Source/WTF/wtf/ThreadSafeLazyUniquePtr.h

/*
 * Copyright (C) 2026 Apple Inc. All rights reserved.
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#pragma once

#include <memory>
#include <wtf/Atomics.h>
#include <wtf/Noncopyable.h>
#include <wtf/Nonmovable.h>
#include <wtf/UniqueRef.h>

namespace WTF {

// ThreadSafeLazyUniquePtr<T> is a unique-ownership pointer that is lazily installed
// once, atomically, via a compare-and-swap — no lock required. If multiple
// threads race to install, only the first install wins and the losers delete
// their candidate and observe the winner.
template<typename T>
class ThreadSafeLazyUniquePtr final {
    WTF_MAKE_NONCOPYABLE(ThreadSafeLazyUniquePtr);
    WTF_MAKE_NONMOVABLE(ThreadSafeLazyUniquePtr);
public:
    ThreadSafeLazyUniquePtr() = default;

    ~ThreadSafeLazyUniquePtr()
    {
        delete m_pointer.load(std::memory_order_relaxed);
    }

    // In general there is no need to fence in func the compareExchangeStrong will ensure any stores
    // to initialize the value will happen before the new pointer becomes visible.
    template<typename Func>
    T& ensure(NOESCAPE const Func& func) const
    {
        T* oldValue = m_pointer.load(std::memory_order_relaxed);
        if (oldValue) [[likely]] {
            // On all sensible CPUs, we get an implicit dependency-based load-load barrier when
            // loading this.
            return *oldValue;
        }

        T* newValue = toRawOwned(func());
        if (T* actual = m_pointer.compareExchangeStrong(nullptr, newValue, std::memory_order_release, std::memory_order_relaxed)) {
            delete newValue;
            return *actual;
        }
        return *newValue;
    }

    T* get() const { return m_pointer.load(std::memory_order_relaxed); }
    explicit operator bool() const { return !!get(); }

private:
    static T* toRawOwned(std::unique_ptr<T>&& ptr) { return ptr.release(); }
    static T* toRawOwned(UniqueRef<T>&& ref) { return ref.moveToUniquePtr().release(); }

    mutable Atomic<T*> m_pointer { nullptr };
};

static_assert(sizeof(ThreadSafeLazyUniquePtr<int>) == sizeof(int*));

} // namespace WTF

using WTF::ThreadSafeLazyUniquePtr;