C++ Singleton Is a Lie

//VS 2019 Pro / 2013 Ultimate
//The cake is a lie, patterns are a lie

#include <iostream>
#include <Windows.h>

int main2();

class SystemClass
{
private:
    int var2;

    SystemClass()
    {
        this->var = 0;
        this->var2 = 0;
    }

public:
    int var;

    void SetVar(int param1)
    {
        this->var = param1;
    }

    void SetVar2(int param1)
    {
        this->var2 = param1;
    }

    void prn()
    {
        std::cout << "this works! " << this->var << std::endl;
    }

    static SystemClass& getInstance()
    {
        static SystemClass theInstance;

        return theInstance;
    }

    void prn2()
    {
        std::cout << "this works2! " << this->var2 << std::endl;
    }
};

class Singleton
{
private:
    /* Here will be the instance stored. */
    static Singleton* instance;

    int var;

    /* Private constructor to prevent instancing. */
    Singleton();

public:
    int var2;

    void SetVar(int param1)
    {
        this->var = param1;
    }

    void SetVar2(int param1)
    {
        this->var2 = param1;
    }

    void prn()
    {
        std::cout << "this works! " << this->var << std::endl;
    }

    void prn2()
    {
        std::cout << "this works2! " << this->var2 << std::endl;
    }

    /* Static access method. */
    static Singleton* getInstance();
};

/* Null, because instance will be initialized on demand. */
Singleton* Singleton::instance = 0;

Singleton* Singleton::getInstance()
{
    if (instance == 0)
    {
        instance = new Singleton();
    }

    return instance;
}

Singleton::Singleton()
{
    this->var = 0;
    this->var2 = 0;
}

int main()
{
    /*__asm
    {
        int 3
    }*/

    std::cout << "########### SINGLETON BY REFERENCE ###########" << std::endl;

    SystemClass& sys = SystemClass::getInstance();

    //SystemClass sys = SystemClass::getInstance();
    sys.SetVar(333);
    sys.SetVar2(666);
    sys.prn();
    sys.prn2();

    //Por referencia
    __asm
    {
        ; int 3
        mov eax, sys

        ; VISUAL STUDIO 2019
        and dword ptr[eax - 0x04], 0x0

        ; VISUAL STUDIO 2013
        ; and dword ptr[eax + 0x08], 0x0
        ; int 3
    }

    /*__asm
    {
        int 3
        mov eax, SystemClass::getInstance
        int 3
    }*/

    //std::cout << sizeof(SystemClass) << std::endl;

    //ZeroMemory(SystemClass, 4);

    sys = SystemClass::getInstance();
    sys.prn();
    sys.prn2();

    main2();

    std::cin.get();

    return 0;
}

int main2()
{
    std::cout << std::endl;
    std::cout << "########### SINGLETON BY POINTER ###########" << std::endl;

    Singleton* s = Singleton::getInstance(); // Ok

    std::cout << s << std::endl;

    s->SetVar(333);
    s->SetVar2(666);
    s->prn();
    s->prn2();

    __asm
    {
        lea eax, [Singleton::instance]
        and dword ptr[eax], 0x0
    }

    Singleton* r = Singleton::getInstance();

    std::cout << r << std::endl;

    r->prn();
    r->prn2();

    return 0;
}