singleton.h

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/*
 * This file is part of Quantum++.
 *
 * MIT License
 *
 * Copyright (c) 2013 - 2020 Vlad Gheorghiu.
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE.
 */

#ifndef INTERNAL_CLASSES_SINGLETON_H_
#define INTERNAL_CLASSES_SINGLETON_H_

namespace qpp {
namespace internal // internal class, do not modify
{
template <typename T>
class Singleton {
  protected:
    Singleton() noexcept = default;

    Singleton(const Singleton&) = delete;

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

    virtual ~Singleton() = default; // to silence base class Singleton<T> has a
                                    // non-virtual destructor [-Weffc++]

  public:
    static T& get_instance() noexcept(std::is_nothrow_constructible<T>::value) {
        // Guaranteed to be destroyed.
        // Instantiated on first use.
        // Thread safe in C++11
        static T instance;

        return instance;
    }

#ifndef NO_THREAD_LOCAL_

    static T& get_thread_local_instance() noexcept(
        std::is_nothrow_constructible<T>::value) {
        // Guaranteed to be destroyed.
        // Instantiated on first use.
        // Thread safe in C++11
        thread_local static T instance;

        return instance;
    }

#endif // NO_THREAD_LOCAL_
};     /* class Singleton */

} /* namespace internal */
} /* namespace qpp */

#endif /* INTERNAL_CLASSES_SINGLETON_H_ */