overflow upon conversion");
-    }
+Expected<bool, ConversionCode> str_to_bool(StringPiece* src) noexcept;
 
-    // Here we know that the number won't overflow when
-    // converted. Proceed without checks.
+template <typename T>
+Expected<T, ConversionCode> str_to_floating(StringPiece* src) noexcept;
 
-    Tgt result = 0;
+extern template Expected<float, ConversionCode> str_to_floating<float>(
+    StringPiece* src) noexcept;
+extern template Expected<double, ConversionCode> str_to_floating<double>(
+    StringPiece* src) noexcept;
 
-    for (; e - b >= 4; b += 4) {
-      result *= 10000;
-      const int32_t r0 = shift1000[static_cast<size_t>(b[0])];
-      const int32_t r1 = shift100[static_cast<size_t>(b[1])];
-      const int32_t r2 = shift10[static_cast<size_t>(b[2])];
-      const int32_t r3 = shift1[static_cast<size_t>(b[3])];
-      const auto sum = r0 + r1 + r2 + r3;
-      assert(sum < OOR && "Assumption: string only has digits");
-      result += sum;
-    }
+template <class Tgt>
+Expected<Tgt, ConversionCode> digits_to(const char* b, const char* e) noexcept;
+
+extern template Expected<char, ConversionCode> digits_to<char>(
+    const char*,
+    const char*) noexcept;
+extern template Expected<signed char, ConversionCode> digits_to<signed char>(
+    const char*,
+    const char*) noexcept;
+extern template Expected<unsigned char, ConversionCode>
+digits_to<unsigned char>(const char*, const char*) noexcept;
+
+extern template Expected<short, ConversionCode> digits_to<short>(
+    const char*,
+    const char*) noexcept;
+extern template Expected<unsigned short, ConversionCode>
+digits_to<unsigned short>(const char*, const char*) noexcept;
+
+extern template Expected<int, ConversionCode> digits_to<int>(
+    const char*,
+    const char*) noexcept;
+extern template Expected<unsigned int, ConversionCode> digits_to<unsigned int>(
+    const char*,
+    const char*) noexcept;
+
+extern template Expected<long, ConversionCode> digits_to<long>(
+    const char*,
+    const char*) noexcept;
+extern template Expected<unsigned long, ConversionCode>
+digits_to<unsigned long>(const char*, const char*) noexcept;
+
+extern template Expected<long long, ConversionCode> digits_to<long long>(
+    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 Expected<__int128, ConversionCode> digits_to<__int128>(
+    const char*,
+    const char*) noexcept;
+extern template Expected<unsigned __int128, ConversionCode>
+digits_to<unsigned __int128>(const char*, const char*) noexcept;
+#endif
 
-    switch (e - b) {
-      case 3: {
-        const int32_t r0 = shift100[static_cast<size_t>(b[0])];
-        const int32_t r1 = shift10[static_cast<size_t>(b[1])];
-        const int32_t r2 = shift1[static_cast<size_t>(b[2])];
-        const auto sum = r0 + r1 + r2;
-        assert(sum < OOR && "Assumption: string only has digits");
-        return result * 1000 + sum;
-      }
-      case 2: {
-        const int32_t r0 = shift10[static_cast<size_t>(b[0])];
-        const int32_t r1 = shift1[static_cast<size_t>(b[1])];
-        const auto sum = r0 + r1;
-        assert(sum < OOR && "Assumption: string only has digits");
-        return result * 100 + sum;
-      }
-      case 1: {
-        const int32_t sum = shift1[static_cast<size_t>(b[0])];
-        assert(sum < OOR && "Assumption: string only has digits");
-        return result * 10 + sum;
-      }
-    }
+template <class T>
+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 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
 
-    assert(b == e);
-    FOLLY_RANGE_CHECK(size > 0, "Found no digits to convert in input");
-    return result;
-  }
+template <typename T>
+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,
+    Expected<T, ConversionCode>>::type
+convertTo(StringPiece* src) noexcept {
+  return str_to_floating<T>(src);
+}
 
