Lazy.h

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// MuldeR's Utilities for Qt
// Copyright (C) 2004-2022 LoRd_MuldeR <MuldeR2@GMX.de>
//
// This library is free software; you can redistribute it and/or
// modify it under the terms of the GNU Lesser General Public
// License as published by the Free Software Foundation; either
// version 2.1 of the License, or (at your option) any later version.
//
// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
// Lesser General Public License for more details.
//
// You should have received a copy of the GNU Lesser General Public
// License along with this library; if not, write to the Free Software
// Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
//
// http://www.gnu.org/licenses/lgpl-2.1.txt

#pragma once

//MUtils
#include <MUtils/Global.h>
#include <MUtils/Exception.h>

//Qt
#include <QThread>
#include <QAtomicPointer>
#include <QAtomicInt>

//CRT
#include <functional>

namespace MUtils
{
    template<typename T> class Lazy
    {
    public:
        Lazy(std::function<T*(void)> &&initializer) : m_initializer(initializer) { }

        T& operator*(void)
        {
            return (*getValue());
        }

        T* operator->(void)
        {
            return getValue();
        }

        bool initialized()
        {
            return (m_state > 1);
        }

        ~Lazy(void)
        {
            if(T *const value = m_value)
            {
                delete value;
            }
        }

    protected:
        __forceinline T* getValue()
        {
            T *value;
            while (!(value = m_value))
            {
                if (m_state.testAndSetOrdered(0, 1))
                {
                    if (value = m_initializer())
                    {
                        m_value.fetchAndStoreOrdered(value);
                        m_state.fetchAndStoreOrdered(2);
                        break; /*success*/
                    }
                    m_state.fetchAndStoreOrdered(0);
                    MUTILS_THROW("Initializer returned NULL pointer!");
                }
                QThread::yieldCurrentThread();
            }
            return value;
        }

    private:
        QAtomicPointer<T> m_value;
        QAtomicInt m_state;
        const std::function<T*(void)> m_initializer;
    };
}