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