-  bool str_to_bool(StringPiece * src);
+template <typename T>
+typename std::enable_if<
+    std::is_integral<T>::value && !std::is_same<T, bool>::value,
+    Expected<T, ConversionCode>>::type
+convertTo(StringPiece* src) noexcept {
+  return str_to_integral<T>(src);
+}
 
-}                                 // namespace detail
+} // namespace detail
 
 /**
  * String represented as a pair of pointers to char to unsigned
  * integrals. Assumes NO whitespace before or after.
  */
-template <class Tgt>
+template <typename Tgt>
 typename std::enable_if<
-  std::is_integral<Tgt>::value && !std::is_signed<Tgt>::value
-  && !std::is_same<typename std::remove_cv<Tgt>::type, bool>::value,
-  Tgt>::type
-to(const char * b, const char * e) {
+    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);
 }
 
-/**
- * String represented as a pair of pointers to char to signed
- * integrals. Assumes NO whitespace before or after. Allows an
- * optional leading sign.
- */
-template <class Tgt>
+template <typename Tgt>
 typename std::enable_if<
-  std::is_integral<Tgt>::value && std::is_signed<Tgt>::value,
-  Tgt>::type
-to(const char * b, const char * e) {
-  FOLLY_RANGE_CHECK(b < e, "Empty input string in conversion to integral");
-  if (!isdigit(*b)) {
-    if (*b == '-') {
-      Tgt result = -to<typename std::make_unsigned<Tgt>::type>(b + 1, e);
-      FOLLY_RANGE_CHECK(result <= 0, "Negative overflow.");
-      return result;
-    }
-    FOLLY_RANGE_CHECK(*b == '+', "Invalid lead character");
-    ++b;
-  }
-  Tgt result = to<typename std::make_unsigned<Tgt>::type>(b, e);
-  FOLLY_RANGE_CHECK(result >= 0, "Overflow.");
-  return result;
+    std::is_integral<Tgt>::value && !std::is_same<Tgt, bool>::value,
+    Tgt>::type
+to(const char* b, const char* e) {
+  return tryTo<Tgt>(b, e).thenOrThrow(
+      [](Tgt res) { return res; },
+      [=](ConversionCode code) {
+        return makeConversionError(code, StringPiece(b, e));
+      });
 }
 
+/*******************************************************************************
+ * Conversions from string types to arithmetic types.
+ ******************************************************************************/
+
 /**
- * Parsing strings to integrals. These routines differ from
- * to<integral>(string) 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>
+FOLLY_NODISCARD inline typename std::enable_if<
+    std::is_arithmetic<Tgt>::value,
+    Expected<StringPiece, ConversionCode>>::type
+parseTo(StringPiece src, Tgt& out) {
+  return detail::convertTo<Tgt>(&src).then(
+      [&](Tgt res) { return void(out = res), src; });
+}
+
+/*******************************************************************************
+ * Integral / Floating Point to integral / Floating Point
+ ******************************************************************************/
+
+namespace detail {
 
 /**
- * StringPiece to integrals, with progress information. Alters the
- * StringPiece parameter to munch the already-parsed characters.
+ * 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>
 typename std::enable_if<
-  std::is_integral<Tgt>::value
-  && !std::is_same<typename std::remove_cv<Tgt>::type, bool>::value,
-  Tgt>::type
-to(StringPiece * src) {
-
-  auto b = src->data(), past = src->data() + src->size();
-  for (;; ++b) {
-    FOLLY_RANGE_CHECK(b < past, "No digits found in input string");
-    if (!isspace(*b)) break;
-  }
+    !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);
+}
 
