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;
}

If this is a header .h file, class implements should be written in .cpp file
Otherwise, compile error occurs.

Good Example to show singleton. Thank you.

thank you,very good

how to implement in header only library.

Is this implementation thread-safe？

Your implementation has a memory leak.

nice example!

Very helpful! Thanks a lot!

Nice, however, I just would say that there is a memory leak. When you create a raw pointer you need to make sure to delete it in the destructor. If not then make sure to new a smart pointer so it gets deleted automatically.

The copy constructor and the copy assignment operator should be declared private, i.e.

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

Otherwise, you will be able to clone your object. If you are using C++ 11, you may leave the copy constructor and the copy assignment operator public but explicitly delete them:

public:
Singleton(const Singleton&) = delete;
Singleton& operator=(const Singleton&) = delete;

Nice, but I have some notes here:

• First, you have memory leak.
• And second, you should declare the copy constructor and the assignment operator of your class as private or delete them explicitly to prevent cloning your object.

how can i access same object every time for my program
#include
using namespace std;
class student
{
private:
int id;
int marks;
public:
void adddata()
{
int i,mks;
cout<<"enter student marks and id:";
cin>>i;
cin>>mks;
id=i;
marks=mks;
print();
}
void print()
{
cout<<"student id num:"<<id<<endl;
cout<<"student marks:"<<marks<<endl;
}

};

int main()
{
student s;

s.adddata();

x.adddata();

return 0;

}

The use of "static" inside the function getInstance() makes things even cleaner:

 * 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:                                                                     
        /* Private constructor to prevent instancing. */                         
        Singleton();                                                             
                                                                                 
    public:                                                                      
        /* Static access method. */                                              
        static Singleton* getInstance();                                         
};                                                                               
                                                                                 
Singleton* Singleton::getInstance()                                              
{                                                                                
    static Singleton instance;                                                   
                                                                                 
    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;                                                 
}

Singletons like this in multi-threaded environments. Worth a read.
https://www.aristeia.com/Papers/DDJ_Jul_Aug_2004_revised.pdf

If you delete constructor with Singleton()=delete; even your internal methods won't be able to call constructor

Your implementation has a memory leak.

In what manner? Since their initialisation singletons are supposed to live as long as the application does, so you don't have to worry about that raw pointer as once the application execution ends, the application memory will be freed by the system anyway.

This is very bad example, first copy assignment and copy ctors should be deleted, also you fail to release memory.