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/*
* Copyright (C) 2013-2019 Apple Inc. All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY APPLE INC. AND ITS CONTRIBUTORS ``AS IS''
* AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
* THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
* PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR ITS CONTRIBUTORS
* BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
* CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
* SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
* INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
* CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
* ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF
* THE POSSIBILITY OF SUCH DAMAGE.
*/
#pragma once
#include <type_traits>
#include <utility>
#include <wtf/AlignedStorage.h>
#include <wtf/ForbidHeapAllocation.h>
#include <wtf/MainThread.h>
#include <wtf/RefCounted.h>
// NeverDestroyed is a smart-pointer-like class that ensures that the destructor
// for the given object is never called, but doesn't use the heap to allocate it.
// It's useful for static local variables, and can be used like so:
//
// MySharedGlobal& mySharedGlobal()
// {
// static NeverDestroyed<MySharedGlobal> myGlobal("Hello", 42);
// return myGlobal;
// }
namespace WTF {
struct AnyThreadsAccessTraits {
static void assertAccess()
{
}
};
struct MainThreadAccessTraits {
static void assertAccess()
{
ASSERT(isMainThread());
}
};
template<typename T, typename AccessTraits> class NeverDestroyed {
WTF_MAKE_NONCOPYABLE(NeverDestroyed);
WTF_FORBID_HEAP_ALLOCATION;
public:
template<typename... Args> NeverDestroyed(Args&&... args)
{
AccessTraits::assertAccess();
MaybeRelax<T>(new (storagePointer()) T(std::forward<Args>(args)...));
}
NeverDestroyed(NeverDestroyed&& other)
{
AccessTraits::assertAccess();
MaybeRelax<T>(new (storagePointer()) T(WTFMove(*other.storagePointer())));
}
operator T&() { return *storagePointer(); }
T& get() { return *storagePointer(); }
T* operator->() { return storagePointer(); }
operator const T&() const { return *storagePointer(); }
const T& get() const { return *storagePointer(); }
const T* operator->() const { return storagePointer(); }
private:
using PointerType = typename std::remove_const<T>::type*;
PointerType storagePointer() const
{
AccessTraits::assertAccess();
return const_cast<PointerType>(m_storage.get());
}
template<typename PtrType, bool ShouldRelax = std::is_base_of<RefCountedBase, PtrType>::value> struct MaybeRelax {
explicit MaybeRelax(PtrType*) { }
};
template<typename PtrType> struct MaybeRelax<PtrType, true> {
explicit MaybeRelax(PtrType* ptr) { ptr->relaxAdoptionRequirement(); }
};
// FIXME: Investigate whether we should allocate a hunk of virtual memory
// and hand out chunks of it to NeverDestroyed instead, to reduce fragmentation.
AlignedStorage<T> m_storage;
};
// FIXME: It's messy to have to repeat the whole class just to make this "lazy" version.
// Should revisit clients to see if we really need this, and perhaps use templates to
// share more of the code with the main NeverDestroyed above.
template<typename T, typename AccessTraits> class LazyNeverDestroyed {
WTF_MAKE_NONCOPYABLE(LazyNeverDestroyed);
WTF_FORBID_HEAP_ALLOCATION;
public:
LazyNeverDestroyed() = default;
template<typename... Args>
void construct(Args&&... args)
{
AccessTraits::assertAccess();
constructWithoutAccessCheck(std::forward<Args>(args)...);
}
template<typename... Args>
void constructWithoutAccessCheck(Args&&... args)
{
ASSERT(!m_isConstructed);
#if ASSERT_ENABLED
m_isConstructed = true;
#endif
MaybeRelax<T>(new (storagePointerWithoutAccessCheck()) T(std::forward<Args>(args)...));
}
operator T&() { return *storagePointer(); }
T& get() { return *storagePointer(); }
T* operator->() { return storagePointer(); }
operator const T&() const { return *storagePointer(); }
const T& get() const { return *storagePointer(); }
const T* operator->() const { return storagePointer(); }
#if ASSERT_ENABLED
bool isConstructed() const { return m_isConstructed; }
#endif
private:
using PointerType = typename std::remove_const<T>::type*;
PointerType storagePointerWithoutAccessCheck() const
{
ASSERT(m_isConstructed);
return const_cast<PointerType>(m_storage.get());
}
PointerType storagePointer() const
{
AccessTraits::assertAccess();
return storagePointerWithoutAccessCheck();
}
template<typename PtrType, bool ShouldRelax = std::is_base_of<RefCountedBase, PtrType>::value> struct MaybeRelax {
explicit MaybeRelax(PtrType*) { }
};
template<typename PtrType> struct MaybeRelax<PtrType, true> {
explicit MaybeRelax(PtrType* ptr) { ptr->relaxAdoptionRequirement(); }
};
#if ASSERT_ENABLED
// LazyNeverDestroyed objects are always static, so this variable is initialized to false.
// It must not be initialized dynamically; that would not be thread safe.
bool m_isConstructed;
#endif
// FIXME: Investigate whether we should allocate a hunk of virtual memory
// and hand out chunks of it to NeverDestroyed instead, to reduce fragmentation.
AlignedStorage<T> m_storage;
};
template<typename T> NeverDestroyed(T) -> NeverDestroyed<T>;
template<typename T> using MainThreadNeverDestroyed = NeverDestroyed<T, MainThreadAccessTraits>;
template<typename T> using MainThreadLazyNeverDestroyed = LazyNeverDestroyed<T, MainThreadAccessTraits>;
} // namespace WTF;
using WTF::LazyNeverDestroyed;
using WTF::NeverDestroyed;
using WTF::MainThreadNeverDestroyed;
using WTF::MainThreadLazyNeverDestroyed;
using WTF::AnyThreadsAccessTraits;
using WTF::MainThreadAccessTraits;
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