Kylin/Universal/Singleton.h

#ifndef CORE_SINGLETON_H
#define CORE_SINGLETON_H

#include "BoostLog.h"
#include <boost/assert.hpp>
#include <memory>

namespace Amass {

using GlobalInstance = std::true_type;
using LocalInstance = std::false_type;

struct Construct {};
struct Reference {};
struct Register {};

template <typename T, typename Style = LocalInstance>
class Singleton;

template <typename T>
class Singleton<T, LocalInstance> {
public:
    template <typename Invoke = Reference, typename InstanceType = T, typename... Args>
    static std::shared_ptr<T> instance(Args &&...args) {
        // LOG(debug) << typeid(T).name() << " address: " << (size_t)&m_instance;
        static_assert(std::is_base_of_v<T, InstanceType>, "InstanceType must be child of T ...");
        if constexpr (std::is_same_v<Invoke, Construct>) {
            if (!m_instance.expired()) {
                throw std::runtime_error("an instance have constructed ...");
            }
            auto instance = std::shared_ptr<T>(new InstanceType(std::forward<Args>(args)...));
            m_instance = instance;
            return instance;
        } else if constexpr (std::is_same_v<Invoke, Reference>) {
            return m_instance.expired() ? nullptr : m_instance.lock();
        } else if constexpr (std::is_same_v<Invoke, Register>) {
            static_assert(sizeof...(args) == 1, "args must equal to 1...");
            m_instance = std::get<0>(std::forward_as_tuple(args)...);
            return std::get<0>(std::forward_as_tuple(args)...);
        } else {
            static_assert(!std::is_same_v<Invoke, GlobalInstance>, "Can not to it!");
        }
    }

    static std::weak_ptr<T> weakPointer() {
        return m_instance;
    }

private:
    Singleton() = delete;
    ~Singleton() = delete;
    Singleton(const Singleton &) = delete;
    Singleton &operator=(const Singleton &) = delete;

private:
    inline static std::weak_ptr<T> m_instance;
};

template <typename T>
class Singleton<T, GlobalInstance> {
public:
    template <typename... Args>
    static T *instance(Args &&...args) {
        if (!m_value) {
            m_value.reset(new T(std::forward<Args>(args)...));
        }
        return m_value.get();
    }

private:
    Singleton() = delete;
    ~Singleton() = delete;
    Singleton(const Singleton &) = delete;
    Singleton &operator=(const Singleton &) = delete;

private:
    inline static std::shared_ptr<T> m_value;
};

} // namespace Amass

#endif // SINGLETON_H