-  auto m = b;
-
-  // First digit is customized because we test for sign
-  bool negative = false;
-  /* static */ if (std::is_signed<Tgt>::value) {
-    if (!isdigit(*m)) {
-      if (*m == '-') {
-        negative = true;
-      } else {
-        FOLLY_RANGE_CHECK(*m == '+', "Invalid leading character in conversion"
-                          " to integral");
-      }
-      ++b;
-      ++m;
+/**
+ * Checked conversion from integral to integral. The checks are only
+ * performed when meaningful, e.g. conversion from int to long goes
+ * unchecked.
+ */
+template <class Tgt, class Src>
+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,
+    Expected<Tgt, ConversionCode>>::type
+convertTo(const Src& value) noexcept {
+  /* static */ if (
+      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 makeUnexpected(ConversionCode::ARITH_POSITIVE_OVERFLOW);
     }
   }
-  FOLLY_RANGE_CHECK(m < past, "No digits found in input string");
-  FOLLY_RANGE_CHECK(isdigit(*m), "Non-digit character found");
-  m = detail::findFirstNonDigit<Tgt>(m + 1, past);
-
-  Tgt result;
-  /* static */ if (!std::is_signed<Tgt>::value) {
-    result = detail::digits_to<typename std::make_unsigned<Tgt>::type>(b, m);
-  } else {
-    auto t = detail::digits_to<typename std::make_unsigned<Tgt>::type>(b, m);
-    if (negative) {
-      result = -t;
-      FOLLY_RANGE_CHECK(result <= 0, "Negative overflow");
-    } else {
-      result = t;
-      FOLLY_RANGE_CHECK(result >= 0, "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 makeUnexpected(ConversionCode::ARITH_NEGATIVE_OVERFLOW);
     }
   }
-  src->advance(m - src->data());
-  return result;
+  return static_cast<Tgt>(value);
 }
 
 /**
- * StringPiece to bool, with progress information. Alters the
- * StringPiece parameter to munch the already-parsed characters.
+ * Checked conversion from floating to floating. The checks are only
+ * performed when meaningful, e.g. conversion from float to double goes
+ * unchecked.
  */
-template <class Tgt>
+template <class Tgt, class Src>
 typename std::enable_if<
-  std::is_same<typename std::remove_cv<Tgt>::type, bool>::value,
-  Tgt>::type
-to(StringPiece * src) {
-  return detail::str_to_bool(src);
+    std::is_floating_point<Tgt>::value && std::is_floating_point<Src>::value &&
+        !std::is_same<Tgt, 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 makeUnexpected(ConversionCode::ARITH_POSITIVE_OVERFLOW);
+    }
+    if (value < std::numeric_limits<Tgt>::lowest()) {
+      return makeUnexpected(ConversionCode::ARITH_NEGATIVE_OVERFLOW);
+    }
+  }
+  return static_cast<Tgt>(value);
 }
 
-namespace detail {
-
 /**
- * Enforce that the suffix following a number is made up only of whitespace.
+ * Check if a floating point value can safely be converted to an
+ * integer value without triggering undefined behaviour.
  */
-inline void enforceWhitespace(const char* b, const char* e) {
-  for (; b != e; ++b) {
-    FOLLY_RANGE_CHECK(isspace(*b), to<std::string>("Non-whitespace: ", *b));
+template <typename Tgt, typename Src>
+inline typename std::enable_if<
+    std::is_floating_point<Src>::value && std::is_integral<Tgt>::value &&
+        !std::is_same<Tgt, bool>::value,
+    bool>::type
+checkConversion(const Src& value) {
+  constexpr Src tgtMaxAsSrc = static_cast<Src>(std::numeric_limits<Tgt>::max());
+  constexpr Src tgtMinAsSrc = static_cast<Src>(std::numeric_limits<Tgt>::min());
+  if (value >= tgtMaxAsSrc) {
+    if (value > tgtMaxAsSrc) {
+      return false;
+    }
+    const Src mmax = folly::nextafter(tgtMaxAsSrc, Src());
+    if (static_cast<Tgt>(value - mmax) >
+        std::numeric_limits<Tgt>::max() - static_cast<Tgt>(mmax)) {
+      return false;
+    }
+  } else if (std::is_signed<Tgt>::value && value <= tgtMinAsSrc) {
+    if (value < tgtMinAsSrc) {
+      return false;
+    }
+    const Src mmin = folly::nextafter(tgtMinAsSrc, Src());
+    if (static_cast<Tgt>(value - mmin) <
+        std::numeric_limits<Tgt>::min() - static_cast<Tgt>(mmin)) {
+      return false;
+    }
   }
+  return true;
+}
+
+// Integers can always safely be converted to floating point values
+template <typename Tgt, typename Src>
+constexpr typename std::enable_if<
+    std::is_integral<Src>::value && std::is_floating_point<Tgt>::value,
+    bool>::type
+checkConversion(const Src&) {
+  return true;
 }
 
