Lazy.h

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// MuldeR's Utilities for Qt
// Copyright (C) 2004-2018 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 <QAtomicPointer>

//CRT
#include <functional>

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

        T& operator*(void)
        {
            T *value;
            while (!(value = m_value.fetchAndAddOrdered(0)))
            {
                if (!(value = m_initializer()))
                {
                    MUTILS_THROW("Initializer returned NULL pointer!");
                }
                if (m_value.testAndSetOrdered(NULL, value))
                {
                    break; /*success*/
                }
                delete value;
                value = NULL;
            }
            return *value;
        }

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

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