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// Copyright 2011 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
// WARNING: Thread local storage is a bit tricky to get right. Please make sure
// that this is really the proper solution for what you're trying to achieve.
// Don't prematurely optimize, most likely you can just use a Lock.
#ifndef BASE_THREADING_THREAD_LOCAL_H_
#define BASE_THREADING_THREAD_LOCAL_H_
#include <memory>
#include "base/dcheck_is_on.h"
#include "base/memory/ptr_util.h"
#include "base/threading/thread_local_internal.h"
#include "base/threading/thread_local_storage.h"
namespace base {
// `thread_local` is only allowed for trivially-destructible types (see
// //styleguide/c++/c++.md#thread_local-variables). This class provides
// thread-scoped management of non-trivially-destructible types. Pointers handed
// to it are owned and automatically deleted during their associated thread's
// exit phase (or when replaced if Set() is invoked multiple times on the same
// thread).
//
// The ThreadLocalOwnedPointer instance itself can only be destroyed when no
// threads, other than the one it is destroyed on, have remaining state set in
// it. Typically this means that ThreadLocalOwnedPointer instances are held in
// static storage or at the very least only recycled in the single-threaded
// phase between tests in the same process.
#if DCHECK_IS_ON()
template <typename T>
using ThreadLocalOwnedPointer = internal::CheckedThreadLocalOwnedPointer<T>;
#else // DCHECK_IS_ON()
template <typename T>
class ThreadLocalOwnedPointer {
public:
ThreadLocalOwnedPointer() = default;
ThreadLocalOwnedPointer(const ThreadLocalOwnedPointer&) = delete;
ThreadLocalOwnedPointer& operator=(const ThreadLocalOwnedPointer&) = delete;
~ThreadLocalOwnedPointer() {
// Assume that this thread is the only one with potential state left. This
// is verified in ~CheckedThreadLocalOwnedPointer().
Set(nullptr);
}
T* Get() const { return static_cast<T*>(slot_.Get()); }
// Sets a new value, returns the old.
std::unique_ptr<T> Set(std::unique_ptr<T> ptr) {
auto existing = WrapUnique(Get());
slot_.Set(const_cast<void*>(static_cast<const void*>(ptr.release())));
return existing;
}
T& operator*() { return *Get(); }
private:
static void DeleteTlsPtr(void* ptr) { delete static_cast<T*>(ptr); }
ThreadLocalStorage::Slot slot_{&DeleteTlsPtr};
};
#endif // DCHECK_IS_ON()
} // namespace base
#endif // BASE_THREADING_THREAD_LOCAL_H_
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