mfbt/OperatorNewExtensions.h

   1 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
   2 /* vim: set ts=8 sts=2 et sw=2 tw=80: */
   3 /* This Source Code Form is subject to the terms of the Mozilla Public
   4  * License, v. 2.0. If a copy of the MPL was not distributed with this
   5  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
   6
   7 /* A version of |operator new| that eschews mandatory null-checks. */
   8
   9 #ifndef mozilla_OperatorNewExtensions_h
  10 #define mozilla_OperatorNewExtensions_h
  11
  12 #include "mozilla/Assertions.h"
  13
  14 // Credit goes to WebKit for this implementation, cf.
  15 // https://bugs.webkit.org/show_bug.cgi?id=74676
  16 namespace mozilla {
  17 enum NotNullTag {
  18   KnownNotNull,
  19 };
  20 }  // namespace mozilla
  21
  22 /*
  23  * The logic here is a little subtle.  [expr.new] states that if the allocation
  24  * function being called returns null, then object initialization must not be
  25  * done, and the entirety of the new expression must return null.  Non-throwing
  26  * (noexcept) functions are defined to return null to indicate failure.  The
  27  * standard placement operator new is defined in such a way, and so it requires
  28  * a null check, even when that null check would be extraneous.  Functions
  29  * declared without such a specification are defined to throw std::bad_alloc if
  30  * they fail, and return a non-null pointer otherwise.  We compile without
  31  * exceptions, so any placement new overload we define that doesn't declare
  32  * itself as noexcept must therefore avoid generating a null check.  Below is
  33  * just such an overload.
  34  *
  35  * You might think that MOZ_NONNULL might perform the same function, but
  36  * MOZ_NONNULL isn't supported on all of our compilers, and even when it is
  37  * supported, doesn't work on all the versions we support.  And even keeping
  38  * those limitations in mind, we can't put MOZ_NONNULL on the global,
  39  * standardized placement new function in any event.
  40  *
  41  * We deliberately don't add MOZ_NONNULL(3) to tag |p| as non-null, to benefit
  42  * hypothetical static analyzers.  Doing so makes |MOZ_ASSERT(p)|'s internal
  43  * test vacuous, and some compilers warn about such vacuous tests.
  44  */
  45 inline void* operator new(size_t, mozilla::NotNullTag, void* p) {
  46   MOZ_ASSERT(p);
  47   return p;
  48 }
  49
  50 #endif  // mozilla_OperatorNewExtensions_h