singleton.h

/*
 *  Copyright (C) 2004-2021 Edward F. Valeev
 *
 *  This file is part of Libint.
 *
 *  Libint 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 3 of the License, or
 *  (at your option) any later version.
 *
 *  Libint 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 Libint.  If not, see <http://www.gnu.org/licenses/>.
 *
 */
 
#ifndef _libint2_include_libint2_util_singleton_h_
#define _libint2_include_libint2_util_singleton_h_
 
#include <memory>
 
namespace libint2 {
  namespace detail {
 
    template <typename T> class managed_singleton {
      public:
        static T* instance() {
          if (not instance_)
            instance_ = std::unique_ptr<T>(new T);
          return instance_.get();
        }
        static bool instance_exists() {
          return instance_.get() != nullptr;
        }
        static void delete_instance() {
          instance_.reset();
        }
      private:
        managed_singleton() = delete;
        static std::unique_ptr<T> instance_;
    };
    template <typename T> std::unique_ptr<T> managed_singleton<T>::instance_;
 
  }  // namespace libint2::detail
} // namespace libint2
 
#endif /* header guard */