-}  // namespace detail
+// Also, floating point values can always be safely converted to bool
+// Per the standard, any floating point value that is not zero will yield true
+template <typename Tgt, typename Src>
+constexpr typename std::enable_if<
+    std::is_floating_point<Src>::value && std::is_same<Tgt, bool>::value,
+    bool>::type
+checkConversion(const Src&) {
+  return true;
+}
 
 /**
- * String or StringPiece to integrals. Accepts leading and trailing
- * whitespace, but no non-space trailing characters.
+ * Checked conversion from integral to floating point and back. The
+ * result must be convertible back to the source type without loss of
+ * precision. This seems Draconian but sometimes is what's needed, and
+ * complements existing routines nicely. For various rounding
+ * routines, see <math>.
  */
-template <class Tgt>
+template <typename Tgt, typename Src>
 typename std::enable_if<
-  std::is_integral<Tgt>::value,
-  Tgt>::type
-to(StringPiece src) {
-  Tgt result = to<Tgt>(&src);
-  detail::enforceWhitespace(src.data(), src.data() + src.size());
-  return result;
+    (std::is_integral<Src>::value && std::is_floating_point<Tgt>::value) ||
+        (std::is_floating_point<Src>::value && std::is_integral<Tgt>::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 result;
+      }
+    }
+  }
+  return makeUnexpected(ConversionCode::ARITH_LOSS_OF_PRECISION);
+}
+
+template <typename Tgt, typename Src>
+inline std::string errorValue(const Src& value) {
+#ifdef FOLLY_HAS_RTTI
+  return to<std::string>("(", demangle(typeid(Tgt)), ") ", value);
+#else
+  return to<std::string>(value);
+#endif
+}
+
+template <typename Tgt, typename Src>
+using IsArithToArith = std::integral_constant<
+    bool,
+    !std::is_same<Tgt, Src>::value && !std::is_same<Tgt, bool>::value &&
+        std::is_arithmetic<Src>::value &&
+        std::is_arithmetic<Tgt>::value>;
+
+} // namespace detail
+
+template <typename Tgt, typename Src>
+typename std::enable_if<
+    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));
+      });
 }
 
 /*******************************************************************************
- * Conversions from string types to floating-point types.
+ * Custom Conversions
+ *
+ * Any type can be used with folly::to by implementing parseTo. The
+ * implementation should be provided in the namespace of the type to facilitate
+ * argument-dependent lookup:
+ *
+ * namespace other_namespace {
+ * ::folly::Expected<::folly::StringPiece, SomeErrorCode>
+ *   parseTo(::folly::StringPiece, OtherType&) noexcept;
+ * }
  ******************************************************************************/
+template <class T>
+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;
+}
 
-/**
- * StringPiece to double, with progress information. Alters the
- * StringPiece parameter to munch the already-parsed characters.
- */
-template <class Tgt>
-inline typename std::enable_if<
-  std::is_floating_point<Tgt>::value,
-  Tgt>::type
-to(StringPiece *const src) {
-  using namespace double_conversion;
-  static StringToDoubleConverter
-    conv(StringToDoubleConverter::ALLOW_TRAILING_JUNK
-         | StringToDoubleConverter::ALLOW_LEADING_SPACES,
-         0.0,
-         // return this for junk input string
-         std::numeric_limits<double>::quiet_NaN(),
-         nullptr, nullptr);
-
-  FOLLY_RANGE_CHECK(!src->empty(), "No digits found in input string");
-
-  int length;
-  auto result = conv.StringToDouble(src->data(), src->size(),
-                                       &length); // processed char count
-
-  if (!std::isnan(result)) {
-    src->advance(length);
-    return result;
-  }
+FOLLY_NODISCARD
+inline Expected<StringPiece, ConversionCode> parseTo(
+    StringPiece in,
+    StringPiece& out) noexcept {
+  out = in;
+  return StringPiece{in.end(), in.end()};
+}
 
