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Created July 21, 2011 20:25
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Singleton example in 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;
}
@rakeshmalviya1985
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Is this implementation thread-safe?

no i will make it

@saravpreet14
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This will make a new object. To prevent that you must delete copy constructor or make it private

Singleton a(*s);
std::cout << &a << std::endl;

@SwarajKetan
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SwarajKetan commented Oct 20, 2020

//I implement this way

class Singleton 
{
private:

    Singleton() { }
    void operator delete(void*) {}; // such that its not deleted accidentally
public:

    Singleton(Singleton&) = delete; // Copy prohibited
    void operator=(const Singleton&) = delete; // Assignment prohibited
    Singleton& operator=(Singleton&&) = delete; // Move assignment
    static Singleton* getInstance();
    // This is a sample method
    std::chrono::system_clock::time_point getTime() const {
        auto now = std::chrono::system_clock::now();
        return now;
    };
};

Singleton* Singleton::getInstance() {
    static Singleton* pInstance_;
    return pInstance_;
}

@philipphenkel
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@SwarajKetan Your singleton gets never created. You could either add a new to your getInstance implementation or avoid pointers completely like here:

Singleton& Singleton::getInstance() {
    static Singleton instance;
    return instance;
}

@asdlei99
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asdlei99 commented Jan 5, 2021

@TravisZhang
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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

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