singleton.hpp

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/*
Copyright (c) 2025 acrion innovations GmbH
Authors: Stefan Zipproth, s.zipproth@acrion.ch
 
This file is part of Cbeam, see https://github.com/acrion/cbeam and https://cbeam.org
 
Cbeam is offered under a commercial and under the AGPL license.
For commercial licensing, contact us at https://acrion.ch/sales. For AGPL licensing, see below.
 
AGPL licensing:
 
Cbeam is free software: you can redistribute it and/or modify
it under the terms of the GNU Affero General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.
 
Cbeam 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 Affero General Public License for more details.
 
You should have received a copy of the GNU Affero General Public License
along with Cbeam. If not, see <https://www.gnu.org/licenses/>.
*/
 
#pragma once
 
#include <cbeam/error/runtime_error.hpp> // for cbeam::error:runtime_error
#include <cbeam/logging/log_manager.hpp> // for CBEAM_LOG_DEBUG
 
#include <cassert> // for assert
 
#include <map>     // for std::map, std::operator!=, std::_Rb_tree_iterator
#include <memory>  // for std::unique_ptr, std::allocator, std::shared_ptr, std::make_shared, std::make_unique
#include <mutex>   // for std::mutex, std::lock_guard
#include <string>  // for std::string, std::operator+, std::basic_string, std::char_traits
#include <utility> // for std::pair, std::forward
#include <vector>  // for std::vector
 
namespace cbeam::lifecycle
{
    struct singleton_control
    {
    public:
        virtual ~singleton_control() = default;

        static void reset()
        {
            CBEAM_LOG_DEBUG("cbeam::lifecycle::singleton_control::reset");
 
            std::lock_guard<std::mutex> lock(_mutex);
            _shutdown = true;
            for (auto& instance : _instances)
            {
                instance.second->release_instance();
                instance.second.reset();
            }
            _instances.clear();
        }

        static void set_operational()
        {
            CBEAM_LOG_DEBUG("cbeam::lifecycle::singleton_control::set_operational");
            _shutdown = false;
        }
 
    protected:
        virtual void release_instance() = 0;
 
        static inline std::map<std::string, std::unique_ptr<singleton_control>> _instances;
        static inline std::mutex                                                _mutex;
        static inline bool                                                      _shutdown{false};
    };

    template <typename T, typename... Args>
    class singleton : public singleton_control
    {
    private:
        struct private_construction_tag
        {
        };
 
    public:
        singleton(private_construction_tag, const std::string& name, Args... args)
            : _name{name}
            , _instance{std::make_shared<T>(std::forward<Args>(args)...)}
        {
            CBEAM_LOG_DEBUG("cbeam::lifecycle::singleton construction: " + _name);
        }
 
        ~singleton() noexcept override
        {
            CBEAM_LOG_DEBUG("cbeam::lifecycle::singleton destruction: " + _name);
        }

        static std::shared_ptr<T> get(const std::string& name, Args... args)
        {
            if (_shutdown)
            {
                CBEAM_LOG_DEBUG("cbeam::lifecycle::singleton::get: " + name + ": refused to create an instance because singleton_control::reset() "
                                                                              "had been called. Use singleton_control::set_operational() to enable again.");
                return nullptr;
            }
            std::lock_guard<std::mutex> lock(_mutex);
 
            auto&      base_instance        = _instances[name];
            const bool create_base_instance = !base_instance;
 
            if (create_base_instance)
            {
                base_instance = std::make_unique<singleton>(private_construction_tag{}, name, std::forward<Args>(args)...);
            }
 
            auto derived_instance = dynamic_cast<singleton*>(base_instance.get());
 
            if (derived_instance)
            {
                return derived_instance->_instance;
            }
            else
            {
                if (create_base_instance)
                {
                    base_instance.reset();
                }
                throw cbeam::error::runtime_error("cbeam::lifecycle::singleton: incompatible singleton type requested.");
            }
        }

        static void release(const std::string& name)
        {
            CBEAM_LOG_DEBUG("cbeam::lifecycle::singleton::release: " + name);
            std::lock_guard<std::mutex> lock(_mutex);
 
            auto it = _instances.find(name);
            if (it != _instances.end())
            {
                it->second.reset();
                _instances.erase(it);
            }
        }
 
    protected:
        void release_instance() override
        {
            CBEAM_LOG_DEBUG("cbeam::lifecycle::singleton::release_instance: " + _name);
            _instance.reset();
        }
 
    private:
        singleton(const singleton&)            = delete;
        singleton(singleton&&)                 = delete;
        singleton& operator=(const singleton&) = delete;
        singleton& operator=(singleton&&)      = delete;
 
        std::string        _name;
        std::shared_ptr<T> _instance;
    };
}