Add numeric_cast for checked integral narrowing casts

base/numeric_cast.h

Index: base/numeric_cast.h
===================================================================
--- base/numeric_cast.h	(revision 0)
+++ base/numeric_cast.h	(revision 0)
@@ -0,0 +1,58 @@
+// 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_NUMERIC_CAST_H_
+#define BASE_NUMERIC_CAST_H_
+
+#include <limits>
+
+#include "base/logging.h"
+
+namespace base {
+
+// numeric_cast<> is analogous to static_cast<> for numeric types, except that
+// 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.
+
+// The main test for whether the conversion will under or overflow.
+template <class Dest, class Source>
+inline bool IsNumericCastableTo(Source source) {
+  typedef std::numeric_limits<Source> source_limits;
+  typedef std::numeric_limits<Dest> dest_limits;
+  COMPILE_ASSERT(source_limits::is_specialized, argument_must_be_numeric);
+  COMPILE_ASSERT(source_limits::is_integer, argument_must_be_integral);
+  COMPILE_ASSERT(dest_limits::is_specialized, result_must_be_numeric);
+  COMPILE_ASSERT(dest_limits::is_integer, result_must_be_integral);
+
+  // Source and Dest are the same type.
+  if (dest_limits::digits == source_limits::digits &&
+      dest_limits::is_signed == source_limits::is_signed) {
+    return true;
+
+  // Dest is wider. Check for underflow if dest is not signed.
+  } else if (dest_limits::digits > source_limits::digits) {
+    return dest_limits::is_signed || source >= 0;
+
+  // Destination is narrower than source.
+  } else {
+    // Check for underflow if source is signed.
+    if (source_limits::is_signed &&
+        source < static_cast<Source>(dest_limits::min()))
+      return false;
+
+    // Check for overflow.
+    return source <= static_cast<Source>(dest_limits::max());
+  }
+}
+
+template <class Dest, class Source>
+inline Dest numeric_cast(Source source) {
+  CHECK(IsNumericCastableTo<Dest>(source));
+  return static_cast<Dest>(source);
+}
+
+}  // namespace base
+
+#endif  // BASE_NUMERIC_CAST_H_