Singleton.h

/*
 * Copyright (c) 2020, the SerenityOS developers.
 * 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 THE COPYRIGHT HOLDERS AND 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 THE COPYRIGHT HOLDER OR 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,
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 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

#pragma once

#include <AK/Assertions.h>
#include <AK/Atomic.h>
#include <AK/Noncopyable.h>
#include <AK/kmalloc.h>
#ifdef KERNEL
#    include <Kernel/Arch/i386/CPU.h>
#endif

#ifndef __serenity__
#    include <new>
#endif

namespace AK {

template<typename T>
struct SingletonInstanceCreator {
    static T* create()
    {
        return new T();
    }
};

template<typename T, T* (*InitFunction)() = SingletonInstanceCreator<T>::create>
class Singleton {
    AK_MAKE_NONCOPYABLE(Singleton);
    AK_MAKE_NONMOVABLE(Singleton);

public:
    Singleton() = default;

    template<bool allow_create = true>
    static T* get(T*& obj_var)
    {
        T* obj = AK::atomic_load(&obj_var, AK::memory_order_acquire);
        if (FlatPtr(obj) <= 0x1) {
            // If this is the first time, see if we get to initialize it
#ifdef KERNEL
            Kernel::ScopedCritical critical;
#endif
            if constexpr (allow_create) {
                if (obj == nullptr && AK::atomic_compare_exchange_strong(&obj_var, obj, (T*)0x1, AK::memory_order_acq_rel)) {
                    // We're the first one
                    obj = InitFunction();
                    AK::atomic_store(&obj_var, obj, AK::memory_order_release);
                    return obj;
                }
            }
            // Someone else was faster, wait until they're done
            while (obj == (T*)0x1) {
#ifdef KERNEL
                Kernel::Processor::wait_check();
#else
                // TODO: yield
#endif
                obj = AK::atomic_load(&obj_var, AK::memory_order_acquire);
            }
            if constexpr (allow_create) {
                // We should always return an instance if we allow creating one
                ASSERT(obj != nullptr);
            }
            ASSERT(obj != (T*)0x1);
        }
        return obj;
    }

    T* ptr() const
    {
        return get(m_obj);
    }

    T* operator->() const
    {
        return ptr();
    }

    T& operator*() const
    {
        return *ptr();
    }

    operator T*() const
    {
        return ptr();
    }

    operator T&() const
    {
        return *ptr();
    }

    bool is_initialized() const
    {
        T* obj = AK::atomic_load(&m_obj, AK::memory_order_consume);
        return FlatPtr(obj) > 0x1;
    }

    void ensure_instance()
    {
        ptr();
    }

private:
    mutable T* m_obj { nullptr }; // atomic
};

}