Singleton.h

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/* Copyright (C) 2019 Wildfire Games.
 * This file is part of 0 A.D.
 *
 * 0 A.D. is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 2 of the License, or
 * (at your option) any later version.
 *
 * 0 A.D. 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 General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with 0 A.D.  If not, see <http://www.gnu.org/licenses/>.
 */

#ifndef INCLUDED_SINGLETON
#define INCLUDED_SINGLETON

#include "lib/debug.h"

/**
 * Template base class for singletons.
 *
 * Usage:
 *      class MyClass : public Singleton<MyClass> {};
 *      MyClass::GetSingleton().MyMethod();
 *
 * Modified from http://gamedev.net/reference/articles/article1954.asp
 */
template<typename T>
class Singleton
{
    NONCOPYABLE(Singleton);
public:
    Singleton()
    {
        ENSURE(!ms_singleton);
        ms_singleton = static_cast<T*>(this);
    }

    ~Singleton()
    {
        ENSURE(ms_singleton);
        ms_singleton = nullptr;
    }

    static T& GetSingleton()
    {
        ENSURE(ms_singleton);
        return *ms_singleton;
    }

    static T* GetSingletonPtr()
    {
        ENSURE(ms_singleton);
        return ms_singleton;
    }

    static bool IsInitialised()
    {
        return ms_singleton != nullptr;
    }

private:
    static T* ms_singleton;
};

template <typename T>
T* Singleton<T>::ms_singleton = nullptr;

#endif // INCLUDED_SINGLETON