#pragma once

#include <cassert>

template<class T>
class singleton
{
    singleton() {}
    ~singleton() { is_destructed = true;  }

    static bool is_destructed;
public:
    static T& instance()
    {
        assert(!is_destructed);
        (void)is_destructed; // prevent removing is_destructed in Release configuration

        static T inst;
        return inst;
    }

    static const T& const_instance() { return instance(); }
};

template<class T>
bool singleton<T>::is_destructed = (singleton<T>::instance(), false); // force creating instance before main() is called