-  for (;; src->advance(1)) {
-    if (src->empty()) {
-      throw std::range_error("Unable to convert an empty string"
-                             " to a floating point value.");
-    }
-    if (!isspace(src->front())) {
-      break;
-    }
-  }
+FOLLY_NODISCARD
+inline Expected<StringPiece, ConversionCode> parseTo(
+    StringPiece in,
+    std::string& out) {
+  out.clear();
+  out.append(in.data(), in.size()); // TODO try/catch?
+  return StringPiece{in.end(), in.end()};
+}
 
-  // Was that "inf[inity]"?
-  if (src->size() >= 3 && toupper((*src)[0]) == 'I'
-        && toupper((*src)[1]) == 'N' && toupper((*src)[2]) == 'F') {
-    if (src->size() >= 8 &&
-        toupper((*src)[3]) == 'I' &&
-        toupper((*src)[4]) == 'N' &&
-        toupper((*src)[5]) == 'I' &&
-        toupper((*src)[6]) == 'T' &&
-        toupper((*src)[7]) == 'Y') {
-      src->advance(8);
-    } else {
-      src->advance(3);
-    }
-    return std::numeric_limits<Tgt>::infinity();
-  }
+FOLLY_NODISCARD
+inline Expected<StringPiece, ConversionCode> parseTo(
+    StringPiece in,
+    fbstring& out) {
+  out.clear();
+  out.append(in.data(), in.size()); // TODO try/catch?
+  return StringPiece{in.end(), in.end()};
+}
 
-  // Was that "-inf[inity]"?
-  if (src->size() >= 4 && toupper((*src)[0]) == '-'
-      && toupper((*src)[1]) == 'I' && toupper((*src)[2]) == 'N'
-      && toupper((*src)[3]) == 'F') {
-    if (src->size() >= 9 &&
-        toupper((*src)[4]) == 'I' &&
-        toupper((*src)[5]) == 'N' &&
-        toupper((*src)[6]) == 'I' &&
-        toupper((*src)[7]) == 'T' &&
-        toupper((*src)[8]) == 'Y') {
-      src->advance(9);
-    } else {
-      src->advance(4);
-    }
-    return -std::numeric_limits<Tgt>::infinity();
+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;
   }
+};
 
-  // "nan"?
-  if (src->size() >= 3 && toupper((*src)[0]) == 'N'
-        && toupper((*src)[1]) == 'A' && toupper((*src)[2]) == 'N') {
-    src->advance(3);
-    return std::numeric_limits<Tgt>::quiet_NaN();
+template <class Error>
+struct ReturnUnit {
+  template <class T>
+  constexpr Expected<Unit, Error> operator()(T&&) const {
+    return unit;
   }
+};
 
-  // "-nan"?
-  if (src->size() >= 4 &&
-      toupper((*src)[0]) == '-' &&
-      toupper((*src)[1]) == 'N' &&
-      toupper((*src)[2]) == 'A' &&
-      toupper((*src)[3]) == 'N') {
-    src->advance(4);
-    return -std::numeric_limits<Tgt>::quiet_NaN();
-  }
+// 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());
+}
 
-  // All bets are off
-  throw std::range_error("Unable to convert \"" + src->toString()
-                         + "\" to a floating point value.");
+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
+
 /**
- * Any string, const char*, or StringPiece to double.
+ * 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_floating_point<Tgt>::value,
-  Tgt>::type
-to(StringPiece src) {
-  Tgt result = to<double>(&src);
-  detail::enforceWhitespace(src.data(), src.data() + src.size());
-  return result;
+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);
+  });
 }
 
