Add numeric_cast for checked integral narrowing casts

base/safe_numerics.h

+// Copyright 2013 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef BASE_SAFE_NUMERICS_H_
+#define BASE_SAFE_NUMERICS_H_
+
+#include <limits>
+
+#include "base/logging.h"
+
+namespace base {
+namespace internal {
+
+template <bool SameSize, bool DestLarger,
+          bool DestIsSigned, bool SourceIsSigned>
+struct IsValidNumericCastImpl;
+
+#define BASE_NUMERIC_CAST_CASE_SPECIALIZATION(A, B, C, D, Code) \
+template <> struct IsValidNumericCastImpl<A, B, C, D> { \
+  template <class Source, class DestBounds> static inline bool Test( \
+      Source source, DestBounds min, DestBounds max) { \
+    return Code; \
+  } \
+}
+
+#define BASE_NUMERIC_CAST_CASE_SAME_SIZE(DestSigned, SourceSigned, Code) \
+  BASE_NUMERIC_CAST_CASE_SPECIALIZATION( \
+      true, true, DestSigned, SourceSigned, Code); \
+  BASE_NUMERIC_CAST_CASE_SPECIALIZATION( \
+      true, false, DestSigned, SourceSigned, Code)
+
+#define BASE_NUMERIC_CAST_CASE_SOURCE_LARGER(DestSigned, SourceSigned, Code) \
+  BASE_NUMERIC_CAST_CASE_SPECIALIZATION( \
+      false, false, DestSigned, SourceSigned, Code); \
+
+#define BASE_NUMERIC_CAST_CASE_DEST_LARGER(DestSigned, SourceSigned, Code) \
+  BASE_NUMERIC_CAST_CASE_SPECIALIZATION( \
+      false, true, DestSigned, SourceSigned, Code); \
+
+// The three top level cases are:
+// - Same size
+// - Source larger
+// - Dest larger
+// And for each of those three cases, we handle the 4 different possibilities
+// of signed and unsigned. This gives 12 cases to handle, which we enumerate
+// below.
+//
+// The last argument in each of the macros is the actual comparison code. It
+// has three arguments available, source (the value), and min/max which are
+// the ranges of the destination.
+
+
+// These are the cases where both types have the same size.
+
+// Both signed.
+BASE_NUMERIC_CAST_CASE_SAME_SIZE(true, true, true);
+// Both unsigned.
+BASE_NUMERIC_CAST_CASE_SAME_SIZE(false, false, true);
+// Dest unsigned, Source signed.
+BASE_NUMERIC_CAST_CASE_SAME_SIZE(false, true, source >= 0);
+// Dest signed, Source unsigned.
+// This cast is OK because Dest's max must be less than Source's.
+BASE_NUMERIC_CAST_CASE_SAME_SIZE(true, false,
+                                 source <= static_cast<Source>(max));
+
+
+// These are the cases where Source is larger.
+
+// Both unsigned.
+BASE_NUMERIC_CAST_CASE_SOURCE_LARGER(false, false, source <= max);
+// Both signed.
+BASE_NUMERIC_CAST_CASE_SOURCE_LARGER(true, true,
+                                     source >= min && source <= max);
+// Dest is unsigned, Source is signed.
+BASE_NUMERIC_CAST_CASE_SOURCE_LARGER(false, true,
+                                     source >= 0 && source <= max);
+// Dest is signed, Source is unsigned.
+// This cast is OK because Dest's max must be less than Source's.
+BASE_NUMERIC_CAST_CASE_SOURCE_LARGER(true, false,
+                                     source <= static_cast<Source>(max));
+
+
+// These are the cases where Dest is larger.
+
+// Both unsigned.
+BASE_NUMERIC_CAST_CASE_DEST_LARGER(false, false, true);
+// Both signed.
+BASE_NUMERIC_CAST_CASE_DEST_LARGER(true, true, true);
+// Dest is unsigned, Source is signed.
+BASE_NUMERIC_CAST_CASE_DEST_LARGER(false, true, source >= 0);
+// Dest is signed, Source is unsigned.
+BASE_NUMERIC_CAST_CASE_DEST_LARGER(true, false, true);
+
+#undef BASE_NUMERIC_CAST_CASE_SPECIALIZATION
+#undef BASE_NUMERIC_CAST_CASE_SAME_SIZE
+#undef BASE_NUMERIC_CAST_CASE_SOURCE_LARGER
+#undef BASE_NUMERIC_CAST_CASE_DEST_LARGER
+
+
+// The main test for whether the conversion will under or overflow.
+template <class Dest, class Source>
+inline bool IsValidNumericCast(Source source) {
+  typedef std::numeric_limits<Source> SourceLimits;
+  typedef std::numeric_limits<Dest> DestLimits;
+  COMPILE_ASSERT(SourceLimits::is_specialized, argument_must_be_numeric);
+  COMPILE_ASSERT(SourceLimits::is_integer, argument_must_be_integral);
+  COMPILE_ASSERT(DestLimits::is_specialized, result_must_be_numeric);
+  COMPILE_ASSERT(DestLimits::is_integer, result_must_be_integral);
+
+  return IsValidNumericCastImpl<
+      sizeof(Dest) == sizeof(Source),
+      (sizeof(Dest) > sizeof(Source)),
+      DestLimits::is_signed,
+      SourceLimits::is_signed>::Test(
+          source,
+          DestLimits::min(),
+          DestLimits::max());
+}
+
+}  // namespace internal
+
+// numeric_cast<> is analogous to static_cast<> for numeric types, except that

[wtc, 2013/01/25 15:40:59]

The cast is checked_numeric_cast not numeric_cast, right?

+// it CHECKs that the specified numeric conversion will not overflow or
+// underflow. Floating point arguments are not currently allowed (this is
+// COMPILE_ASSERTd), though this could be supported if necessary.
+template <class Dest, class Source>
+inline Dest checked_numeric_cast(Source source) {
+  CHECK(internal::IsValidNumericCast<Dest>(source));
+  return static_cast<Dest>(source);
+}
+
+}  // namespace base
+
+#endif  // BASE_SAFE_NUMERICS_H_