// Copyright 2023 PingCAP, Inc.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

#pragma once


namespace ext
{
/** Example (1):
  *
  *    class Derived : public ext::Singleton<Derived>
  *    {
  *        friend class ext::singleton<Derived>;
  *        ...
  *    protected:
  *        Derived() {};
  *    };
  *
  * Example (2):
  *
  *    class Some
  *    {
  *        ...
  *    };
  *
  *    class SomeSingleton : public Some, public ext::Singleton<SomeSingleton> {}
  */
template <typename T>
class Singleton
{
public:
    static T & instance()
    {
        /// C++11 has thread safe statics. GCC and Clang have thread safe statics by default even before C++11.
        static T instance;
        return instance;
    }

protected:
    Singleton() = default;

public:
    Singleton(const Singleton &) = delete;
    Singleton & operator=(const Singleton &) = delete;
};

} // namespace ext