static - Singleton Class in C++

Question

I have used singleton calss following the example:

singleton class

But i get the error as "Unresolved external symbols"

this is the code i tried out:

#include<iostream>
using namespace std;

class singleton
{
    int value;
    static singleton *instance;
protected:
    singleton()
    {
        value=0;
    }
public:
    static void initialize()
    {
        if(instance==NULL)
            singleton();
        else
            cout<<"An instance of singleton already exist...";
    }
    static singleton& getInstance()
    { 
        return *instance; 
    }
    int getValue() 
    { 
        return value; 
    }

};

void main()
{
    singleton::initialize();
}

A little bit explanation on Singleton classes would be really great. The scenario its used. advantages and drawbacks. Alternatives to Singleton. etc

Answer 1

For a start, I think:

singleton();

should be:

instance = new singleton();

The way you have it, you're not actually storing the newly instantiated object so instance will always be null.

It's also good form to explicitly set statics with:

singleton *singleton::instance = 0;

(outside the class definition).

In fact, it's possibly better to start with the baseline singleton code and work your way up from there. This is a standard-form pointer version:

#include <iostream>

class singleton {
    protected:
        static singleton *instance;
        singleton() { }
    public:
        static singleton *getInstance() {
            if (instance == 0)
                instance = new singleton();
            return instance;
        }
};
singleton *singleton::instance = 0;

int main() {
    singleton *s1 = singleton::getInstance();
    singleton *s2 = singleton::getInstance();
    std::cout << s1 << '
';
    std::cout << s2 << '
';
    return 0;
}

You can see that both pointers are the same from the output:

0xbc0358
0xbc0358

Or the reference version, since that seems to be what you're aiming for:

#include <iostream>

class singleton {
    protected:
        static singleton *instance;
        singleton() { }
    public:
        static singleton& getInstance() {
            if (instance == 0)
                instance = new singleton();
            return *instance;
        }
};
singleton *singleton::instance = 0;

int main() {
    singleton &s1 = singleton::getInstance();
    singleton &s2 = singleton::getInstance();
    std::cout << &s1 << '
';
    std::cout << &s2 << '
';
    return 0;
}