Added integer parsing functions in base/string_to_number.h

webrtc/base/string_to_number.h

+/*
+ *  Copyright 2017 The WebRTC Project Authors. All rights reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#ifndef WEBRTC_BASE_STRING_TO_NUMBER_H_
+#define WEBRTC_BASE_STRING_TO_NUMBER_H_
+
+#include <string>
+#include <limits>
+
+#include "webrtc/base/optional.h"
+
+namespace rtc {
+
+// This file declares two families of functions to parse numbers from strings,
+// one set for integers and one set for floats. The standard C library functions
+// either fail to indicate errors (atoi, etc.) or are a hassle to work with
+// (strtol, sscanf, etc.). The standard C++ library functions (std::stoi, etc.)
+// indicate errors by throwing exceptions, which are disabled in WebRTC.
+//
+// Integers are parsed using one of the following functions:
+//   rtc::Optional<int-type> StringToNumber(const char* str, int base = 10);
+//   rtc::Optional<int-type> StringToNumber(const std::string& str,
+//                                          int base = 10);
+//
+// These functions parse a value from the beginning of a string into one of the
+// fundamental integer types, or returns an empty Optional if parsing
+// failed. Values outside of the range supported by the type will be
+// rejected. By setting base to 0, one of octal, decimal or hexadecimal will be
+// detected from the string's prefix (0, nothing or 0x, respectively).
+// If non-zero, base can be set to a value between 2 and 36 inclusively.
+//
+// Floating-point values are parsed similarly to integers, but don't support
+// explicitly specifying the base:
+//   rtc::Optional<float-type> StringToNumber(const char* str);
+//   rtc::Optional<float-type> StringToNumber(const std::string& str);
+
+template <typename T>
+typename std::enable_if<std::is_integral<T>::value && std::is_signed<T>::value,
+                        rtc::Optional<T>>::type
+StringToNumber(const char* str, int base = 10) {

[kwiberg-webrtc, 2017/02/16 00:27:24]

You probably want to static_assert that long long is at least as large as your
type. (Hmm. Is that guaranteed by the standard? Even if it is, it's probably
good to assert it to give readers peace of mind...)

[ossu, 2017/02/16 10:19:26]

Makes sense. The tests should catch any case where that is not true already,
though.

[kwiberg-webrtc, 2017/02/16 11:59:42]

Yes, but it's easier for a human to see that the static_assert tests actually
catch everything.

+  RTC_DCHECK(str);
+  char* end = nullptr;
+  errno = 0;
+  const long long int value = std::strtoll(str, &end, base);
+  if (end != str && errno == 0 && value >= std::numeric_limits<T>::min() &&
+      value <= std::numeric_limits<T>::max()) {
+    return rtc::Optional<T>(static_cast<T>(value));
+  }
+  return rtc::Optional<T>();
+}
+
+template <typename T>
+typename std::enable_if<std::is_integral<T>::value &&
+                            std::is_unsigned<T>::value,
+                        rtc::Optional<T>>::type
+StringToNumber(const char* str, int base = 10) {
+  RTC_DCHECK(str);
+  // Explicitly discard negative values. std::strtoull parsing causes unsigned
+  // wraparound. We cannot just reject values that start with -, though, since
+  // -0 is perfectly fine, as is -0000000000000000000000000000000.
+  bool is_negative = false;
+  for (const char *ptr = str; isspace(*ptr) || *ptr == '-'; ++ptr) {
+    if (*ptr == '-') {
+      is_negative = true;
+    }
+  }

[kwiberg-webrtc, 2017/02/16 00:27:23]

You allow multiple minus signs---that's probably not a good idea. (In fact, I'd
argue that it may make sense to reject "-0" and similar when the result type is
unsigned, but I don't feel strongly about it.)

[ossu, 2017/02/16 10:19:26]

I should check, but I'd expect stroull would reject it if it were invalid. My
check for - is only to reject slightly more things.

[kwiberg-webrtc, 2017/02/16 11:59:42]

Oh, I see. I thought you were discarding the minuses, but you still pass them to
std::strtoull.

+  char* end = nullptr;
+  errno = 0;
+  const unsigned long long int value = std::strtoull(str, &end, base);
+  if (end != str && errno == 0 && value >= std::numeric_limits<T>::min() &&
+      value <= std::numeric_limits<T>::max() && (value == 0 || !is_negative)) {
+    return rtc::Optional<T>(static_cast<T>(value));
+  }
+  return rtc::Optional<T>();
+}
+
+template <typename T>
+typename std::enable_if<std::is_floating_point<T>::value,
+                        rtc::Optional<T>>::type
+StringToNumber(const char* str);
+
+template<>
+rtc::Optional<float> StringToNumber<float>(const char* str) {
+  char *end = nullptr;
+  errno = 0;
+  const float value = std::strtof(str, &end);
+  if (end != str && errno == 0) {
+    return rtc::Optional<float>(value);
+  }
+  return rtc::Optional<float>();
+}
+
+template<>
+rtc::Optional<double> StringToNumber<double>(const char* str) {
+  char *end = nullptr;
+  errno = 0;
+  const double value = std::strtod(str, &end);
+  if (end != str && errno == 0) {
+    return rtc::Optional<double>(value);
+  }
+  return rtc::Optional<double>();
+}
+
+template<>
+rtc::Optional<long double> StringToNumber<long double>(const char* str) {
+  char *end = nullptr;
+  errno = 0;
+  const long double value = std::strtold(str, &end);
+  if (end != str && errno == 0) {
+    return rtc::Optional<long double>(value);
+  }
+  return rtc::Optional<long double>();
+}

[kwiberg-webrtc, 2017/02/16 00:27:23]

You could probably unify these three if you made a templated wrapper for
strtof/strtod/strtold.

[ossu, 2017/02/16 10:19:26]

Yeah, that's true. I'll look into that. As-is, the implementations need to be
moved into a .cc-file, since fully specialized templates act like normal
functions; I got multiple definitions of the same symbol when linking in another
CL I'm working on that uses this.

I'm considering putting all implementations in a .cc file, and explicitly
instantiating them for the basic types. So we don't risk ending up inlining this
everywhere it's used.

[kwiberg-webrtc, 2017/02/16 11:59:42]

Sounds good.

+
+// The std::string overloads only exists if there is a matching const char*
+// version.
+template <typename T>
+auto StringToNumber(const std::string& str, int base)
+    -> decltype(StringToNumber<T>(str.c_str(), base)) {
+  return StringToNumber<T>(str.c_str(), base);
+}
+
+template <typename T>
+auto StringToNumber(const std::string& str)
+    -> decltype(StringToNumber<T>(str.c_str())) {
+  return StringToNumber<T>(str.c_str());
+}

[kwiberg-webrtc, 2017/02/16 00:27:23]

Could you ditch this one if you defaulted base to 10 on line 125?

[ossu, 2017/02/16 10:19:26]

Alas, no, there's no base parameter to the floating-point variants.

+
+}  // namespace rtc
+
+#endif  // WEBRTC_BASE_STRING_TO_NUMBER_H_