Singleton example in C++

gistfile1.c++

/*
 * Example of a singleton design pattern.
 * Copyright (C) 2011 Radek Pazdera

 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.

 * This program 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 General Public License for more details.

 * You should have received a copy of the GNU General Public License
 * along with this program. If not, see <http://www.gnu.org/licenses/>.
 */

#include <iostream>

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

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

    public:
        /* 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()
{}

int main()
{
    //new Singleton(); // Won't work
    Singleton* s = Singleton::getInstance(); // Ok
    Singleton* r = Singleton::getInstance();

    /* The addresses will be the same. */
    std::cout << s << std::endl;
    std::cout << r << std::endl;
}

https://stackoverflow.com/questions/55490024/stdcall-once-when-should-it-be-used
I think you should use std::call_once to prevent multi-thread problems