Singleton.h

/*
 * Gamedev Framework (gf)
 * Copyright (C) 2016-2021 Julien Bernard
 *
 * This software is provided 'as-is', without any express or implied
 * warranty.  In no event will the authors be held liable for any damages
 * arising from the use of this software.
 *
 * Permission is granted to anyone to use this software for any purpose,
 * including commercial applications, and to alter it and redistribute it
 * freely, subject to the following restrictions:
 *
 * 1. The origin of this software must not be misrepresented; you must not
 *    claim that you wrote the original software. If you use this software
 *    in a product, an acknowledgment in the product documentation would be
 *    appreciated but is not required.
 * 2. Altered source versions must be plainly marked as such, and must not be
 *    misrepresented as being the original software.
 * 3. This notice may not be removed or altered from any source distribution.
 */
#ifndef GF_SINGLETON_H
#define GF_SINGLETON_H

#include <cassert>
#include <utility>

namespace gf {
#ifndef DOXYGEN_SHOULD_SKIP_THIS
inline namespace v1 {
#endif

  template<typename T>
  class SingletonStorage;

  template<typename T>
  class Singleton {
  public:
    Singleton()
    : m_single(nullptr)
    {
    }

    Singleton(const Singleton&) = delete;

    Singleton(Singleton&&) = delete;
    Singleton& operator=(const Singleton&) = delete;

    Singleton& operator=(Singleton&&) = delete;

    T& operator()() {
      assert(m_single);
      return *m_single;
    }

    void reset() noexcept {
      m_single = nullptr;
    }

    bool isValid() const noexcept {
      return m_single != nullptr;
    }

  private:
    friend class SingletonStorage<T>;

    T *m_single;
  };

  template<typename T>
  class SingletonStorage {
  public:
    template<typename ... Args>
    SingletonStorage(Singleton<T>& ref, Args&&... args)
    : m_storage(std::forward<Args>(args)...) {
      assert(ref.m_single == nullptr);
      ref.m_single = &m_storage;
    }

  private:
    T m_storage;
  };

#ifndef DOXYGEN_SHOULD_SKIP_THIS
}
#endif
}

#endif // GF_SINGLETON_H