OperatorNewExtensions.h (2228B)
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