-/*******************************************************************************
- * Integral to floating point and back
- ******************************************************************************/
+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()); });
+}
 
 /**
- * Checked conversion from integral to flating point and back. The
- * result must be convertible back to the source type without loss of
- * precision. This seems Draconian but sometimes is what's needed, and
- * complements existing routines nicely. For various rounding
- * routines, see <math>.
+ * 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, class 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),
-  Tgt>::type
-to(const Src & value) {
-  Tgt result = value;
-  auto witness = static_cast<Src>(result);
-  if (value != witness) {
-    throw std::range_error(
-      to<std::string>("to<>: loss of precision when converting ", value,
-                      " to type ", typeid(Tgt).name()).c_str());
-  }
-  return result;
+template <class Tgt>
+Expected<Tgt, detail::ParseToError<Tgt>> tryTo(StringPiece* src) {
+  Tgt 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{};
+  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); });
 }
 
 /*******************************************************************************
  * Enum to anything and back
  ******************************************************************************/
 
-#if defined(__GNUC__) && __GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 7)
-// std::underlying_type became available by gcc 4.7.0
-
 template <class Tgt, class Src>
-typename std::enable_if<std::is_enum<Src>::value, Tgt>::type
-to(const Src & value) {
-  return to<Tgt>(static_cast<typename std::underlying_type<Src>::type>(value));
+typename std::enable_if<
+    std::is_enum<Src>::value && !std::is_same<Src, Tgt>::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_enum<Tgt>::value, Tgt>::type
-to(const Src & value) {
-  return static_cast<Tgt>(to<typename std::underlying_type<Tgt>::type>(value));
+typename std::enable_if<
+    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); });
 }
 
-#else
-
 template <class Tgt, class Src>
-typename std::enable_if<std::is_enum<Src>::value, Tgt>::type
-to(const Src & value) {
-  /* static */ if (Src(-1) < 0) {
-    /* static */ if (sizeof(Src) <= sizeof(int)) {
-      return to<Tgt>(static_cast<int>(value));
-    } else {
-      return to<Tgt>(static_cast<long>(value));
-    }
-  } else {
-    /* static */ if (sizeof(Src) <= sizeof(int)) {
-      return to<Tgt>(static_cast<unsigned int>(value));
-    } else {
-      return to<Tgt>(static_cast<unsigned long>(value));
-    }
-  }
+typename std::enable_if<
+    std::is_enum<Src>::value && !std::is_same<Src, Tgt>::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, Tgt>::type
+typename std::enable_if<
+  std::is_enum<Tgt>::value && !std::is_same<Src, Tgt>::value, Tgt>::type
 to(const Src & value) {
-  /* static */ if (Tgt(-1) < 0) {
-    /* static */ if (sizeof(Tgt) <= sizeof(int)) {
-      return static_cast<Tgt>(to<int>(value));
-    } else {
-      return static_cast<Tgt>(to<long>(value));
-    }
-  } else {
-    /* static */ if (sizeof(Tgt) <= sizeof(int)) {
-      return static_cast<Tgt>(to<unsigned int>(value));
-    } else {
-      return static_cast<Tgt>(to<unsigned long>(value));
-    }
-  }
+  return static_cast<Tgt>(to<typename std::underlying_type<Tgt>::type>(value));
 }
 
-#endif // gcc 4.7 onwards
-
 } // namespace folly
-
-// FOLLY_CONV_INTERNAL is defined by Conv.cpp.  Keep the FOLLY_RANGE_CHECK
-// macro for use in Conv.cpp, but #undefine it everywhere else we are included,
-// to avoid defining this global macro name in other files that include Conv.h.
-#ifndef FOLLY_CONV_INTERNAL
-#undef FOLLY_RANGE_CHECK
-#endif
-
-#endif /* FOLLY_BASE_CONV_H_ */